Nitrogen transformations in Lake Bonney, Antarctica : dynamics in a non-turbulent environment
by Christopher Dee Woolston
A thesis submitted in partial fulfillment of the requirements of the degree of Master of Science in
Biological Sciences
Montana State University
© Copyright by Christopher Dee Woolston (1994)
Abstract:
A thesis submitted in partial fulfillment of the requirements of the degree of Master of Science in
Biological Sciences NITROGEN TRANSFORMATIONS IN LAKE BONNEY, ANTARCTICA:
DYNAMICS IN A NON-TURBULENT ENVIRONMENT
by
C hristopher Dee W oolston
A thesis su b m itted in p a rtia l fu lfillm en t
o f th e req u irem en ts o f th e d eg ree
of
M aster o f Science
in
Biological Sciences
MONTANA STATE UNIVERSITY
Bozeman, M ontana
A ugust 1994
HSOi
ii
APPROVAL
o f a thesis su b m itted by
C hristopher Dee Woolston
This thesis h as b een re a d b y each m e m b er o f th e thesis
com m ittee a n d has been fo u n d to be satisfactory reg ard in g
con ten t, English usage, form at, citations, bibliographic style, a n d
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G rad u ate Studies.
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D ate
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Ill
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iv
ACKNOWLEDGMENTS
Sincere thanks go to Dr. Jo h n Priscu w ho m a d e it possible
fo r m e to w ork in A ntarctica a n d p atien tly allow ed m e to
succeed. I also w ish to th a n k all those who p ro v id e d assistance
in th e field: M ary Voytek, Alix Cockcroft, Kate Wing, V an
K albach, th e staff o f C rary Laboratory, a n d m a n y o th e r
em ployees of A.S.A. P articular th an k s go to R ichard B artlett,
th e Lake B bnney rese arch assistan t w ho w orked d ilig en tly to
keep th e cam p a n d th e science ru n n in g sm oothly.
Discussions w ith Mike Briggs, Tom Sharp, a n d Rob
Edw ards g reatly im p ro v ed m y rese arch a n d th is thesis.
C om m ents from com m ittee m em bers Dr. W arren Jones, Dr.
Calvin Kaya, Dr. G ordon McFeters, a n d Dr. Lynn Irb y w ere
in stru ctiv e a n d appreciated.
I w ish to th a n k m y fam ily: N eom a N elson-D aniels, C arrie
W oolston, Mom a n d Dad, Bill a n d Peg W oolston, Bob a n d Carol
Lind, Edwin a n d Carrie Dover, arid Carwin a n d T heone Dover.
T h eir su p p o rt a n d en couragem ent m ade m y ed u catio n possible.
Several friends, n o tab ly Pat Joyce, Shawn M orrison, Steve
Bradley, a n d Rob Edwards, p ro v id ed sh elter a n d san ity w h en I
n e e d e d it.
Finally, th an k s go to m y wife, Blythe. H er u n d e rsta n d in g
o f science was a n inspiration, a n d h e r u n d e rsta n d in g o f m e was
a blessing.
This rese arch was su p p o rted b y N.SE. g ra n t DPP9117907 (aw ard ed to J.C. Priscu) a n d the G ary Lynch M em orial
A w ard.
V
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS................................................................................... ...iv
LIST OF TABLES..................................................................................................viii
LIST OF FIGURES......... ................................................................
ABSTRACT.............................................
x
xii
INTRODUCTION......................................................................................................I
PREVIOUS RESEARCH ON LAKE BONNEY AND OTHER
DRY VALLEY LAKES.................................
6
HYPOTHESES AND OBJECTIVES..................................................................... ....9
DESCRIPTION OF STUDY SITE............................................ .............................11
Geography and Geology...................................................................11
Climate................................................................................... 11
Morphology............................................................................12
PART I: ROUTINE DATA COLLECTION AND OVERVIEW OF
15N EXPERIMENTS..............................................................................
13
ROUTINE DATA COLLECTION METHODS........................................... 14
Parameters Analyzed........................................................................14
Water Sampling Procedure................................................. 14
Chemical and BiologicalMeasurements........................... 15
Chlorophyll a.........................................
15
Nutrients.................................................................................16
Particulate Carbon and Nitrogen...........................
18
Primary Productivity.......................................
18
Physical Parameters...........................................
20
vi
TABLE OF CONTENTS, CONT
Page
RESULTS.................................................................................................. .23
Nutrients.........................................................................
’.23
Primary Productivity and Chlorophyll a.................... „23
Particulate Carbon and Nitrogen..............................
23
Irradiance............................................................................... 27
GENERAL OVERVIEW OF 15N EXPERIMENTS................................... 32
Uptake Measurements......................................................................32
I^N Determination...........................................................................36
PART II: UPTAKE AND REGENERATION OF DISSOLVED
INORGANIC NITROGEN.......................................................................... 39
INTRODUCTION........................................................................................ 40
METHODS................................................................................................... 46
Isotope Dilution.................................................................................46
Size Fractionated Regeneration........................................ 47
Substrate Effects on Regeneration.......... ....................... 48
I^N Analysis of Filtrate......... ............................................ 49
Calculation of Regeneration................................................54
Production of Ammonium from Serine.........................55
Substrate Kinetics...,............................................................. 57
Water Column Uptake..........................................................64
Time Course...................................................................
65
RESULTS.................................................................................................... 67
Isotope Dilution.........................................
67
Size Fractionation..................................................................72
Substrate Effect on Regeneration..................................... 73
Ammonium Production from Serine............................... 74
Substrate Kinetics...,..............................
77
Water Column Uptake.......................................................... 87
vii
TABLE OF CONTENTS, CONT.
Time Course.................................................
Page
87
DISCUSSION.............................................................................................. 97
Regeneration.........................................
97
Substrate Kinetics.....................
98
Water Column Uptake...............
100
Time Course.........................................................................103
CONCLUSION AND AREAS OF FURTHER RESEARCH..................... 106
PART HI: IRRADIANCE REQUIREMENTS OF DlN UPTAKE......................108
INTRODUCTION...................................................................................... 109
Hypotheses...................................................................................... 112
METHODS......................
113
RESULTS.............................
117
DISCUSSION............................................................................................. 134
CONCLUSION AND AREAS FOR FURTHER RESEARCH................... 145
GENERAL CONCLUSION....................................................................,„.146
REFERENCES............................................................................................147
v iii
L IS T O F T A B L E S
T a b le
Page
1. Dates and depths of routine data collection;..................
2.
21
Extinction coefficents in east and west lobe
water columns, 1993........................................................................ 29
/
3.
4.
Averages and ranges of daily irradiance
(pmol m"2 s"1) at 4 east lobe depths
during austral spring, summer,
fall, and winter, 1993...................................................................... 30
Percent isotope discrimination for extraction
of NH4+ with zeolite at various
enrichments................... i.........................................................53
5.
6.
enrichments (|iM) for substrate kinetics
experiments..............................................
58
enrichements (pM) for water column uptake
experiments...................................................................................... 65
7.
Average rates of regeneration (pM h"*) from
isotope dilution experiments.......................................................... 72
8.
Average rates of regeneration from size
fractionated isotope dilution
experiments..... ...............
73
Average rates of regeneration (pM h"l) from
substrate effects on regeneration
experiments...............................
74
9.
ix
LIST OF TABLE, CONT.
T able
Page
10.
Percentage of added serine converted
toNH4+.................................................................
77
11.
Original and regeneration corrected parameters
of Michaelis-Menten curves......................................................... 80
12.
Average uptake and slopes of linear regressions
of substrate kinetics experiments................................................ 82
13.
Upward diffusive flux and uptake rates ( pM h"*)
of DIN at 5 m and 13 m in each lobe, 1993..........................86
14.
Nitrogen uptake vs irradiance paTamters
from hyperbolic curves.......................
128
Slope, dark uptake, and maximum uptake for nitrogen
uptake vs irradiance experiments described by
linear regressions...................................................................
129
15.
16.
Uptake paramters for selected 1993 uptake
vs irradaince experiments corrected for
regeneration........................................... .................,...................... 132
17.
Threshold irradiance (Ij) and estimated
portion of Noy 17, 1993
that I< It..................!....................................................................... 133
X
L IS T O F F IG U R E S
F ig u re
'
Page
1.
DIN and SRP concentrations (p.M) in east
and west lobes of Lake Bonney,
1993..................................................................................................24
2.
Chlorophyll a (Hg I"*) and primary productivity
(mg C m"3 d"^) in east and west
lobes of Lake Bonney, 1993...........................................................25
3. PC=PN ratios (pMrpM) in east and west
lobes of Lake Bonney...................................................................... 26
4.
Natural log of irradiance (pmol m ^ s"*)
in east and west lobes of
Lake Bonney..................................................................................... 28
5.
Estimated irradiance throughout Nov 17, 1993
at 5, 13, 17, and 25m east lobe.....................................................31
6. I^ N atom-percent of filter vs ^ N
atom-percent of filtrate in
kill experiments..................
7.
35
Measured ^ N atom-percents
vs standards.......................................................................................37
8. Efficiency of NILt+ extraction using zeolite
with and without a rotary
evaporator...............................
51
xi
LIST OF FIGURES, CONT.
F ig u re
Page
9.
atom-percent and NH4+ concentration
of the filtrate over time in isotope
dilution experiments, 1992-1993.............. .............................. ...68
10.
atom-percent and NH4"1" concentration
of the filtrate over time in serine
experiments, 1992-1993..................................................................75
11. Substrate kinetics of NH4+ uptake, 1993............
78
12. Water column DIN uptake, 1992-1993.........................................88
13. Time course of DIN uptake, 1992-1993.................................
92
14. DIN uptake vs. irradiance, 1992-1993.......................................118
15. Estimated C:N uptake ratios vs irradiance................................ 143
NITROGEN TRANSFORMATIONS IN LAKE BONNEY, ANTARCTICA
Christopher D. Woolston, 1994
ABSTRACT
This study was designed to determine sources of
dissolved inorganic nitrogen (DIN) and microbial strategies for
DIN utilization in a non-turbulent environment (perennially
ice-covered Lake Bonney, Antarctica).
Water samples were inoculated with ^ N labeled DIN and
incubated either in situ or in an incubator. Experiments were
designed to test DIN uptake over a range of substrate
concentrations, irradiances, and incubation periods. Additional
experiments were designed to measure in situ NH4"*"
regeneration, as well as the effects of free amino acids,
ambient NIfy+ concentration , and phytoplankton on NIfy+
regeneration rates.
Experiments designed to measure in situ NXfy+
regeneration rates were inconclusive; apparently rates were
below the detection limits of the method. The efficiency of
conversion of serine to free N Ify+ increased with depth.
Removal of phytoplankton had no discernible effect on
regeneration rates.
NIfy+ was the preferred source of DIN at all depths.
In
situ uptake rates of N tfy+ in the trophogenic zone generally
exceeded the rate of supply of NXfy+ by diffusion, indicating
that NFfy+ regeneration was a major source of DIN in the
trophogenic zone. Three mechanisms for enhancing uptake of
DIN (particularly NEfy+ ) in relatively nutrient poor waters were
observed: increased microbial affinity for NFfy+ , surge uptake
of DIN within minutes of DIN enrichment, and increased
affinity of DIN uptake for irradiance.
Microplankton in the shallow, relatively nutrient poor
waters of Lake Bonney appear well acclimated and adapted to
utilize the ambient supply of DIN.
1
INTRODUCTION
It h a s long b een recognized th a t supply of dissolved inorganic
n itro g e n (DIN) often controls p roductivity of m arin e p h y to p la n k to n
(e.g. D ugdale a n d Goering 1967). Recently, it has becom e
in creasin g ly ev id en t th a t p h y to p lan k to n grow th can b e lim ited b y
n itro g e n in fre sh w ate r system s as well (D odds e t ah 1991; P riddle e t
ah 1986; Priscu a n d Priscu 1984).
T he ecological im portance of DIN has b een th o ro u g h ly
d o cu m en ted , b u t th e com plexity of m ost en v iro n m en ts h as
co n fo u n d ed attem p ts to isolate physiological responses to n u trie n t
su p p ly in n a tu ra l system s (Sm etacek e t al. 1990) . T u rb u len ce a n d
grazing p re ssu re create conditions of disequilibrium w hich p re v e n t
m axim al utilization o f n u trien ts a n d a d d significant com plexity to
p h y to p la n k to n dynam ics (Sm etacek e t al. 1990).
Even in som e relatively qu iescen t A ntarctic lakes (e.g.
H eyw ood Lake, Signey Island) d u rin g ice-cover, m in o r tu rb u len c e
a n d su b se q u en t fluctuations in th e lig h t field g reatly com plicate
d e te rm in a tio n of th e factors th a t co n tro l p ro d u ctiv ity (Hawes 1983).
Sim ilarly, losses d u e to grazing m u st be co n sid ered in m o st
investigations o f aquatic n u trie n t cycles a n d p h y to p la n k to n
p roductiv ity . Grazing rates m u st be q uantified fo r accu rate
calculations o f critical d e p th a n d n u trie n t control o f p rim a ry
p ro d u c tiv ity (Sm etacek e t al. 1990). F urtherm ore, Lynch a n d
S hapiro (1981) fo u n d th a t grazing significantly affects com m unity
succession in resp o n se to n u trie n t additions. Like ,turbulence, b u t to
2
a lesser extent, grazing creates disequilibrium th a t in h ib its
p h y to p la n k to n acclim ation o r ad ap tatio n to n u trie n t resources.
T he p e re n n ia lly ice-covered lakes in th e M cM urdo d ry valley
reg io n o f S outh V ictoria Land, A ntarctica, provide a n o p p o rtu n ity to
o bserve n a tu ra l tran sfo rm atio n s of DIN in th e absence o f tu rb u len ce
o r grazing p ressu re. This stu d y focuses o n Lake Bonney, a
m erom ictic lake lo cated in th e u p p e r T aylor Valley.
Like o th e r lakes in th e M cM urdo d ry valleys, Lake B onney is
c h a ra c te riz e d b y extrem e h y d rau lic stability. In th is lake, m ixing
occurs alm o st exclusively on a m olecular scale a n d tu rb u le n t m ixing
is n e a rly n o n -ex isten t (Spigel et al. 1990). T here a re two m ain causes
fo r this stability. Firstly, th e p e re n n ia l 4 m ice-cover p rev en ts d ire c t
w in d -in d u ced mixing. Secondly, chem ical g rad ien ts w ith in th e lake
le a d to stro n g d en sity stratification (Spigel e t a l.1 9 9 0 ).
G razing p re ssu re in Lake Bonney is lim ited b y th e n e a r absen ce
o f u p p e r tro p h ic levels. Some zooplankton have b e e n reco rd ed ,
including ciliated zooplanktors of the genus Strombidium a n d
Sphaerophyra (Parker e t al. 1982). These organism s a re p rim arily
asso ciated w ith b en th ic m ats a n d are ra re to ab sen t in th e pelagic
e n v iro n m e n t (P arker e t al. 1982).
h i th e absence o f grazing p ressu re, p h y to p lan k to n p ro d u ctio n
can b e m o re d irectly rela ted to tem p eratu re, light, a n d n u trie n t
availability (Cullen 1990). These th re e factors h av e a com plex
re g u la to ry effect o n productivity, a n d m uch co n tro v ersy h as
su rro u n d e d attem p ts to isolate one facto r as th e cause o f seasonal
succession o f p h y to p lan k to n populations (McCombie 1960, Cullen
1990). At a physiological level, th e y a re n o t m u tu ally exclusive: all
organism s re q u ire n u trie n ts fo r biosynthesis, a n d te m p e ra tu re a n d
3
lig h t g o v ern th e ab ility of p h y to p lan k to n to utilize available
su b strate. O bservation of n u trie n t control of p rim a ry p ro d u ctiv ity
co u ld th e re fo re m o st read ily b e accom plished in a system w here
lig h t a n d te m p e ra tu re rem a in relatively co n stan t o v er th e doubling
tim e of a n organism .
For th e se reasons, Lake Bonney is a n ideal site fo r m o n ito rin g
p h y to p la n k to n u tilization of n u trien ts. The h y d rau lic stab ility o f Lake
B onney p rev en ts ra p id tem poral v ariatio n in te m p eratu re. Due to
th e p e re n n ia l 4 m ice-cover, w hich provides in su latio n as well as
p re v e n ts tu rb u len ce, changes in te m p e ra tu re a t a given d e p th are
v e ry g rad u al. Even in relativ ely tu rb u le n t m arin e h ab itats, seasonal
te m p e ra tu re changes in A ntarctic w aters do n o t significantly affect
m icro b ial h etero trp p h ic activity (EUis-Evans 1982).
F u rth erm o re, d u rin g th e au stral spring a n d sum m er,
m ic ro p lan k to n in Lake Bonney grow in a relatively co n sta n t Ught
regim e, h i all aq u atic system s, th e Ught regim e flu ctu ates according
to first o rd e r effects (factors th a t govern solar flux, e.g. latitu d e,
w eath er, a n d fight extinction w ithin th e w ater colum n) a n d second
o rd e r effects (e.g., tu rb u le n t displacem ent of p h y to p la n k to n a n d
p h y to p la n k to n m igration). In Lake Bonney, these effects com bine to
fo rm te m p o ra l stabU ity in th e fight regim e o ver th e fife sp an of
m ic ro p lan k to n . At h ig h latitudes, th e re is little diel v a ria tio n in
irrad ian ce; fo u r m o n th s of darkness a n d fo u r m o n th s o f d aylight a re
se p a ra te d b y two m o n th periods of twilight. The in te n sity o f fight
th a t re a c h e s th e w ater colum n of Lake Bonney v aries w ith cloud
cover a n d th e changing tran sp aren cy o f th e ice, b u t th e se v ariatio n s
a re m in o r co m p ared to th e diel fig h t/d a rk cycles fo u n d in low
la titu d e system s. H ydraulic stability m inim izes seco n d o rd e r effects
4
a n d th u s p ro v id es p h y to p lan k to n a relatively co n sta n t lig h t
en v iro n m en t.
T he stab ility o f th e w ater colum n h as sh ap ed th e stru c tu re a n d
com position of planktonic com m unities. Most o f th e p h y to p lan k to n
in Lake B onney are flagellates (principally Chlamydomonas
subcaudata, Cryptomonas sp., a n d Ochromonas sp.) (Sharp 1993),
w hile diato m s a re ra r e (Parker a n d Sim m ons 1983, P ark er e t al.
1977, S h arp 1993). A pparently, n a tu ra l selection stro n g ly favors
m otile algae in this quiescent environm ent.
h i a d d itio n to shaping com m unity com position, th e stability of
th e w ate r colum n h as allow ed fo r specialized grow th a n d survival
strategies. As p rev io u sly d escribed, th ese strategies in c lu d e m o tility
a n d efficient p h o to sy n th esis a t low irrad ian ce (Priscu e t al. 1990).
P otentially, w ater colum n stability could also allow fo r efficient
utilizatio n o f inorganic n u trien ts b y p h y to p lan k to n a n d efficient
reg en e ra tio n o f NH4+ b y bacteria.
T he p u rp o se o f this stu d y was to assess tran sfo rm atio n s of
dissolved in o rg an ic n itro g en b y several d ifferen t m icrobial
com m unities in Lake Bonney. The m ain b o d y o f this thesis h as b e e n
d iv id e d in to th re e p arts. P art I describes th e m e th o d s a n d resu lts of
ro u tin e d a ta collection. These results are necessary fo r
in te rp re ta tio n o f all experim ents discussed in P arts II a n d III. P art
II in c lu d es ex p erim en ts d esig n ed to assess sources o f DIN as well as
m icro b ial strateg ies to en h an ce DIN utilization. P art III focuses o n
th e ro le o f irra d ian c e in regulating DIN up tak e a n d th e d eg ree to
w hich m icro p lan k to n h av e ad a p ted to utilize irra d ian c e fo r DIN
u p ta k e. Following P art III is a n overall conclusion th a t b riefly
in te g ra te s th e re su lts o f P art I, II, a n d III. The re m a in d e r o f this
5
in tro d u c to ry section will discuss aspects rele v an t to all p a rts o f th e
thesis: p rev io u s re se a rc h o n Lake Bonney a n d o th e r d ry v alley lakes,
h y p o th e ses a n d objectives, a n d a description o f th e stu d y site.
6
PREVIOUS RESEARCH ON LAKE BONNEY AND OTHER
DRY VALLEY LAKES
C aptain R.F. Scott's discovery of Lake Bonney in 1903 m ark ed
th e begin n in g o f d ry valley lake re se a rc h (Parker e t al. 1982). Owing
to th e ir rem oteness, th o ro u g h scientific investigation of th e lakes d id
n o t beg in u n til th e early 1960's. Liquid w ater b e n e a th th e ice covers
o f Lake B onney a n d Lake V anda was sam p led in 1961; th is was th e
first a tte m p t a t lim nological analysis of d ry valley lakes (A ngino a n d
A rm itage 1963).
S everal early investigations o f Lake Bonney fo cu sed o n
d istrib u tio n of n u trie n ts (A rm itage a n d House 1962, A ngino a n d
A rm itage 1963, A ngino e t al. 1963, G oldm an 1964, Y am agata e t al.
1967). T he ex trem e salinity of Lake Bonney's d e e p e r w aters
a p p a re n tly caused m ethodological difficulties; re p o rts of n u trie n t
co n cen tratio n s w ere inconsistent a n d often co n trad icto ry (F ortner e t
al. 1986, S harp 1990).
T he first studies o f dynam ic biological processes in d ry valley
lakes w ere co n d u cted b y G oldm an (1964). G oldm an's m easu rem en ts
o f p rim a ry p ro d u ctiv ity w ithin th e lake in d icated a h ig h q u an tu m
yield o f photosynthesis. He concluded th a t th e p h y to p la n k to n h a d
a d a p te d to low am b ien t light levels.
In th e 1970's, several studies d o cu m en ted th e d istrib u tio n of
m ic ro p lan k to n in d ry valley lakes (Koob a n d Leister 1972, P arker e t
al. 1977, P arker e t al. 1982). Koob a n d Leister (1972) re p o rte d th re e
d istin ct zones of p rim ary productivity a n d p h y to p lan k to n ab u n d an ce.
7
Later studies a tte m p te d to co rrelate p ro d u ctiv ity in d r y v alley
lakes w ith lig h t a n d n u trie n t supply. V incent (1981) suggested th a t
n u trie n ts p lay ed th e p rim ary role in reg u latio n o f m icro p lan k to n
grow th. Specifically, h e p ro p o sed th a t th e p h y to p la n k to n o f Lake
Fryxell (lo cated in th e low er T aylor Valley) w ere n itro g e n deficient
a n d th a t reg en eratio n of NH4+ in large p a r t fueled p ro d u ctiv ity in
th e trophogenic zone. G reen e t al. (1989) n o ted th a t th e allochtonous
in p u ts o f P O 4'3 to Lake Fryxell g reatly exceeded NO3', a n d n itro g en ,
co m p ared to p h o sp h o ru s, was m ore efficiently cycled w ithin th e lake.
T ho ro u g h investigation of th e factors th a t co n tro l p ro d u ctiv ity
in Lake B onney b eg an in 1989. Priscu e t al. (1990) h y p o th esized
th a t th e p h o to sy n th etic ap p aratu s of Lake Bonney p h y to p la n k to n
sh o u ld b e precisely a d a p te d to th e relatively co n sta n t a m b ie n t light
fields. P h o to sy n th esis/irrad ian ce experim ents co n d u cted initially b y
Priscu e t al. (1988) a n d confirm ed b y Lizotte a n d Priscu (1992)
d e m o n stra te d th a t p h y to p lan k to n w ere in d e ed sh ad e ad ap ted ;
how ever, p h y to p la n k to n in th e relativ ely n u trie n t-p o o r surface
w aters a p p e a re d to h a rv e st light less efficiently th a n d e e p e r
p o p u latio n s. Lizotte a n d Priscu (1993) d eterm in ed q u a n tu m yield of
p h o to sy n th esis th ro u g h m easurem ents o f n a tu ra l fluorescence,
chlorophyll concentration, ph y to p lan k to n ab so rp tio n spectra,
p h o to sy n th etic efficiency, a n d spectral irrad ian ce. Interestingly,
q u a n tu m yields in creased d ram atically w ith increasing p ro x im ity to
th e chem ocline. Models co n stru cted in a n attem p t to p re d ic t p rim a ry
p ro d u ctiv ity fro m n a tu ra l fluorescence, irradiance, a n d q u an tu m
yields w ere unsuccessful. A pparently, p ro d u ctiv ity o f th e u p p e r
8
tro p h o g en ic zone is largely influenced b y n u trie n t su p p ly (liz o tte
a n d Priscu 1993).
S harp (1993) fo rm u lated a one-dim ensional lig h t-d e p en d en t
grow th m o d el of p rim a ry productivity. This m o d el assu m ed th a t
lig h t availability, a n d n o t n u trie n t supply, lim its grow th. T he m o d el's
p red ictio n s o f gross chlorophyll specific grow th rate s d id n o t
co rre la te w ell w ith actu al grow th rates, p a rtic u larly in su rface
w aters. For exam ple, chlorophyll specific grow th ra te s w ith in th e
chem ocline exceeded grow th rate s in th e relatively h ig h irra d ian c e
surface w aters. Once again, it was concluded th a t n u trie n t supply in
th e u p p e r trophogenic zone lim ited photosynthesis (Sharp 1993).
N u trien t bioassays co n d u cted b y Sharp (1993), d esig n ed to d etect
deficiency o f n itro g en o r p hosphorus, w ere inconclusive; how ever,
re c e n t bioassays h av e show n th a t p h y to p lan k to n p h o to sy n th esis in
th e u p p e r trophogenic zone can be en h an ced significantly b y n u trie n t
en ric h m e n t (Priscu, u n p u b lish ed data).
9
HYPOTHESES AND OBJECTIVES
Ju st as Priscu e t al. (1990) h y p o th esized th a t p h y to p la n k to n
sh o u ld be precisely a d a p te d to am b ien t light levels, I p ro p o se th a t
th e level o f physiological ad ap tatio n s fo r inorganic n itro g en u p tak e
will reflec t th e n u tritio n a l status of th e com m unity. In situations
w h ere n itro g e n lim itatio n exists (if any), p h y to p la n k to n will h av e
e n h a n c e d efficiency o f inorganic n itro g en uptake, p a rtic u larly u p tak e
of NH4+ . Specifically, this stu d y was designed to te st th e following
hypotheses:
1. NH4+ is th e p re fe rre d source of DIN fo r all Lake Bonney
p lanktonic com m unities.
2. R egeneration of NH4+ provides a m ajor n itro g en source fo r
p h y to p la n k to n in th e u p p e r trophogenic zone.
3. "R egenerated" NO3" an d NO2" p rovide a m in o r source o f n itro g en
fo r p h y to p lan k to n in th e u p p er trophogenic zone.
4. N itrogen d eficient com m unities a re capable o f ra p id sh o rt te rm
u p tak e o f NH4+, and, to a lesser extent, N O g'and NO2™.
5. N itrogen u p tak e is extrem ely "shade adapted," i.e., quickly
sa tu ra te d w ith resp e ct to irrad ian ce, p articu larly in n itro g e n
d e p le te d su rface w aters.
6. P hotoin h ib itio n of n itro g en uptake, w hen p resen t, occurs a t a n
irra d ia n c e su b stan tially above am b ien t levels.
10
T he follow ing objectives w ere u n d e rta k e n to te st th e
hypotheses:
1. M easure m axim um up tak e rates of NH4+, NOg", a n d NO2"
th ro u g h o u t th e w ater colum n.
2. M easure th e u p tak e response to d ifferen t co n cen tratio n s o f NH4+,
NOg", a n d NO2".
3. M easure rate s o f reg en eratio n o f NH4+.
4. M easure th e tim e course uptake of NH4"1", NOg", a n d NO2".
5. Q uantify th e irrad ian ce req u irem en ts fo r u p tak e of NHq+, NOg",
a n d N02~.
11
DESCRIPTION OF STUDY SITE
G eography an d Geology
T he M cM urdo d ry valleys h e betw een th e MacKay a n d Koettiitz
glaciers a t la titu d e 77° 10'S to 77° 4 5 'S, longitude 160° 20'E to 160°
OOrE (C hinn 1993). At 3700 km ^, th e valleys re p re s e n t th e larg est
ice-free a re a o n th e co ntinent. The T aylor VaUey w as originally
c a rv e d b y th e T aylor V glaciation approxim ately fo u r m illion years
ago, a n d n u m ero u s sm aller glaciations h av e sh ap ed th e v ah ey since
th e n (Heywood 1984, C hinn 1993). The Taylor V ahey h as b een
la rg e ly ice-free fo r a t least 100,000 years, a n d th e b asin s o f th e
p re s e n t d a y T ay lo r V ahey lakes (Lake B onney, Lake Fryxell, Lake
H oare, Lake Chad, a n d M umm y Pond) w ere fo rm ed 100,000 to
500,000 y ears ago (Chinn 1993).
T he m ajo r rock types fo u n d in th e m o d ern -d ay T aylor V ahey
in clu d e dolerites, m arbles, granite, basalts, gneisses, schists,
sandstones, a n d m etasedim ents (Claridge a n d CampbeU 1977;
H eyw ood 1984). U ltram aphic dikes are p ro m in e n t featu re s of th e
la n d su rro u n d in g Lake Bonney, evidence o f p ast vulcanism (Lawson,
p erso n al com m unication).
Climate
T he M cM urdo d ry valleys co n stitu te one of th e d rie st
ecosystem s in th e w orld. Precipitation in th e fo rm o f snow m ay
re a c h 10 cm a y ear, b u t m o st of th e m o istu re is quickly lo st to
sublim atio n (G reen et al. 1989). Potential ablation ra te s are
r
12
ap p ro x im ately 30 tim es g reater th a n th e average a n n u a l
p recip itatio n (Chinn 1993). Katabatic w inds, w hich blow fro m th e
P olar P lateau a n d can exceed 130 km h "1, m a in ta in th e relativ e
h u m id ity below 50% (Heywood e t al. 1984). The a n n u a l m e an
te m p e ra tu re is b etw een -20 to -25° C (Heywood e t al. 1984).
M orphology
Lake B onney, w ith a surface a re a o f ap p ro x im ately 4 k m 2
(C hinn 1993), is th e larg est lake in th e T aylor Valley. It consists of
two lobes (th e east lobe a n d th e w est lobe) w hich a re co n n ected b y a
n a rro w ( 4 0 m w ide), shallow (14 m deep) sill. T he w est lobe, th e
sm aller o f th e two, is ab u tte d b y th e T aylor Glacier. Both lobes h av e
a m axim u m d e p th o f approxim ately 40 m. Several stream s (ru n o ff
fro m th e Taylor, Hughes, LaCroix, Sollas, Rhone, Calkin, M atterhorn,
a n d M arr glaciers) flow in te rm itte n tly in to th e lake d u rin g late
a u stra l spring a n d a u stra l sum m er. The lake h as n o outflows; w ater
is lo st th ro u g h ab latio n a n d sublim ation of th e p ere n n ia l 4 m thick
ice-cover (C hinn 1993).
13
Part I
Routine Data Collection and Overview of
Experiments
14
ROUTINE DATA COLLECTION METHODS
P aram eters A nalyzed
R outine collections w ere co n d u cted in each lo b e ev ery 10 d
th ro u g h o u t th e 1992-1993 field season (h ereafter re fe rre d to as th e
"1992 season") a n d ev ery 14 d th ro u g h o u t th e 1993 field season.
W ater sam ples w ere analyzed fo r chlorophyll a (CHL); p rim a ry
p ro d u c tio n (PPR); NH44", NOg", an d NO2" (DIN); soluble reactive
p h o sp h o ru s (SRP) a n d particulate carb o n a n d n itro g en (PC a n d PN).
A n in v e n to ry of sam pling dates a n d d ep th s is p re se n te d in T able I.
All d e p th s a re fro m th e piezom etric level, th e level o f th e w ater
surface in th e sam pling hole (approxim ately 27 cm below th e to p of
th e ice).
W ater Sam phng P rocedure
Early in each season, sam pling holes w ere d rille d w ith a 10 cm
o r 25 cm d ia m e te r ice auger. A fter drilling, holes w ere en larg ed w ith
a "hot finger," co p p er tubing th ro u g h w hich h o t glycol was circulated.
T he h o t fin g er was reap p lied to th e holes periodically th ro u g h o u t th e
season to p re v e n t narrow ing o r refreezing. After ap p licatio n of th e
h o t finger, holes w ere allow ed to cool fo r a t least 24 h b efo re
sam pling. A p o rta b le stru ctu re was p laced over th e sam pling hole.
For ro u tin e d a ta collection, sam ples w ere collected w ith a 2.2
lite r Niskin sam pling b o ttle w ith teflon-coated springs. T he b o ttle
w as fa ste n e d to m e te re d 1 /8 " airc ra ft cable a n d w as low ered a n d
ra ise d w ith a m a n u al w inch. The Niskin b o ttle was in v e rte d sev eral
15
tim es b efo re rem o v al o f w ater to elim inate g rad ien ts w ith in th e
bottle.
W ater sam ples w ere collected th ro u g h a sh o rt piece of ru b b e r
tubing a ttac h ed to th e nozzle of the Niskin. One I fro m each d e p th
was tra n sfe rre d in to a I liter HDPE b o ttle fo r analysis o f DIN, SRP,
CHL, a n d CHN. Once full, each I liter HDPE bottle was p laced in a
cooler fo r tra n sp o rt to laboratories o n th e lakeshore. A p o rtio n o f th e
w ate r rem a in in g in th e Niskin was u sed to fill p rim a ry p ro d u ctiv ity
b o ttle s (3 145 m l Pyrex glass screw -top bottles). Two o f th e b o ttles
w ere clear a n d one was opaque. The tubing o n th e Niskin was
in se rte d to th e b o tto m o f each b o ttle to p re v e n t in tru sio n of gasses
in to th e sam ple. Once full, th e bottles w ere placed in a d a rk box u n til
in o c u lated w ith I^C-COg"2. yyq bottles w ere acid w ash ed a n d rin se d
th re e tim es w ith sam ple w ater b efo re final collection.
Chemical an d Biological M easurem ents
Chlorophyll a
Collection of CHL an d n u trie n t sam ples com m enced w ithin 2 h
o f collection. T he lab o rato ry was cool a n d dim d u rin g filtratio n o f
CHL a n d n u trie n ts. For th e 1992 season, 2 200 m l aliq u o ts w ere each
filte re d o n to p reco m b u sted (450° C fo r 2 h) 25 m m W h atm an GF/F
glass fib e r filters u n d e r low v acu u m (< 0.5 atm ). T he filtra te was
collected in a n acid w ashed E hrlem yer flask. A p o rtio n of th e filtrate
w as u se d to p re-rin se a 125 m l HDPE n u trie n t b o ttle; th e re s t was
tra n sfe rre d to th e b o ttle fo r fu tu re n u trie n t analysis. D uring th e
1993 season, two 100 m l aliquots w ere filtered fo r CHL analysis, a n d
th e filtrate was collected directly in to th e 125 m l HDPE n u trie n t
16
b o ttle th ro u g h th e use of vacuum cham bers (bell jars). Each CHL
filte r was g en tly fo ld ed in h a lf (CHL side in) a n d p la ced in a glassine
envelope. The envelopes w ere w rap p ed in alu m in u m foil a n d k e p t
fro zen u n til analysis a t C rary lab o rato ry , McMurdo Station. N utrient
b o ttles w ere also k e p t fro zen u n til analysis. Analysis o f CHL a n d
n u trie n ts o cc u rred w ithin two m o n th s of collection.
For CHL analysis, th e frozen filters w ere p laced in 20 m l
scintillation vials. T en m l o f 90% acetone w ere a d d e d to each vial,
a n d th e sam ples w ere vortex ed fo r I m in. Tests co n d u c ted in 1989
in d icate th a t this is a n ap p ro p riate m eth o d of CHL ex tractio n fo r th e
p h y to p la n k to n in Lake Bonney (Iizo tte a n d Priscu, u n p u b lish e d
d ata). CHL co n cen tratio n of th e extract was m easu red
fluoro m etrically w ith a T u rn e r Design m odel 10 AU H uorom eter
ca lib ra te d w ith sta n d a rd concentrations of p u rified CHL (Sigma
C hem ical). M easurem ents w ere m ad e before a n d a fte r acidification
(0.2 m l I N HC1) to co rrect for phaeopigm ent fluorescence.
N utrien ts
N u trie n t sam ples w ere thaw ed b efo re analysis. All n u trie n t
assays w ere co n d u cted o n 10 m l sam ples. For NH4+ analysis,
sam ples fro m 15 m a n d d eep er in th e east lobe a n d 13 m a n d d e e p e r
in th e w est lobe w ere d ilu ted 1:10 ( I m l lake w ater + 9 m l d eio n ized
w ater). This was d o n e to p re v e n t salt in terferen ce w ith th e color
form ing reactio n a n d to keep NH4+ concentrations w ithin the lin ear
ra n g e o f th e m eth o d (Sharp 1993). T he blue in d o p h en o l reactio n
betw een NH4+, phenol, a n d hypochlorite a t high pH was u sed to
%
17
d eterm in e NH4+ concentration (Solorzano 1969). A bsorbance o f th e
sam ple was m e asu red using a I cm p ath len g th cell a n d a Beckman
sp ectrop h o to m eter.
For N O 2' analysis, sam ples fro m 15 m a n d d e e p e r in th e east
lo b e w ere d ilu ted 1:10 ( I m l lake w ater + 9 m l d eio n ized w ater).
Sam ples fro m th e w est lobe w ere n o t diluted. NO2" co n cen tratio n s in
th e w est lo b e w ere consistently less th a n I pM; d ilu tio n o f th e
sam ple co u ld th erefo re decrease NO2" con cen tratio n s below th e lim it
o f detectio n . R epeated recovery tests indicate g re a te r th a n 90%
reco v e ry o f a d d e d NO2' a t d ep th s below 15 m. C oncentration of
NO2" was m e asu red w ith th e N ED /sulfanilam ide d iazo d zatio n
re a c tio n (APHA 1985).
Sam ples fo r NO3™ analysis fro m 15 m a n d d e e p e r in th e east
lobe a n d 13 m a n d d e e p e r in th e w est lobe w ere d ilu te d 1:10. AU o f
th e NO3" in th e sam ple was red u ce d to NO2" b y shaking th e sam ples
w ith spongy cadm ium a n d am m onium chloride EDTA b u ffer a t pH
8.3 (Jones, 1984). The co ncentration o f NO2" was th e n d eterm in ed
w ith th e N ED /sulfanilam ide reaction.
Soluble reactiv e p h o sp h o ru s sam ples fro m 15 m a n d d e e p e r in
th e e a st lo b e w ere dU uted 1:2 (5 m l lake w ater + 5 m l d eionized
w ater). Sam ples fro m th e w est lobe w ere n o t dU uted; reco v ery tests
in d ic ated g re a te r th a n 90% efficiency o f SRP d etectio n in u n d ilu ted
sam ples. SRP was m e asu red w ith th e am m onium
m o ly b d a te /p o ta ssiu m antim onyl ta rtra te m eth o d (Downes 1978).
18
Following th e m eth o d s of Sharp (1993), th e red u ctio n step to p re v e n t
a rse n a te in te rfe re n c e was om itted.
Particulate C arbon a n d Nitrogen
Collection of PC a n d PN com m enced after filtering of all CHL
a n d n u trie n t sam ples. For each d ep th , one 500 m l aliq u o t was
filte re d o n a p reco m b u sted 25 m m GF/F glass fib er filte r a t low
v acu u m (< 0.5 atm ). Each filter was rin sed w ith ap p ro x im ately 20 m l
o f d eio n ized w ater (DIW) to rem ove inorganic n itro g en fro m th e
filter. Filters w ere a ir d rie d fo r several d in alu m in u m w eigh-boats
b efo re tra n sp o rt to C rary Laboratory. U pon arriv al a t th e lab o rato ry ,
filters w ere fro zen (-45° C) u n til analysis. Sam ples fro m th e 1992
seaso n w ere tra n sp o rte d frozen to MSU a n d an aly zed w ith in 6
m o n th s o f collection. Samples fro m th e 1993 season w ere an aly zed
a t C rary la b o ra to ry w ithin 3 m onths o f collection.
P articu late filters w ere acidified b efo re analysis to rem o v e
rem ain in g inorganic carbon. Flash-com bustion gas ch ro m ato g rap h y
(Carlo E rba m o d el 1500 elem ental analyzer) was u se d to m easu re th e
n itro g e n a n d ca rb o n co n ten t of th e filters w rapped in tin foil
(com bustio n catalyst). A reas u n d e r chro m ato g rap h ic peaks w ere
co m p ared to areas o b tain ed from stan d ard s of iso th io u rea (1992) o r
acetan ilid e (1993). S tandards w ere w eighed ± 1.0 ng w ith a C ahn
m o d el 35 electrobalance. All m easu rem en ts w ere co rrected fo r
ca rb o n a n d n itro g en contam ination o f the filters a n d th e foil.
P rim ary P ro d u ctiv ity
P hotosynthesis was m easu red b y in situ u p ta k e o f ^^C-C02. A
Glison 1000 m l p ip e tte was u sed to inoculate p rim a ry p ro d u ctiv ity
19
b o ttles w ith
stock. Bottles w ere sealed tig h tly a n d
in v e rte d several tim es afte r ad d itio n o f I^C-COg
to m ix th e
sam ple.
T he p ro d u ctiv ity bottles w ere su sp en d ed fo r 24 h a t th e d e p th
of collection. Studies conducted b y Sharp (1993) in d ic ated th a t
m e asu rem e n ts o f daily n e t p h o tosynthesis b ased o n a single 24 h
in c u b atio n d id n o t differ significantly fro m m easu rem en ts b ased o n
th re e 8 h incubations. The in cu b atio n hole was lo cated aw ay fro m
th e sam pling h u t a n d was covered to p re v e n t excess irra d ia n c e fro m
en terin g th e w ater colum n. The bottles w ere p laced in th e hole
b etw een 0 7 0 0 h a n d 0 8 0 0 h local tim e, w hen th e su n w as b eh in d
m o u n tain s. This re d u c e d th e possibility th a t algal p h o to sy n th etic
ability w ould b e d am ag ed b y in cid en tal c l o s u r e to su n lig h t b efo re
incubation.
T he b o ttles w ere re tu rn e d to th e lakeshore la b o ra to ry
im m ed iately a fte r rem o v al fro m th e lake. AU b o ttles w ere k ep t cold
a n d d a rk b efo re a n d d u rin g filtration. Samples w ere filte re d on
p re c o m b u ste d 25 m m GF/F W hatm an glass fib er filters a t low
v ac u u m (<0.5 atm ). FUters w ere tra n sfe rre d to 20 m l scintillation
vials a n d acidified w ith 0.5 m l 3 N HCl to rem ove unassim U ated ^ C .
T he vials w ere d rie d (40-50° C) fo r I to 2 d a n d sealed fo r sh ip m en t
to C rary lab o rato ry , McMurdo.
A ctivity o f th e filters was d e te rm in e d b y Uquid scintU latibn
spectroscopy a t C rary Laboratory. T en m l of CytoScint ES (ICN
P harm aceuticals) Uquid scintiUation cocktaU was a d d e d to each vial
b efo re analysis. Counts p e r m in w ere co m pared to a q u en ch curve fo r
20
conversion to disintegrations per min (DPM).
Acetone was used as
the quenching agent and ^ C toluene was the standard.
DPM was converted to rate of carbon uptake through the
following equation:
mg C n r 3 day l =
(DIC)* ( DPMc - DPMd ) * 1.06 * pCi * IO3 liter
(I)
(2.2 * IO6 DPM) * (jiCi added)* m3*t
where DIC (dissolved inorganic carbon) is expressed in mg I" I , 1.06
is a correction for isotope discrimination, DPM c is disintegrations per
min in the clear bottles,
DPM d is disintegrations per min in the dark
bottle, 2.2 * I O^ is the DPM per pCi, IO3 converts liters to m3 , and t
is the length of incubation (d).
Physical Parameters
Lake profiles of temperature, oxygen, and irradiance were
collected on the day of each routine data collection. Irradiance was
measured at I m intervals with a Li-Cor 4 tc quantum sensor
attached to a Li-Cor model LI-1000 data logger. In addition,
irradiance on the ice surface was continuously monitored by a Li-Cor
2TC quantum sensor attached to a Campbell 21 x data logger. In situ
irradiance was measured every 10 min at 10 m east lobe from 17
Jan 1993 to 31 Dec 1993 by a with a Li-Cor 4 tc quantum sensor
attached to a Campbell 21 x data logger.
Irradiance readings from the
Campbell were calibrated to correspond with Li-Cor
1000 readings.
21
T able I. Dates a n d d ep th s o f collection fo r various p aram eters.
EAST. LOBE
Sam pling D ates
CHL PPR
SRP DIN
PC/PN
24 Nov 1992
B
A
B
B
B
4 Dec
C
A
C
C
C
14 Dec
C
A
C
C
C
24 Dec
C
A
C
C
C
4 Ja n 1993
C
A
C
C
C
27 Oct
E
D
E
E
E
10 Nov
E
D
E
E
E
24 Nov
E
D
E
E
E
7 Dec
E
D
E
E
E
21 Dec
E
D
E
E
E
A= 4, 5, 6, 8, 10, 12, 13, 15, 16, 17, 18, 20 m.
B= 4, 5, 6, 8, 10, 12, 13, 15, 16, 17, 18, 20, 23, 26, 29, 32,
35 m .
C= 4, 5, 6, 8, 10, 12, 13, 15, 16, 17, 18, 20, 22, 25, 30, 32,
35 m .
D= 4, 6, 8, 10, 12, 1 3 ,1 5 ,1 8 , 20, 22 m.
E= 4, 6, 8, 10, 12, 13, 15, 18, 20, 22, 25, 30, 35, 38 m.
22
T able I , cont. Dates a n d d epths of collection fo r various p aram eters.
WEST LOBE
Sam pling D ates
CHL PPR
SRP DIN PC/PN
26 Nov 1992
B
A
B
B
B
6 Dec
B
A
B
B
B
26 Dec
B
A
B
B
B
6 Ja n 1993
B
A
B
B
B
7 Jan
B
NA
B
B
B
29 Oct
D
C
D
D
D
12 Nov
D
C
D
D
D
26 Nov
D
C
D
D
D
9 Dec
D
C
D
D
D
23 Dec
D
C
D
D
D
A= 4, 5, 6, 8, 10, 12, 13, 14, 15, 17 m.
B= 4, 5, 6, 8, 10, 12, 13, 14, 15, 17, 20, 22, 25, 30, 35 m.
C= 4, 6, 8, 10, 12, 13, 14, 15, 17, 20 m.
D= 4, 6, 8, 10, 12, 13, 14, 15, 17, 20, 22, 25, 30, 35, 38 m .
23
RESULTS
N utrients
Profiles o f NELj+, NOg", NO2", a n d SRP from 1993 ro u tin e d a ta
collections in each lobe are sum m arized in Figure I. C oncentrations
o f all n u trie n ts a re low above th e first chem ocline, a n d n u trie n t
g rad ie n ts a re gen erally strong w ithin a n d directly b e n e a th th e
chem ocline. SRP concentrations in each lobe an d NO2" concentrations
in th e w est lo b e ra re ly exceed I
P rim ary P roductivity a n d Chlorophyll a
P rim ary p ro d u ctiv ity a n d CHL ( Figure 2) w ere closely
c o rre la te d in u p p e r 20 m o f each lobe. Below 20 m , p ro d u ctiv ity w as
u n d e te c ta b le d espite th e presence o f chlorophyll.
In th e ea st lobe, GHL typically reac h ed its m axim um
co n c e n tra tio n ju st below th e ice (4-6 m ). T here was o ften an o th er,
less p ro m in e n t, chlorophyll a p eak w ithin th e first chem ocline (10-13
m ). P rim ary p ro d u ctiv ity follow ed th e sam e g en eral p a tte rn , except
th a t p rim a ry p ro d u ctiv ity peaks w ithin th e first ch em ocline w ere
m o re p ro m in e n t th a n th e p ro d u ctiv ity peaks fro m 4-6 m. CHL
co n c en tratio n s in th e u p p e r 20 m o f th e w est lobe consistently
exceeded co n cen tratio n s of th e east lobe.
P articulate C arbon a n d Nitrogen
R ep resen tativ e profiles o f PC=PN ratio s are illu stra te d in Figure
3. PC=PN is always g reater th a n th e Redfield ratio of 6.6 (by atom s),
24
Depth (
N u t r i e n t C o n c e n t r a t i o n (//M)
Figure I . NH4"1", NO2", NOg", an d SRP concentrations (^M) in east a n d
w est lobes o f Lake B onney, 1993.
25
PPR ( m g C m 3 d J)
0
1
2
3
4
5
PPR ( m g C m
0
1
2
d J)
3
4
QO Nov 12 1 9 9 3
w e st lobe
D epth
Nov 10 1 9 9 3 e a s t lobe
—O—
0. 0
CHL (//g I
4
8
1.5
2.0
CHL (A/g I ')
)
PPR ( m g C m 3 d :)
0
0.5
12
PPR ( m g C m
0
1
1
2
d
)
2
D epth
(m)
Dec 9 19 93
w est lobe
Dec 7 1 9 9 3 e a s t lobe
Figure 2. C hlorophyll a (^g I '1) an d p rim ary p ro d u ctiv ity (mg C m~3
d"1) in ea st a n d w est lobes of Lake Bonney, 1993.
26
C:N C o n t e n t of P a r t i c u l a t e s (yuMi/zM)
0
10
20
30
0
10
20
30
No v 10 1 9 9 $
. E a s t Lobe
Oct 27 1993
x E a s t L o be
D epth (
6
0
10
20
30
Nov 12 1 9 9 3
I
W e st L o b e
0
10
20
30
40
No v 2 6 1 9 9 3
West Lobe
Figure 3. PCrPN ratio s (nMr^M) in east a n d w est lobes o f Lake
Bonney, 1993.
27
.
th e ra tio associated w ith b alan ced grow th (Redfield 1957). However,
th e p ro p o rtio n of PC o r PN attrib u tab le to viable p h y to p la n k to n is
u n clear; m e asu red PC:PN ratios m ay be significantly in flu en ced b y
d e tritu s a n d bacteria.
Irra d ia n c e
Profiles o f th e n a tu ra l log of irrad ian ce are illu stra te d in Figure
4. T h ere w ere two d istin ct layers o f tu rb id ity as m e a su re d b y lig h t
a tte n u a tio n : th e ice-cover a n d th e w ater colum n. Extinction
coefficients o f p h o tosynthetically available ra d ia tio n (PAR),
d e te rm in e d w ith th e following eq u atio n , w ere relativ ely co n stan t
w ith d ep th :
Kpar = Indo/ Iz)/ z
w h e re
(2 )
Kpa r = extinction coefficient o f PAR (m "I)
Iq = irra d ia n c e a t an in itial d e p th (nmoles q u a n ta m"2
s 'I )
Iz = irra d ia n c e a t d e p th z (nmoles q u a n ta m~2 s"l)
Extinction coefficients fo r b o th lobes during th e 1993 season
a re su m m arized in T able 2. As th e season p ro g ressed fro m au stral
28
I n PAR ( / / mo l m 2 s ^
0
2
4
6
8
In PAR ( / / mo l m 2 s ^
0
2
4
6
8
Dec 2 2 1 9 9 3
e a s t lobe
Depth (
O c t 31 199 3
e a s t lobe -
I n PAR ( / / mo l m
0
2
4
6
s
8
In PAR ( / / mo l m
0
2
4
s )
6
Dec 9 1 9 9 3
west lobe
D e p t h ( m)
Oct 28 1993
w est lobe
)
Figure 4. N atural log of irrad ian ce (nmol m~2 s '1) in ea st a n d w est
lobes o f Lake B onney, 1993.
8
spring to a u stra l sum m er, th e re was n o clear tem p o ra l tre n d in
Kpa r in eith e r lobe.
T able 2 . Extinction coefficients (Kpa r ) in w est a n d ea st lobe w ater
colum ns, 1993 d ata. Coefficients calculated from lig h t atten u a tio n
fro m 5 m to 25 m , w ith th e exception of Nov 25 w est lo b e (5 m to 16
m).
EAST LOBE
Date
Kpa r (m"1)
Oct 3 1
0.102
Nov 10
0.134
Nov 25
0.080
Dec 9
0.124
Dec 22
0.125
WEST LOBE
Date
Oct 28
Nov 13
Nov 25
Dec 9
Dec 22
Kpa r (m"1)
0.191
0.188
0.158
0.201
0.124
30
A verages a n d ran g es of calculated in situ d aily irra d ia n c e s a t 5
m , 13 m , 17 m, a n d 25 m in th e east lobe are sum m arized in Table 3.
Irrad ia n ce s w ere calculated fro m d aily irrad ian ces a t 10 m w ith
E quation 2 assum ing Kp ai- = 0.1
T able 3. A verages a n d ranges (in p aren th eses) o f d aily irra d ian c e
(nm ole m '2 s~l) d u rin g a u stra l sum m er, fall, w in ter and-spring,
1993. All irra d ian c es ro u n d e d to th e n ea re st w hole n u m b e r.
Sm
Z
Jan-M arch
A pril-June
July-Sep
Oct-Dec
18
2
3
18
(2-30)
(0-16)
(0-12)
(6-42)
13 m
17 m
8
I
I
8
5
I
I
6
(1-13)
(0-7)
(0-5)
(3-18)
(1-9)
(0-4)
(0-4)
(2-12)
25 m
2
0
0
3
(0-4)
(0-2)
(0-1)
(1-6)
T he d aily progression of calculated in situ irra d ian c es a t 4 e a st
lobe d e p th s o n Nov 17, 1993 are illu strated in Figure 5. Irrad ian ces
w ere calcu lated fro m 10 m in in terv al read in g s o f irra d ia n c e a t 10 m
w ith E quation 2 assum ing Kp a r =
o.l.
31
Nov 17 1993
e a s t lobe
D
12 0 0
1800
2400
Local Time (h)
Figure 5. C alculated irrad ian ce at 10 m in intervals o n Nov 17, 1993
a t 5 m , 13 m , 17 m, a n d 25 m east lobe.
32
GENERAL OVERVIEW OF 15N EXPERIMENTS
U ptake M easurem ents
AU u p ta k e a n d re g en e ra tio n experim ents u tilized th e stable
iso to p e I^ N as a label. U ptake rate s in u p tak e ex p erim en ts
(including su b stra te kinetics, tim e course, w ater colum n, a n d u p tak e
vs irra d ia n c e experim ents) w ere calculated b y m e a su re m e n t o f th e
accum ulation of 1^N into particulates collected o n W hatm an GF/F
glass fib e r fUters. U ptake rate s w ere calculated using th e foUowing
equation:
(15Np -0 .4 4 2 )
V=
(3)
t * ( (((15Na + (0.366 * 14N* 0.01)) *100)/ ( 14N + 15Na ))" 0.366)
w h e re
V= specific u p tak e ra te ( h 'l )
15]sjp = I Sn a tom -percent o f p articu lates a fte r in c u b atio n
I^N a = co ncentration ((UVl) of 1^N lab eled n u trie n t a t
beginning of incubation.
I 4N =
n a tu ra l co n cen tratio n (|iM) of n u trie n t a t
beginning of incubation,
t =
tim e (h)
0.422 = estim ated b ack g ro u n d 15N ato m -p ercen t of
particulates.
0.366 = global average back g ro u n d 15N ato m -p ercen t
33
All
filters w ere rin se d w ith ap p ro x im ately 20 m l DIW
im m ed iately afte r filtratio n to rem ove excess in o rg an ic nitro g en .
ato m -p ercen ts of rin se d a n d u n rin se d filters w ere co m p ared to te st
th e effects o f rinsing. The
ato m -p ercen t o f th e filters
consisten tly d ecreased afte r rinsing, indicating th a t th e n e t effect of
rin sin g w as rem oval o f n on-assim ilated n itro g en . How ever, th is te st
could n o t d eterm in e if rinsing resu lted in cell b reak ag e a n d release
o f assim ilated nitrogen. Tests co n d u cted b y Sharp (1993) in d icated
th a t rin sin g th e sesto n fro m 17 m east lobe w ith DIW d id n o t
significantly red u ce organic carb o n content; p resu m ab ly th e sam e is
tru e of organic nitrogen.
A biotic a d so rp tio n of n itro g en to th e filters was m o n ito re d w ith
fre q u e n t "kill" tests. In th ese tests, sam ples w ere tr e a te d w ith
appro x im ately 20 m l form alin fo r a final co n cen tratio n o f
appro x im ately 5% before ad d itio n of n u trien ts. At th is con cen tratio n ,
re te n tio n of inorganic n itro g en o n th e filter is lim ited to abiotic
a d so rp tio n to th e seston a n d to th e filter. Kill ex p erim en ts w ere
co n d u c te d c o n c u rren tly w ith m ost u p tak e experim ents a n d receiv ed
id e n tic al rinses. In som e cases, kill sam ples w ere in c u b a te d fo r 24 h;
in o th e r cases, kill sam ples w ere in c u b ate d fo r 5-10 m in. The len g th
of in c u b atio n h a d n o discernible effect on th e final
ato m -p ercen t
o f th e filter. A pparently, abiotic ad so rp tio n is n o t a tim e d e p e n d e n t
process, a t le ast w ithin th e tim e fram e o f these experim ents.
Results o f 1992 kill experim ents in d icated m ethodological
p ro b lem s, p resu m a b ly because th e form alin u sed w as o f varying
q u ality (in som e experim ents, th e form alin was h ig h ly polym erized).
For instan ce, u p tak e rate s of NHq+ in form alin tre a te d sam ples o ften
34
exceeded d a rk u p tak e o f NILj+. Fifty kill experim ents co n d u cted
o v er th e 1993 season fro m a single b atch of n o n -p o ly m erized
fo rm alin p ro v id ed m o re realistic m easurem ents o f abiotic ad so rp tio n .
T hese experim ents indicate n o significant difference in ad so rp tio n of
NHzj.+ a n d N O 3 ". It should be n o te d th a t form alin can quickly reac t
w ith NILj.+ to fo rm h ex am eth y len etetram in e a n d /o r form am ide
(C hiayvareesajja a n d Boyd 1993). This reactio n m ig h t affect
ad so rp tio n of NHzj+ to filters. If abiotic ad so rp tio n of NHzj+ is
significantly g reater in non-form alin tre a te d sam ples co m p ared to
fo rm alin tre a te d sam ples, th e abiotic ad so rp tio n o f NHq+ in sam ples
in this stu d y could b e significantly u n d erestim ated , th u s leading to
significant o v erestim atio n of up tak e rates. This is p a rtic u la rly tru e
w ith sam ples below 13 m in each lobe w here am b ien t NHq+ pools
w ere large, a n d a sm all am o u n t of ^ N H q + ad h erin g to th e filter o r
p a rtic u la te m a tte r could significantly increase estim ated u p tak e
rates.
T here was n o significant correlation (r^ = 0.06, p > 0.05)
b etw een ^ N ato m -p ercen t of th e filtrate a n d -^ N a to m -p e rc en t of
th e sesto n in kill experim ents (Figure 6). The average (+ sta n d a rd
deviation) ^ N ato m -p ercen t of all NHq+ a n d NO3" kill experim ents
co n d u c ted in 1993 was 0.442% ± 0.160%. This average was
s u b tra c te d fro m all ^ N ato m -p ercen t readings in all 1993 u p tak e
ex perim en ts as a n approxim ate correction fo r abiotic ad h esio n a n d
b a c k g ro u n d ^ N ab u n d an ce. In instances w here th e m e asu red ^ N
N a to m -p e rc e n t particulate
35
I .2
I .0
0.8
0. 6
0.4
0. 2
0.0
0
10
15
20
30
40
50
N a t o m —p e r c e n t s o l u t i o n
Figure 6 . I^N ato m p erce n t p articu late vs
ato m p e rc e n t
solution fo r NO3" a n d NFLp kill experim ents from 1993. Line
re p re se n ts lin e a r regression.
36
atom-percent was below 0.442%, the uptake rate was assumed to be
zero. Owing to the uncertainty of kill experiments conducted during
1992, uptake rates from this season were corrected with the global
average background
atom-percent of 0.366 %.
This correction is
not significantly different from the correction used for 1993 samples
(p > 0.05).
Note that all p values in this study were determined with
a t distribution.
N H 4 + regeneration rates were calculated by measurement of
dilution of ^ N H 4 + . Measurement of dissolved
required
extraction of NIfy+ from solution, a process described below.
I ^ N Determination
I^N atom-percent ( ( ^ N : ^ N + ^ N ) * 100) was measured with
an atomic emission spectrometer following Dumas combustion of the
sample (Timperley and Priscu 1986).
Samples achieve maximum
discharge with approximately 10 pg N (Timperley and Priscu 1986);
the amount of sample analyzed was adjusted accordingly.
Calibration
of the instrument was periodically monitored with ^ N standards.
A
combined standard curve containing all standard measurements over
a two year period show a good fit with a linear least squares
regression (r^=
0.9997, Figure 7), indicating that instrument
y= 1.0234x + 0.1570
Standard
15
N a t o m —p e r c e n t
Figure 7. S tan d ard
atom p erce n t vs -^N atom p e rc e n t
m e a su re d w ith atom ic em ission spectro m eter. Line re p re se n ts lin e a r
reg ressio n . All sta n d a rd m easu rem en ts from 1992 a n d 1993
included.
38
accu racy d id n o t v a ry significantly o v er this p erio d . All sam ple -^N
a to m -p e rc e n t m easu rem en ts w ere co rrected using th is co m bined
s ta n d a rd curve.
PART II
Uptake and Regeneration of Dissolved Inorganic Nitrogen.
40
INTRODUCTION
It is well estab lish ed th a t in n itro g e n lim ited m a rin e system s,
p h y to p la n k to n are capable of physiological ad ju stm en t to en h an ce
u tilizatio n o f DIN. For instance, m an y p h y to p lan k to n a re capable of
ra p id s h o rt te rm NHLi+ up tak e ra te s (Priscu 1987, H arriso n e t al.
1989, G oldm an a n d G hbert 1982). If n itro g en starv ed p h y to p la n k to n
a re exposed to a pulse of NHzi+, th e sh o rt-term (< I h) u p tak e ra te of
N H zi+ m a y b e 2 0 tim es larg er th a n th e grow th ra te (H arrison e t al.
1989). In co n trast, u p tak e of n u trien ts such as NOg", NO2", a n d Si
often lags fo r h o u rs after addition, indicating a n in d u ctio n p erio d
n ecessary fo r activating u p tak e (H arrison et al. 1989). In general, th e
tr e n d o f u p ta k e ra te o ver tim e is a n in d icato r o f th e n u trie n t h isto ry
a n d physiological state of th e com m unity (Dugdale 1977, G oldm an
a n d GUbert 1982, Priscu 1987, H arrison e t al. 1989).
In a d d itio n to maxim izing tim e d e p e n d e n t responses, n itro g en
deficien t p h y to p la n k to n are capable o f increasing affinity a n d long
te rm u p ta k e ra te s fo r D IN , p articu larly NHzi+ (Suttle a n d H arrison,
1988, M cCarthy 1981). Utilization of NHzi+ supports "reg en erated
production" (Dugdale a n d Goeiing 1967), an d th e rate s of
re g e n e ra tio n d ictate th e sustainable levels of p ro d u ctiv ity . Indeed,
re g e n e ra tio n ra te s a re closely associated w ith p rim a ry p ro d u ctiv ity
ev en in relativ ely n u trie n t rich m arin e coastal w aters (G ard n er e t al.
1993).
41
Because p ro d u ctiv ity in fresh w ater system s is o ften assu m ed a
p rio ri to b e lim ited b y p h o sp h o ru s, th e re h av e b ee n relativ ely few
investigations o f n itro g en reg en eratio n a n d physiological u p tak e
re sp o n se in fresh w ater system s. However, th e assu m p tio n th a t all
fre sh w a te r system s a re p h o sp h o ru s lim ited is sim plistic; a n u m b e r
o f fre sh w a te r system s h av e rece n tly b e e n show n to b e p rim a rily
n itro g e n lim ited (e.g. D odds et al 1991, Priscu a n d Priscu 1984).
Several A ntarctic lakes in th e d ry valleys o f S outh V ictoria
L and w ere su sp ected to be p rim arily n itro g en d eficien t (P riddle e t al.
1985, Priscu 1989, W oolston a n d Priscu 1993). Evidence fo r
p o te n tia l n itro g en deficiency in clu d ed relatively low N:P supply
ratios, in c re ase d pro d u ctiv ity following DIN en rich m en t, a n d
in c re a se d DIN u p tak e rate s following DIN en ric h m en t (P riddle e t al.
1985, W oolston a n d Priscu 1993).
T he p e rm a n e n tly ice-covered lakes of th e d ry valleys o f South
V ictoria Land, A ntarctica have b ee n h y pothesized to b e n itro g en
deficient, a n d several featu res of th ese lakes facilitate analysis o f th e
processes of n itro g e n up tak e a n d reg en eratio n . These featu res
in clu d e lack o f u p p e r trophic levels, h y d rau lic stability, a n d strong
chem ical gradients.
In m a n y aquatic system s, th e process of NHq+ reg en e ra tio n is
p rim arily associated w ith zooplankton excretion. Koike e t al. (1986)
estim ated th a t over 95% o f th e NHq+ reg en erated in th e euphotic
zone o f A ntarctic coastal w aters was p ro d u ced b y m icrozooplankton
a n d n e t zooplankton. Koike et al. (1986) n o ted th a t th e ro le of
b a c te ria a n d p h y to p lan k to n in NHq+ reg en eratio n is larg ely u n know n
a n d difficult to discern; b o th b acteria a n d p h y to p lan k to n can
42
assim ilate as well as p ro d u ce NH44". F urtherm ore, b a c te ria h av e h ig h
n u tritio n a l req u irem e n ts a n d a high affinity fo r scarce n u trie n ts
(H arrison 1992). These observations h av e led som e rese arch ers to
conclude th a t b ac teria m ay b e inefficient rem in eralizers o f n itro g en
in m o st aquatic environm ents (H arrison 1992).
T h ere is n o d o u b t th a t b acteria are theoretically cap ab le of
significant reg en eratio n . Bacterial p ro d u ctio n m ay b e su p p o rted
largely b y dissolved organic m a tte r (DOM) leaked fro m
p h y to p la n k to n (Angelo 1989), a n d d eam ination of DOM reg en erates
NHq+. F urtherm ore, NHq+ is p ro d u ced as a b y p ro d u ct o f bacterial
decom position of phytoplankton. However, G oldm an (1987) n o te d
th a t b ac teria l reg en e ra tio n o f NHq+ m ay be extrem ely inefficient
w h en th e C:N ra tio of th e organic su b strate is 10:1 o r g reater. In
such instances, NHq+ is p resum ably re ta in e d b y th e b acteria. Since
this ratio is often exceeded in n atu re, G oldm an concluded th a t
b acterial reg en eratio n o f NHq+ is insignificant in m o st aquatic
system s.
Zooplankton are ap p aren tly ra re o r ab sen t in th e p eren n ially
ice-covered lakes of th e McMurdo d iy valleys; NHq + reg en e ra tio n in
th e se lakes is th e re fo re reg u lated b y b ac teria a n d p h y to p lan k to n .
Som e rese arch ers h av e suggested th a t th e lack of zo o p lan k to n w ould
d im in ish th e ex ten t a n d efficiency o f reg en eratio n . P ark er an d
Sim m ons (1985) claim ed th a t th e sim plified food ch ain s in A ntarctic
aq u atic system s m inim ized th e p o ten tial fo r n u trie n t recycling,
likew ise, M atsum oto (1993) claim ed th a t p ro d u ctiv ity of d ry valley
lakes is lim ited b y a "paucity o f n u trie n t cycling." How ever, Priscu e t
43
al. (1989) fo u n d th a t rate s of NH4"1" reg en eratio n e ith e r eq u aled o r
exceeded ra te s o f NHq+ u p tak e in Lakes Fryxell a n d V anda. This
observatio n , in co n cert w ith high m easu red NHq+ u p ta k e rates, led
th e m to conclude th a t regeneration of NHq+ supports p ro d u ctio n in
th e se lakes. T he n itro g en dynam ics in th ese lakes w ere te rm e d
"functionally sim ilar" to th e dynam ics of system s w h ere re g e n e ra tio n
is d o m in a te d b y zooplankton (Priscu e t al. 1989). EUis-Evans (1983)
also re p o rte d significant n u trie n t reg en eratio n in a n A ntarctic lake
a n d sp ecu lated th a t b acterial activity was responsible fo r th e b u lk of
n u trie n t cycUng.
A lthough NHq+ is considered to b e a re g e n e ra te d n u trien t, it
can b e quickly co n v erted to o th e r form s o f DDL T n low pro d u ctiv ity
system s, n itrificatio n could convert patches of NHq+ in to p atch es of
NO3" a n d NO2' b efo re DIN uptake b y p h y to p lan k to n . T herefore,
NO3" a n d NO2" in such system s could be con sid ered to b e
re g e n e ra te d n u trien ts.
This stu d y investigates uptake o f DIN a n d re g e n e ra tio n of NHq+
in several distinct, verticaU y stratified planktonic com m unities in
Lake Bonney. Glacial m elt stream s a re th e p rim a ry ex tern al source
o f n u trie n ts to Lake Bonney. These stream s h a d in te rm itte n t flow
d u rin g th e 1992 season a n d relativ ely low flow d u rin g th e 1993
season (p erso n al observation). Lake Bonney d u rin g th e 1993 season
co u ld th e re fo re be co n sid ered to be a closed system w ith resp e ct to
n u trie n ts. M icroplankton in a closed, n o n -tu rb u le n t sy stem utilize
44
n u trie n ts fro m two sources: stead y m olecular diffusion a n d
biologically m ed iated regeneration.
As in Lakes V an d a a n d Fryxell, NH4+ in Lake B onney is
re g e n e ra te d alm ost en tirely b y b acteria a n d p h y to p lan k to n . Strong
g rad ien ts in n u trie n t supply a n d stratification of p h y to p la n k to n a n d
b a c te ria l com m unities in Lake Bonney allow fo r in v estig atio n of th e
in te ra c tio n s b etw een u p tak e a n d reg en e ra tio n in a v a rie ty of
en v iro n m en ts. Specifically, m y stu d y exam ined n itro g e n u p tak e a n d
re g e n e ra tio n in th e com m unities o f th e u p p e r tro phogenic zone (5
m ), th e m id d le trophogenic zone (13 m) a n d th e low er trophogenic
zone (17 m ea st lobe). R egeneration a n d u p tak e was also m e asu red
a t 20 m w est lobe a n d 25 m in each lobe; a t these d ep th s,
p h y to p la n k to n p ro d u ctiv ity is u n d etectab le (Sharp 1993).
My stu d y was d esigned to te st th e following h y p o th eses:
1. NH4+ is th e p re fe rre d source of DIN fo r all Lake Bonney
p lanktonic com m unities.
2. R egeneration of NLLj+ provides a m ajor n itro g en source for
p h y to p la n k to n in th e u p p e r trophogenic zone.
3. R egen erated NO3" a n d NO2" provide a m in o r source of n itro g en
fo r p h y to p lan k to n in th e u p p er trophogenic zone.
4. N itrogen deficient p h y to p lan k to n a re capable of ra p id sh o rt te rm
u p tak e o f NH4+, an d , to a lesser extent, NO3" an d NO2".
~\
To te s t th ese h ypotheses, th e following objectives w ere
u n d erta k en :
45
1. Q uantify th e am b ien t a n d m axim al rate s o f u p tak e o f
a n d NO2".
2. Q uantify th e affinity fo r NH4+, NOg-, a n d NO2-.
3. Q uantify th e am b ien t rates of reg en eratio n of NH4"1".
4. D eterm ine th e sources of reg en erated NH4"1".
NOg
46
METHODS
Isotope Dilution
Water was collected at each depth in a 5 liter Niskin bottle and
gently decanted into 4.2 liter bottles (glass bottles were used for the
1992 season, polycarbonate bottles were used for the 1993 season).
The bottles were kept dark in a cooler and quickly returned to the
lakeshofe laboratory.
Aliquots were removed for RN, PC, DIN, and
CHL analysis.
The bottles were then inoculated with ^ N H ^ C l (99 atompercent).
Water front 5 m east lobe, 5 m west lobe, 13 m east lobe,
13 m west lobe, 17 m east lobe, and 20 m west lobe were enriched
with 10 (imoles of ^ N H ^ C l for a final ^ N H ^ C l concentration of
approximately 2.5 |iM.
Water from 25 m east lobe and 25 m west
lobe were inoculated with 15 pmoles of ^ N H ^ C l for a final ^ N H ^ C l
concentration of approximately 3.75 pM.
Bottles were inverted
several times after addition of ^ N H ^ C l to mix the isotope. Within 5
min of addition, 250 ml of each sample was filtered on a
precombusted 25 mm Whatman GF/F glass fiber filter at low vacuum
(< 0.5 atm). The filter funnel was attached to a vacuum chamber, and
the filtrate was collected in an acid-washed 250 ml HDPE bottle.
filter was dried, then frozen, and returned to MSU for analysis of
particulate ^ N .
The filtrate was frozen and returned to Crary
The
47
la b o ra to ry fo r NH44" analysis. The rem aining filtrate was fro zen a n d
r e tu rn e d to MSU fo r dissolved
analysis.
Following in itial filtration, th e bottles w ere p laced in a n
in c u b a to r. AU b o ttles w ere w rap p ed in n e u tra l d en sity screening to
sim ulate in situ irrad ian ce. Irrad ian ce w ithin th e in c u b a to r was
m o n ito re d w ith a Ii-C or 4n q u an tu m sen so r a tta c h e d to a Ii-C or
m o d e l LI-IOOO d a ta logger. 250 m l aliquots w ere rem o v e d
ap p ro x im ately ev ery 8 h fo r filtratio n as d escrib ed above.
Size F ractio n ated R egeneration
W ater w as coUected in a 5 Uter Niskin b o ttle a t 5 m in b o th
lobes. W ater was gently d ecan ted in to 4.2 Uter b o ttles (glass b o ttles
w ere u se d fo r th e 1992 season, p o ly carb o n ate b o ttle w ere u sed fo r
th e 1993 season). The bottles w ere k ep t d ark in a co o ler a n d quickly
r e tu rn e d to th e lak esh o re lab o rato ry . AUquots w ere rem o v ed fo r
CRN, DIN, a n d CHL analysis.
Before ad d itio n of 10 nmoles of ^^NHqCl (final ad d itio n
ap p ro x im ately 2.7 nM), th e sam ple was filtered th ro u g h a 0.7 ^m
G elm an fUter. Based o n previous m icroscopic ex am ination of Lake
B onney sam ples (Sharp 1993), this filtratio n rem o v ed p h y to p la n k to n
a n d ep ip h y tic b a c te ria b u t n o t free-Uving b acteria. A 250 m l aU quot
was filte re d o n to a p reco m b u sted 25 m m W hatm an GF/F glass fib e r
filte r less th a n five m in after inoculation w ith 15 NHqCL T he filtrate
was coUected in a n acid w ashed 250 m l HDPE bottle. T he in cu b atio n
b o ttles w ere in c u b ate d in a n incu b ato r. N eutral d en sity screening
w as u se d to sim ulate in situ irrad ian ce. A dditional 250 m l aUquots
48
w ere rem o v e d fo r filtratio n approxim ately ev ery 16 h fo r 3 d. T he
filters w ere a ir d ried , frozen, a n d re tu rn e d to MSU fo r -^N analysis.
S u b strate Effects o n R egeneration
W ater w as collected w ith 2 casts of a 9 liter N iskin b o ttle a t 5
m in ea ch lobe. W ater was gently d ec an te d in to 3, 4.2 lite r b o ttles
(glass bo ttles w ere u sed fo r th e 1992 season, p o ly c arb o n ate b o ttle
w ere u se d fo r th e 1993 season). T he b o ttles w ere k e p t d a rk in a
cooler a n d re tu rn e d to th e lakeshore lab o rato ry . A fiquots w ere
rem o v ed fo r CHN, DIN, an d CHL analysis.
For each lobe, th e bottles w ere in o cu lated w ith varying
am o u n ts o f 15N H qC l: 5 junoles (final ad d itio n approxim ately 1.4 fiM),
10 nm oles (final ad d itio n approxim ately 2.7 nM), a n d 15 nmoles
(final a d d itio n approxim ately 4 nM). The bottles w ere in v e rte d
sev eral tim es. Less th a n five m in a fte r inoculation, a 250 m l aliq u o t
fro m ea ch b o ttle was filtered th ro u g h a p reco m b u sted 25 m m
W hatm an GF/F filter. The filtrate was collected in a n acid w ashed
250 m l HDPE bottle. The in cu b atio n bottles w ere in c u b a te d in a fight
a n d te m p e ra tu re co n tro lled incu b ato r. N eutral d en sity screening was
u se d to sim ulate in situ irrad ian ce. An ad d itio n al 250 m l aliq u o t w as
re m o v e d fro m each b o ttle fo r filtratio n ap p roxim ately ev ery 16 h.
T he filters w ere air d ried , frozen, a n d re tu rn e d to MSU fo r 15N
analysis. T he filtrate was frozen a n d re tu rn e d to MSU fo r NHq+ a n d
15N analysis.
49
15 N A nalysis of Filtrate
NH44" was ex tracted w ith activ ated zeolite (Ionsiv W-85; U nion
C arbide), a m olecular sieve w ith a h igh affinity fo r NH4+. Following
th e m e th o d s o f Angelo (1989), 0.1 mg activated zeolite (4 h a t 200°
C) p e r m l of filtrate was ad d ed . A fter zeolite ad d itio n , th e sam ple
was m ixed th o ro u g h ly a n d th e n allow ed to settle fo r 30 m in. The
sam ple w as m ixed again a n d filtered o n a p reco m b u sted 25 m m
W hatm an GF/C glass fib er filter a t low vacuum (<0.5 atm ). The
zeolite w as tra p p e d b y th e filter, a n d th e filter was d rie d a t 55° C fo r
24 h b efo re
ato m -p ercen t analysis.
T he efficiency of NFLf1" extraction b y zeolite v aries as a fu n ctio n
o f salinity. Tests co n d u cted b y Angelo (1989) in d icated th a t
ex tractio n efficiency in fresh w ater ap p ro ach ed 90%, b u t extraction
efficiency in salinities typical o f seaw ater (6.3 mM NaCl) w ere less
th a n 50%. At I M NaCl, extraction of NH4+ was u n d etectab le (Angelo
1989).
T he efficiency o f NFLfh extraction b y zeolite is a n im p o rta n t
p a ra m e te r. Atomic em ission analysis can only be co n d u c ted o n a
lim itpd ra n g e o f n itro g en levels; it is th erefo re n ecessary to b e able
to co n tro l th e am o u n t of nitro g en th a t has b een tra p p e d b y the
zeolite. As p rev io u sly stated, 15N ato m -p ercen t analysis v ia atom ic
em ission sp ectro m etry is optim ized a t ap p roxim ately 10
n itro g en .
50
To te st th e efficiency of NH4+ extraction b y zeolite in Lake
B onney sam ples, NH44" con cen tratio n o f filtered w ater was m e asu red
b efo re a n d a fte r zeolite extraction. W ater was collected d u rin g th e
1992 season a n d sto red frozen in I -gallon collapsible HDPE
co n tain ers. At MSU, approxim ately 200 m l was filte re d a n d th e
filtrate collected in HDPE bottles. A fter rem oval of 4 aliquots fro m
each b o ttle fo r NH4+ analysis, activated zeolite was a d d e d to each
sam ple. As p rev io u sly described, th e zeolite suspension was m ixed,
allow ed to settle fo r 30 m in, m ixed again, an d filtered. A v acuum
c h a m b e r was u sed fo r filtering, a n d th e filtrate was collected d irectly
in to a n acid w ashed HDPE bottle. Four aliquots w ere rem o v ed fro m
each b o ttle fo r NHq+ analysis. Efficiency of NHq+ extraction was
d e te rm in e d w ith th e following eq u atio n (Angelo 1989):
% efficiency = 100 [I - (([NHq+Jp0St X(NHq+Ip re)"1)]
w h e re
(4)
(NHq+Ipost = concentration o f NHq+ after zeolite
extraction
[NHq+Jp re = concentration of NHq+ before zeolite
extraction.
Tests w ere co n d u cted to d eterm in e if d istillation in creased
efficiency of NHq+ extraction in saline samples. NaCl a n d NHqCl w ere
a d d e d to d eio n ized w ater to sim ulate a ran g e of Lake Bonney
en v iro n m en ts. The sam ples w ere b ro u g h t to pH 10.0 w ith NaOH a n d
steam distilled in a ro ta ry ev ap o rato r (60 RPM, 90° C) fo r 4 0 m in.
NHq+ w as ex tracted fro m th e distillate w ith th e zeolite m e th o d
51
50
I
I
12
I
8
1
O
6
I
V
0
20
2
13 m
e a s t lobe
__1_______ 1_______ 1__
0
4
40
60
0
0
80
filtrate
5 m
v
O
e a s t lobe
__1_______ 1 _______ 1__
20
40
60
80
+
Time (h)
Time (h)
1.5 -
OOO
17 m
e a s t lobe
Time (h)
25 m
east
Time (h)
Figure 8. NH4"1" extraction efficiencies using zeolite w ith a n d w ith o u t
a ro ta ry e v a p o ra to r. Salinity in lake sam ples was m e a su re d w ith a
refra cto m e ter.
/ Umol NH
V
O
V v
5 v-
0
V
v
1
9
O
4
0
20
8
8
O
C
O
10
O
O
N a t o m —p e r c e n t f i l t r a t e
O
0
30
12
I
O
1 -Oj
40
1
52
p rev io u sly d escrib ed . Results fro m zeolite extractio n tests, w ith a n d
w ith o u t th e use o f th e ro ta ry ev ap o rato r, are illu stra te d in Figure 8.
It sh o u ld b e n o te d th a t zeolite itself contains n itro g en . T h rough
atom ic em ission spectrom etry, Priscu (u n p u b lish ed d ata) d eterm in ed
th a t each m g of activ ated zeolite contains 0.03 fig n itro g en . In
applicatio n s d escrib ed h ere, n itro g en in th e zeolite will le a d to a
slight ( ap p roxim ately 2%) u n d erestim atio n o f th e I^N-NHzJ+ atom p e rc e n t o f th e filtrate.
G ard n er e t al. (1993) re p o rte d a m eth o d utilizing h igh
p erfo rm a n ce liq u id ch ro m ato g rap h y fo r analysis o f ato m -p ercen t of
dissolved n itro g en . This m eth o d allows fo r efficient
atom -
p e rc e n t d etectio n o f sm all sam ple volum es (< 5 m l), b u t was n o t
reco m m en d e d fo r situations w here th e labeled n itro g en a d d e d w ould
b e larg e relativ e to am b ien t nitrogen. T herefore, th is g en erally
prom ising m e th o d is u nsuitable fo r analysis of re g e n e ra tio n in th e
su rface w aters o f Lake B onney.
A n o th er p o ten tially com plicating facto r associated w ith th e use
o f zeolite is isotope discrim ination. Zeolite has a h ig h e r affinity fo r
14NHa+ th a n fo r 15NHa+, a n d the m agnitude of this effect increases
w ith increasin g salinity. Angelo (1992) estim ated th a t isotopic
d iscrim in atio n ra n g e d fro m 5% in fresh w ater to ap p ro x im ately 15%
a t 6 mM NaCl to 90% a t I M NaCl.
Isotope d iscrim in atio n was d eterm in ed co n c u rre n tly w ith som e
tests o f ex tractio n efficiency. After rem oval of 4 aliquots fo r NHa+
analysis, m e asu red am ounts o f 15NHa+ w ere ad d e d to each bottle.
Before zeolite ad d itio n , fo u r ad d itio n al aliquots w ere rem o v ed fo r
53
NH4+ analysis. This allow ed fo r precise estim ation o f th e
atom -
p e rc e n t en rich m en t. A ctivated zeolite was th e n u sed to ex tract NH4+
as p rev io u sly d escrib ed . Isotope d iscrim ination was d e te rm in e d
w ith th e following eq u atio n (Angelo 1989):
% discrim in atio n = 100 [ (-^N fii - I^N zeo)
w h e re
(5)
= know n ato m -p ercen t en ric h m en t o f th e filtrate
I^N zeo = m easu red ato m -p ercen t en ric h m e n t o f the
zeolite
Isotope discrim in atio n values a t various d ep th s are su m m arized in
T able 4.
T able 4. P ercen t isotope discrim ination fo r extraction o f NHq+ w ith
zeolite a t v ary in g
D ep th (m)
5
5
13
13
17
25
5
20
20
.
Lobe
East
East
-East
East
East
East
W est
W est
W est
enrichm ents.
% discrim ination______Initial atom -% -filtrate
76.8
56.7
78.4
79.0
81.0
74.5
76.3
94.8
55.7
22.2
49.8
41.1
47.3
17.8
24.9
25.1
4.9
3.1
It sh o u ld b e n o te d th a t reg en e ra tio n is d e te rm in e d b y ra te o f
ch an g e o f 15N atom -percent, n o t th e 15N ato m -p ercen t a t a n y given
54
tim e p o in t. C onstant rate s of isotope discrim ination w ith in a given
e x p e rim en t will th erefo re n o t affect calculations o f reg en e ra tio n .
Zeolite provides a co n venient m eth o d fo r analysis o f dissolved
15 n H4 +. C om pared to o th er published m ethods of NH44" extraction
(e.g., m icrodiffusion, p recip itatio n w ith a m ercu ry salt, a n d extractio n
in to in d o p h e n o l blue) (Kristiansen a n d Paasche, 1989), th e use of
zeolite is relativ ely sim ple. It is n o t tim e o r la b o r in te n siv e a n d does
n o t re su lt in th e p ro d u ctio n of h azard o u s waste. However, th e re are
sev eral issues th a t m u st b e ad d ressed before ex p erim en tal
applicatio n of zeolite.
As Figure 8 illustrates, zeolite ex tractio n efficiencies w ere low,
occasionally even below detection, fo r th e highly saline d eep w ater of
Lake Bonney.
However, th e d eep w ater of Lake B onney also
contains h ig h concentrations of NH4+; therefore, th e zeolite ex tracted
sufficient n itro g e n fo r
analysis d esp ite low efficiency.
Calculation of Regeneration
In all re g e n e ra tio n experim ents, reg en e ra tio n w as calcu lated
b etw een each tim e p o in t w ith th e following eq u atio n s (Angelo 1989,
Laws 1984):
55
I n ( 1SNftZ1SNf0 ) (S0 - St )
r=
(6)
;
St * S0
;
St = S0
In (StZS0 ) t
I n ( *15NfoZ15Nft) S 0
(7)
r=
t
w h e re
r = NH4+ reg en eratio n ra te (|xM h "1)
1 5 N f 0 = 1 5 N-NHz).+ ato m -p ercen t excess o f filtrate a t
sta rt of tim e step
1 5 N ft = 1 5 N-NHz)+ ato m -p ercen t excess o f filtra te a t e n d
of tim e step
S0 = con cen tratio n of NH4+ (^M) a t sta rt o f tim e step
St = concentration of NHzj.+ (^M ) a t en d o f tim e step
S ta n d a rd deviations o f reg en eratio n re p o rte d in this stu d y a re
c o m p u te d fro m th e v arian ce of reg en e ra tio n rates b etw een each tim e
point.
P ro d u ctio n of A m m onium from Serine
W ater was collected a t each d e p th in a 5 lite r N iskin bottle.
W ater w as g en tly d e c a n te d in to 4.2 lite r bottles (glass b o ttles w ere
u se d fo r th e 1992 season, p o ly carb o n ate bottle w ere u se d fo r th e
1993 season). T he bottles w ere k ep t d a rk in a cooler a n d re tu rn e d to
th e la k esh o re la b o ra to ry w ithin 30 m in. Aliquots w ere rem o v e d fo r
56
CHN, DIN, a n d CHL analysis. A dditional w ater was rem o v e d u n til 2 I
re m a in e d in each bottle.
Each b o ttle was in o cu lated w ith 2 m l of I mM ^^N -Serine (99
ato m -p ercen t, final, ad d itio n approxim ately I jaM). T he b o ttles w ere
in v e rte d sev eral tim es. Less th a n five m in afte r in o cu latio n , a 250
m l aliq u o t fro m each b o ttle was filtered th ro u g h a p reco m b u sted 25
m m W hatm an GF/F filter. The filtrate was collected in a n acid
w ash ed 250 m l HDPEJbottie. The in cu b atio n bottles w ere in cu b ated
a t ap p ro x im ate in situ te m p eratu res a n d w rap p ed in n e u tra l d en sity
screening to sim ulate in situ irrad ian ce. An ad d itio n al 250 m l aliq u o t
w as rem o v e d fro m each b o ttle ap p roxim ately ev ery 16 h fo r
filtratio n . T he filters w ere air dried, frozen, a n d re tu rn e d to MSU fo r
I ^ n analysis. The filtrate was also fro zen a n d r e tu rn e d to MSU fo r
NHq+ a n d 1^N analysis.
T he p ercen tag e o f serine converted into NHq+ w as calculated
fro m a m odification of G ardner et al.'s (1993) equation:
NH4-V o d = 100* « 15Nf [NH4-1I ) + ( 15Np [PN])) [Ser]-I
w h ere
(8 >
NELq^prod = percentage of serine co n v erted in to NHq+
I^Nf= I^N H q+ ato m -p ercen t of filtrate
[NHq+] = concentration of NHq+ in th e filtrate ( iaM)
15Np = ato m -p ercen t of th e seston
[PN] = concentration o f p articulate n itro g en in th e seston
(iiM)
[Ser] = concentration of
-^ N -s e r in e
a d d e d ( iaM).
57
E quation 8 is b ased o n th e assum ption th a t a negligible am o u n t
o f la b ele d serin e is assim ilated in to th e seston. However, th e ab ility
o f p h y to p la n k to n a n d b acteiio p lan k to n to assim ilate dissolved
organic n itro g en (DON) is highly variable; uptake rate s can ran g e
fro m u n d etectab le to 50% th e ra te of NHq+ up tak e (Bronk a n d G ilbert
1993). M ore in fo rm atio n ab o u t DON dynam ics in Lake Bonney is
n e e d e d fo r com plete in te rp retatio n of this experim ent. T he results o f
E quation 8 sh o u ld be considered a m axim um estim ate; th e actual
p erce n tag e o f serine co n v erted to NHq+ is p ro b ab ly lower.
S u b strate Kinetics
W ater was collected w ith a 9 lite r Niskin. To re d u c e gas
exchange, w ater fro m 13 m a n d d e e p e r was tra n sfe rre d th ro u g h a
sip h o n hose to a clean 20 liter carboy. W ater fro m 5 m was a d d e d to
th e ca rb o y th ro u g h a funnel. In all cases, th e carb o y w as k e p t o u t o f
d ire c t su n lig h t d u rin g th e transfer. T he carboy was th e n w rap p ed in
a d a rk b la n k e t a n d quickly re tu rn e d to th e lak esh o re lab o rato ry .
W ater fro m th e carb o y was u sed to fill 10 to 14 lite r
p o ly c arb o n ate bottles. Rem aining w ater was an aly zed fo r n u trie n ts,
CHN, a n d CHL. The polycarbonate bottles w ere e n ric h e d w ith varying
levels of 15NHqCl, N a15NOg, o r N a15N O ^ N utrient ad d itio n s are
sum m arized in Table 5.
Sam ples fro m th e 1992 season w ere a ttac h ed to a sp re a d e r b a r
a n d in c u b a te d in situ a t th e d e p th of collection. In th e 1993 season,
sam ples w ere in c u b ate d in a light a n d te m p eratu re co n tro lled
in c u b ato r; n e u tra l d en sity screening was used to ap p ro x im ate in situ
irra d ia n c e . Irrad ia n ce w ithin th e in c u b a to r was m o n ito re d w ith a Ii-
58
Cor 4 Jt q u a n tu m sen so r atta c h e d to a Li-Cor m o d el LI-IOOO d a ta
logger. T em p e ra tu re inside th e in c u b a to r was m e a su re d w ith a
th e rm o m e te r in a w ater bath .
T able 5.
en rich m en ts fo r su b strate kinetics.
nM 15N
NO2'
.
5 m e a st lobe
0.1
0.3
0.7
1.0
1.5
1.7
2.0
3.0
7.0
10.0
- 15.0
20.0
0.1
0.3
0.7
1.0
1.5
1.7
2.0
3.0
7.0
10.0
15.0
20.0
0.05
0.1
0.3
0.7
1.0
1.5
1.7
2.0
3.0
7.0
10.0
15.0
OJ
0.3
0.7
1.0
1.5
1.7
2.0
3.0
7.0
10.0
15.0
20.0
2.0
3.0
7.0
10.0
15.0
20.0
30.0
40.0
0.05
0.1
0.3
0.7
1.0
1.5
1.7
2.0
3.0
7.0
10.0
15.0
13 m e a st lobe
NH4+
NOf
Location
—
—
—
—
59
T able 5, cont.
en rich m en ts fo r su b strate kinetics.
lM 15N
Location
NH4+
NO3™
NO2
17 m e a st lobe
0.1
2.0
3.0
7.0
10 . 0 .
0.05
0.1
0.3
0.7
15.0
1.0
20.0
1.5
1.7
0.3
0.7
1.0
1.5
1.7
2.0
3.0
7.0
25 m e a st lobe
30.0
40.0
2.0
10.0
3.0
7.0
15.0
10.0
20.0
15.0
10.0
12.0
15.0
17.5
20.0
25.0
30.0
35.0
40.0
50.0
60.0
70.0
10.0
2.0
15.0
3.0
7.0
10.0
15.0
20.0
30.0
40.0
50.0
60.0
70.0
20.0
30.0
40.0
60
Table 5, cont. 15N enrichments for substrate kinetics.
nM 15N
Location
NHLj+
NOg-
NOz
5 m w est lobe
0.1
0.05
0.05
0.3
0.7
0.1
0.1
0.3
0.7
0.3
0.7
1.0
1.0
1.5
1.7
1.5
1.7
2.0
2.0
10.0
3.0
7.0
3.0
7.0
15.0
10.0
10.0
20.0
15.0
15.0
1.0
1.5
1.7
2.0
3.0
7.0
13 m w est lo b e
2.0
2.0
0.05
3.0
7.0
10.0
15.0
3.0
7.0
10.0
15.0
0.1
20.0
20.0
30.0
40.0
30.0
40.0
1.5
1.7
2.0
3.0
7.0
0.3
0.7
1.0
10.0
15.0
61
Table 5, cont. 15N enrichments for substrate kinetics.
nM 15N
Location
NFLfl"
NO3-
20 m w est lo b e
10.0
15.0
20.0
30.0
40.0
50.0
60.0
70.0
2.0
3.0
7.0
10.0
15.0
20.0
30.0
40.0
— — —
— — —
— — —
— — —
—
— — —
--------
—
25 m w est lo b e
10.0
15.0
20.0
30.0
40.0
50.0
60.0
70.0
----
--
0.1
0.5.
1.0
1.5
3.0
7.0
10.0
15.0
6
—
— — —
— — —
—
— — —
■
*
—
NO2"
0.05
0.1
0.3
0.7
1.0
1.5
1.7
2.0
3.0
7.0
10.0
15.0
0.05
0.1
0.3
0.7
1.0
1.5
1.7
2.0
3.0
7.0
10.0
15.0
A fter ap p ro x im ately 24 h of in cu b atio n , sam ples w ere filte re d
o n to p reco m b u sted 47 m m W hatm an GF/F glass fib e r filters u n d e r
62
low v ac u u m (<0.5 atm ). A fter filtratio n , filters w ere rin se d w ith
ap p ro x im ately 20 m l DIW to rem ove inorganic n itro g en . Filters w ere
a ir d rie d in alu m in u m w eigh-boats fo r several days b efo re being
tra n s p o rte d to C rary lab o rato ry . A fter arriv al a t C rary lab o rato ry ,
filters w ere fro zen (-45° C) before tra n sp o rt to MSU fo r fin al analysis.
W here ap p ro p riate , calculated u p tak e ra te s w ere fitte d to
su b stra te c o n c en tratio n w ith th e M ichaelis-M enten equ atio n :
V=
V m ax * S
---------------
(9)
Kg + S
w h e re
V = specific ra te of u p tak e (h"1)
S = su b strate co n cen tratio n (pM)
V m ax = ra te of u p tak e a t satu ratin g levels o f S (h"1)
Ks = su b stra te level a t w hich V = 0.5 Vm ax (th e h a lf
satu ratio n constant) (fiM).
U ptake ra te s a t an y level of S can b e o b ta in ed fro m this
eq u a tio n once Vm ax a n d Ks are know n ( M urphy 1980, Priscu a n d
Priscu 1984). To aid in co nstruction of th e M ichaelis-M enten curve,
it w as assu m ed th a t V=O w hen S=0. This d atu m p o in t w as in clu d ed
in all d a ta sets d escrib ed b y M ichaelis-M enten kinetics. W hen
u p ta k e ra te s d id n o t conform to M ichaelis-M enten kinetics, tre n d s
w ere e stim a te d w ith a lin e ar least sq u ares regression.
R eg en eratio n can significantly affect m easu rem en ts of
su b stra te kinetics. M odels co n stru cted b y G arside (1984) in d icate
th a t isotope d ilu tio n of tracer am ounts (10% am bient) o f -^N
63
ad d itio n s in a n oligotrophic system can lead to 50% o r g reater
u n d e re stim a tio n of u p tak e after 12 h o f incubation. Conversely,
id en tical ra te s o f isotope dilution o f large
en rich m en ts (10 x
am bient) do n o t significantly affect calculation o f u p tak e.
Clearly,
th e d ifferen tial effects o f reg en eratio n could p ro d u ce ap p a re n t, y et
artificial, su b stra te effects o n uptake.
NH4+ u p tak e ra te s w ere corrected fo r changes in NEL}+ specific
activity fro m 14NIfy+ reg en eratio n w ith th e following eq u atio n
(Angelo 1989):
r_
-I
+
(I-Wh
v C
)
Vc
(10)
Vc
w h e re
k = V t S"1
r= NHq+ reg en eratio n ra te fyM h ' 1)
Vc= specific u p tak e ra te co rrected fo r isotope d ilu tio n
( h "1)
O th er p aram ete rs are as in Equation 9. Vc was ad ju ste d u n til
th e eq u atio n approxim ated 0 ( ± 0.005). The fu n ctio n is v ery
sensitive to changes in Vc; to achieve th e d esired re su lt (0 ± 0.005), it
w as n ecessary to estim ate Vc to a t le ast 5, a n d o ften 6, decim al
places. Note th a t p red ic ted u p tak e ra te s d escrib ed b y m ath em atical
64
functions, n o t m e asu red u p tak e rates, w ere ad ju sted fo r
reg en eratio n .
W ater Colum n U ptake
W ater sam ples w ere collected a t selected d ep th s th ro u g h o u t
th e w ate r co lum n w ith a 5 liter Niskin bottle. One lite r fro m each
d e p th w as tra n sfe rre d to a I liter HDPE b o ttle fo r analysis of
b ac k g ro u n d n u trie n ts, PC a n d PN. Rem aining w ater w as u se d to fill 2,
I lite r p o ly c arb o n ate bottles. T he p o ly carb o n ate b o ttles w ere k e p t in
a sealed cooler before inoculation. Each bottle was in o cu lated w ith a
m e asu red volum e of 10 mM solution of eith er l^NH^Cl, N a ^ N O g , 0r
Na^^N02.
ad d itio n s a n d d ep th s sam pled are sum m arized in T able 6 .
N u trie n t en rich m en ts w ere designed to satu rate u p ta k e of n itro g en
(i.e., d esig n ed to m easu re Vmax) (W oolston a n d Priscu 1993). A fter
in o cu latio n , th e b o ttles w ere in v e rte d several tim es a n d su sp en d ed
a t th e d e p th of collection. Incubation b eg an betw een 2200 a n d 0000
local tim e, d u rin g w hich tim e th e lake d id n o t receive d ire c t sunlight.
Bottles w ere rem o v ed fro m th e lake afte r 24 h o f in cu b atio n .
Sam ples w ere filtered on p reco m b u sted 47 m m W h atm an GF/F
glass fib e r filters u n d e r low vacuum (<0.5 atm ). A fter filtration,
filters w ere rin se d w ith ap p roxim ately 20 m l DIW to rem o v e
in o rg an ic n itro g en . Filters w ere d rie d in alu m in u m w eigh-boats fo r
several d b efo re being tra n sp o rte d to C rary lab o ra to ry . A fter arriv al
a t C rary la b o ra to ry , filters w ere fro zen (-45° C) b efo re tra n s p o rt to
MSU fo r fin al analysis.
65
Table 6 .
Enrichments for water column experiment.
IiM 15N
EAST LOBE
____________________________
D epth
NH44"
NOg"
NO2"
5
10
13
17
22
25
30
35
1.0
1.0
1.0
15.0
15.0
50.0
50.0
50.0
1.0
2.0
5.0
6.0
50.0
50.0
50.0
50.0
0.2
0.2
0.2
0.2
30.0
30.0
. 30.0
30.0
0.5
5.0
30.0
30.0
30.0
30.0
30.0
30.0
7.0
15.0
15.0
15.0
15.0
0.2
0.5
15.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
WEST LOBE
5
13
15
17
20
25
30
35
Tim e Course
W ater was collected in a 9 liter Niskin b o ttle a n d im m ed iately
d e c a n te d in to in cu b atio n bottles. All 1992 sam ples a n d w est lobe
1993 sam ples w ere in c u b ate d in 4 lite r bottles (glass in 1992,
66
p o ly c a rb o n a te in 1993). East lobe 1993 sam ples w ere in c u b a te d in I
lite r p o ly carb o n ate bottles. D uring th e 1993 season, a 500 m l aliq u o t
fro m each d e p th was rem oved fo r analysis of CHN a n d DIN. For
NOg" u p tak e, b o ttles w ere en rich ed w ith satu ratin g am o u n ts o f
e ith e r N a^^N 02 d r N a^N O g (i.e. V=Vm ax)(W oolston a n d Priscu,
1993). For NHq+ uptake, one bottle fro m each d e p th was en rich ed
w ith satu ratin g am o u n ts of ^N H q C l (W oolston a n d Priscu, 1993). An
ad d itio n a l b o ttle fro m each d e p th was en rich ed w ith a su b satu ratin g
am o u n t o f 1^NHqCl (»10% am bient). A fter inoculation, th e bottles
w ere in v e rte d several tim es. Less th a n five m in a fte r in o cu latio n , a n
aliq u o t (500 m l d u rin g 1992, 250 m l d u rin g 1993) fro m each b o ttle
w as filte re d th ro u g h a p reco m b u sted W hatm an GF/F filte r (47 m m
fo r 1992, 25 m m fo r 1993). The in cu b atio n b o ttles w ere in c u b ate d
in a n in cu b ato r. N eutral density screening was u sed to sim ulate in
situ irra d ia n c e . A dditional aliquots w ere rem o v ed fro m each b o ttle
fo r filtra tio n ap p ro x im ately every 4 h . The filters w ere a ir d ried ,
frozen, a n d re tu rn e d to MSU fo r I ^N analysis.
67
RESULTS
Isotope d ilu tio n
Changes in 1% -N H 4+ atom -percent an d NELf1" co n cen tratio n o f
th e filtra te o v er tim e are illu strated in Figure 9. D ue to w idely
sc a tte re d 15N-NELf1" ato m -p ercen t m easurem ents, calcu lated
re g e n e ra tio n ra te s v arie d greatly betw een tim e p o in ts in each
e x p e rim en t a n d could n o t be ad eq u ately m odeled w ith e ith e r a lin e a r
o r exponen tial function. T herefore, reg en eratio n w as calculated
b etw een each tim e p o in t, a n d th e reg en e ra tio n ra te s w ere av erag ed
(see M orrisey a n d Fisher 1988). A verage re g e n e ra tio n ra te s a n d
s ta n d a rd deviations a re sum m arized in Table 7. D espite large
coefficients of v ariance, a general tre n d is evident: reg e n e ra tio n
ra te s in crease w ith d e p th in each lobe.
68
50
5 m e a s t lobe
_1_____ 1_____ 1_
0
20
40
60
80
0
^
4
13 m e a s t l o b e
_1_____ 1_____ 1_
20
40
60
80
Time (h)
ooo
17 m
e a s t lobe
Time (h)
25 m
e a s t lobe
Time (h)
Figure 9. NH4 + (m-M) and
atom-percent in the filtrate for isotope
dilution experiments, east lobe 1992.
t HN I ouitV
Time (h)
2
O
V V ^
O
w 8 v ~ 4
v V
0
4
8
O
1
0
V
O
8
O
V
I2
I
0
I
C
0
v v
I
6
Q
V
F
8
O
O
20
10
12
I
O
N a t o m - p e r c e n t filtrate
I
O
O
O
30
_ l_-Oj
40
I
69
50
40
1O 0 '
(b- o °
o'
°
o
o
8
30
20
V
I0
„
V
V
V
V
V
v
v
V"
5 m w est lobe
filtrate
N a t o m - p e r c e n t filtrate
12
___ i_____ I_____ I
0
20
40
60
80
0
20
40
60
80
Time (h)
Time (h)
Tt2 0 0 2 .0
200 v
v v 5 v v ^ © O- 150 1.5
20 m w e st lobe
Time (h)
m w e st lobe
Time
Figure 9, cont. NH4 + (^M) and 15N atom-percent in the filtrate for
isotope dilution experiments, west lobe 1992.
70
12 2. 0
V v V v v
13 m
5 m e a s t lobe
Time (h)
e a st lobe
filtrate
v
Time (h)
2.0
i
r
v v
T
v v v
1 .5
v
1 .0
0 °
v "Y 150
v
I 00
° n°
OOQ
O
0.5
25 m
17 m e a s t l o b e
_I_____ I_____ I_____
0. 0
0
Time (h)
e a s t lobe
20
40
60
80
Time (h)
Figure 9, cont. NH4 4" (^M) and 15N atom-percent in the filtrate for
isotope dilution experiments, east lobe 1993.
<] yU-M NH4
N a t o m - p e r c e n t filtrate
I .0 ^
71
I
I
12
I
6
I
I
12
I
O
O
OO
O
-
-
8
m
N
^
5
m
}
°
?
v
w e s t
v V
20
40
0
4
-
lo b e
__ 1
_______ 1_______ 1__
0
o °
60
0
_
13
m
w e s t
lo b e
__ 1_______ 1_______ 1__
0
80
w
f ilt r a t e
10
O O
O
V V V
0
20
60
80
Time (h)
2.0
.
N H .
Time (h)
40
C)
1.5\ r
O
2 0 0 2.0
v 'v V *
I
V v
V
o °
C O
0.5
150
1.5' -
100
I . O c1
—
2 0
m
w e s t
50
-
v
O
v 5
n 0
0.0 _____ 1_____ 1_____ 1_____ 0
0
20
40
60
80
200
150
O
100
O
0.5
50
lo b e
Time (h)
V V
0
0 0
—
250
I
7
O
1 .0
I
2 5
0.0
0
m
w e s t
lo b e
1
1
1
20
40
60
0
80
Time (h)
Figure 9, cont. NH4 + (nM) and 15N atom-percent in the filtrate for
isotope dilution experiments, west lobe 1993.
j j M
a t o m - p e r c e n t
f i l t r a t e
c
V
72
Table 7. Average rates of NH4 + regeneration (^M h"l) ( + I
s ta n d a rd deviation) in isotope d ilu tio n experim ents a fte r 24 a n d 80 h
of incubation.
D epth (m)
Lobe
r8 0
r 24
1992 ex p erim en ts
5
East
-0.03 ± 0.18
-0.01 ±
0.26
13
East
-0.04 ± 0.20
-0.03 ±
0.16
17
East
-0.02 ± 0 .2 1
25
East
1.25 ± 1.07
0.73 ±
1.24
5
W est
-0.02 ± 0.05
-0.01 ±
0.10
13
W est ,
0.06 + 0.66
0.01 ± 0.72
25
W est
4.32 ± 1 3 .8 3
-1.17 ± 10.09
0.05 ± 0.40
1993 ex p erim en ts
. -0.06 ±
0.23
5
East
-0.18 ± 0.26
13
East
0.10 ± 0.60
17
East
0.13 ± 1.12
-0.35 ±
1.16
25
East
-3.96 ± 5.36
-0.85 ±
7.58
5
. W est
-0.11 ± 0.11
-0.02 ±
0.14
13
W est
0.11 ± 0.15
0.04 ± 0.17
20
W est
5.72 ± 6.06
2.76 ± 6.64
25
W est
7.56 ± 17.67
2.13 ± 1 3 .7 7
0.01 ± 0.47
Size Fractionation
Average regeneration rates from size fractionation experiments
after 24 and 48 h of incubation are summarized in Table 8. As with
73
isotope dilu tio n experim ents, th e sta n d a rd deviations of reg en e ra tio n
estim ates a re larg e relativ e to th e average re g e n e ra tio n rates,
m aking it difficult to d eterm in e a tren d .
T able 8. A verage reg en eratio n rate s (fxM h " l, + s ta n d a rd deviation)
o f size fra ctio n a ted sam ples after 24 a n d 48 h of in cu b atio n .
D epth
Lobe
r2 4
r 48
5
ea st
-0.12 ± 0.08
-0.02 ± 0.19
5
w e st
-0.25 ± 0.39
-0.07 ± 0 .3 1
S u b strate Effect o n R egeneration
As w ith th e p rev io u s experim ents,
ato m -p e rc en t
m e asu rem e n ts fo r su b strate effects o n isotope d ilu tio n ex p erim en ts
w ere w idely scattered. These d a ta (Table 9) in d icate th a t NELj+
reg en e ra tio n increases w ith increasing NH4+ concentration,
p a rtic u la rly in th e ea st lobe; how ever, large coefficients of v arian ce
o f re g e n e ra tio n estim ates p re v e n t definitive in te rp re ta tio n .
74
T able 9 . NH4+ reg en eratio n rates ( ± I sta n d ard deviation) from
su b stra te effects o n reg en eratio n experim ents. R egeneration values
av erag ed fro m 80 h incubation.
ID
r
5
5
5
5
5
5
-0.02 ± 0 .1 4
0.03 ± 0 .1 0
0.02 ±0.06
0.00 ± 0.07
-0.03 ± 0.24
-0.02 ± 0 .0 7
East low en ric h m e n t
East m e d iu m en ric h m en t
East h ig h en rich m en t
W est low e n ric h m e n t
W est m e d iu m en ric h m en t
W est h ig h en ric h m en t
.
A m m onium p ro d u ctio n from serine
Changes in 15N-NH4+ atom -percent an d NH4+ co n cen tratio n of
th e filtrate a re illu strate d in Figure 10. The average p ercen tag es of
15N-Serine co n v erted to NH4+ fro m several tim e p o in ts d u rin g a 50 h
in cu b atio n are sum m arized in Table 10. The po ten tial fo r NH44"
p ro d u c tio n fro m serine generally in creased w ith d ep th . Since
a m b ie n t serin e co n cen tratio n s also increase w ith d e p th fro m 5 m to
25 m in th e e a st lo b e ( fro m 0.04 to 0.14
respectively) (Priscu,
u n p u b lish e d d ata), it can be assum ed th a t th e am o u n t o f NH4+
p ro d u c e d fro m d eam in atio n of serine increases w ith d ep th .
75
13m
ato m -p ercen t
N atom -percent
•
O NH
HN W"'
10
Time (h)
20
30
40
50
60
70
60
70
Time (h)
17 m
*HN n„
N atom -percent
25 m
10
20
30
40
50
Time (h)
60
70
10
20
30
40
50
Time (h)
Figure 10. NH4 "1" (nM) and ^N-NHzJ+ atom-percent in the filtrate for
serine experiments, 1992.
N a t o m —p e r c e n t
76
,uM NH
+
T im e (h)
20
30
Ti m e (h)
T im e (h)
T im e (h)
10
20
30
40
50
0
10
40
50
/zM NH
N a t o m —p e r c e n t
O
Figure 10, cont. NH4"1" (pM) an d 15N-NFLf1" ato m -p ercen t in the
filtra te fo r serin e experim ents, 1993.
77
T able 10. A verage p ercen tag e of ad d e d l^N-serine co n v e rte d to
15 n H4 + (± I sta n d a rd deviation) after 50 h of incubation.
East Lobe
% serine converted
D epth (m)
1992
1993
5
2.37 ± 0 .6 3
1.11 ± 0 .2 9
13
0.48 ± 0.08
1.88 ± 0 .7 0
17
2.34 ± 0 .2 1
6.33 ± 2.04
25
16.23 ± 3 .4 0
41.38 (one reading)
S u b strate k inetics
M ichaelis-M enten kinetics could be used to m o d el NH4+ u p tak e
a t 5 a n d 13 m in b o th lobes (Figure 11). NfLj + u p tak e a t d e e p e r
d e p th s d id n o t conform to M ichaelis-M enten kinetics; a lin e a r least
sq u a re s reg ressio n w as u sed to describ e u p tak e tre n d s a t th ese
d ep th s. M ichaehs-M enten p aram eters, Vm ax a n d Ks, a re
su m m arized in T able 11. P aram eters w ere d e te rm in e d using
M a rq u a rd t's algorithm . R eported sta n d a rd deviations of M ichaelisM enten p a ra m e te rs are equ iv alen t to th e sta n d a rd e r r o r of th e
estim ate.
Large coefficients of variance of isotope d ilu tio n m easu rem en ts
(> 100%) p re v e n t precise regeneration corrections o f NHq+ uptake.
However, there is no question that some regeneration does occur and
that regeneration does, to some extent, affect measurements of
78
7O
Oct 31 1 9 9 3
X
Nov 14 1 9 9 3
5 m e a st lobe
13 m e a s t l o b e
NH
U p t a k e (h
I
0
5
10
15
20
0
25
5
10
I
I
3. 0
I
No v I 1 9 9 3
I
25
O
i
i
i
Nov 14 1 9 9 3
2. 4
13 m w e s t l o b e
5 m w e s t lobe
^ X -O
20
/^M NH4+
/^M NH4
I
15
I .8
___ ©-----I .2
—
—
(B) O
0 .6
l
0
i
i
5
i
10
i
15
yuM NH4+
0.0 c
20
25
0
----------1---------- 1----------1
—
10
20
30
40
AiM NH4+
Figure 11 . S u b strate kinetics of NH4+ uptake. D ata fitte d w ith
M ichaelis-M enten eq u atio n using M arq u ard t’s algorithm .
79
su b stra te kinetics o f NH44* uptake. Positive re g e n e ra tio n ra te s w ere
n e e d e d to assess th e po ten tial influence of reg en eratio n o n NH44"
su b stra te kinetics. The 24 h average fro m 1993 ex p erim en ts was
u se d to estim ate in situ NH4+ reg en eratio n rates, a n d th e 24 h
av erag e plu s o n e sta n d a rd d ev iation was u sed as a n estim ate o f th e
h ig h e st in situ NH4+ reg en eratio n rates allow ed b y th e d ata. In
in stan ces w h ere th e average reg en e ra tio n ra te was n eg ativ e (e.g. 5 m
w est lobe), o n ly th e average plus one sta n d a rd d ev iatio n was u sed
fo r a d ju stm e n t of u p tak e rates. P aram eters from c o rre c te d curves,
along w ith u n co rrected p aram eters, a re sum m arized in T able 11.
To re ite ra te , th eo retical u p tak e rate s along th e M ichaelisM enten curve, n o t th e original u p tak e m easu rem en ts, w ere co rrected
fo r reg en e ra tio n . Because th e y w ere d eriv ed fro m id ealized curves,
th e sta n d a rd deviations o f "corrected" p aram eters d o n o t in d icate th e
d eg ree of fit o f th e original d a ta to th e M ichaehs-M enten curve.
S ta n d a rd deviations of co rrected p aram ete rs h av e two causes.
Firstly, b ecau se co rrectio n fo r reg en eratio n h as a relativ ely large
effect o n low e n ric h m en t sam ples co m p ared to h ig h en ric h m en t
sam ples, reg en e ra tio n correction changes th e shape o f th e curve; th e
co rre c te d curve can n o longer b e precisely d escrib ed b y M ichaelisM enten kinetics. Secondly, because E quation 10 n e v e r eq u aled 0,
th e re was a sm all (< 5%) degree of e rro r in estim ation o f co rrected
u p ta k e ra te s .
U ptake of NO3" a n d NO2" n e v e r ap p ro x im ated M ichaelisM enten kinetics, a n d th e slopes o f lin e a r least sq u a re regressions
w ere n e v e r significantly d ifferen t fro m zero, w ith th e exception o f 5
80
m e a st lobe. These results indicate th a t am b ien t u p ta k e o f NO3" a n d
NO2™ is g en erally su b strate satu rated .
T able 11 . O riginal a n d corrected p aram ete rs of M ichaeUs-M enten
Curves. AU u p ta k e values m uldpU ed b y IOzK ID labels include
d ep th , lobe (E o r W), a n d n u trie n t analyzed.
r
Vmax x IOz*
Ks
1993 E xperim ents
5ENH 4+
0 .0 0
0.08
12.57 ± 2.54
11.69 ±0.02
3.89 ±2.09
1.77 ±0.11
16.49 ±2.36
16.49 ±2.36
0.95 ±0.58
0.95 ±0.58
12.53 ± 2.84
11.53 ±0.13
12.98 ±0.00
7.47 ±3.63
4.17 ±0.13
0.64 ±0.01
13.60 ±0.66
12.72 ±0.75
13.16 ±1.51
33.80 ±2.83
24.76 ±2.86
21.44 ±5.102
5W N H 4+
0 .0 0
0 .0 0 1
13E N H 4+
0 .0 0
0 .1 0
0.70
13W N H 4+
0 .0 0
0 .1 1
0.26
81
Vm ax a n d Ks values fo r NH4+ u p tak e a t 5 m in each lobe w ere
n o t significantly changed b y correction fo r isotope dilution; co rrected
v alu es w ere w ithin one sta n d a rd d ev iatio n of o riginal estim ates.
In
th e case of 5 m w est lobe, E quation 10 ap p ro x im ated zero w hen
Vc=V; th e re fo re, n o ad ju stm en t was necessary. A t 5 m in each lobe,
th e original, u n c o rre c te d M ichaelis-M enten p a ra m e te rs w ere u sed fo r
estim atio n o f a m b ien t u p tak e rates. At 13 m in each lobe, Vm ax of
NH4+ u p ta k e was n o t significantly changed b y co rrectio n for
reg en eratio n ; how ever, Ks significantly d ecreased a fte r co rrectio n fo r
reg en e ra tio n . T herefore, th e M ichaehs-M enten p a ra m e te rs o b ta in ed
a fte r co rrectio n w ith average re g en e ra tio n rate s w ere u se d to
estim ate a m b ie n t u p tak e ra te s a t 13 m in each lobe.
Of ex p erim en ts th a t w ere d escrib ed b y a le a st sq u ares lin e a r
reg ressio n (see T able 1 2 ) , m ost slopes w ere n o t significantly
d iffe re n t th a n 0 (p > 0.05). The average V of th ese ex p erim en ts is
- p re s e n te d as a n estim ate o f Vm ax. A few ex p erim en ts w ere
d escrib ed b y slopes significantly d ifferen t th a n 0 (p < 0.05); Vm ax in
th e se ex p erim en ts is estim ated to b e th e h ig h est y v alu e o f th e
reg ressio n w ithin th e experim ental ran g e of su b stra te co n cen tratio n s.
D espite th e a p p a re n tly significant resp o n se to in creasin g su b strate
co n cen tratio n s in som e experim ents, extrapolation o f regressions to
estim ate u p ta k e a t am b ien t n u trie n t co n cen tratio n is u n w a rra n te d .
T he y-axis in te rc e p t of these regressions is n o t re p o rte d because it is
ecologically u n d efin ed ; th ere can b e n o uptake w hen th e su b strate
co n c en tratio n equals zero, a n d u p tak e can n o t be negative. However,
a h o f th e se reg ressio n s have y-axis in tercep ts th a t a re significantly
82
d iffe re n t fro m zero. T he fact th a t p o rtio n s of these reg ressio n s a re
m eaningless in d icates th a t extrapolations are n o t justified. In
sum m ary, o nly
N H zj+
up tak e a t 5 an d 13 m in b o th lobes v aried
significantly w ith su b strate co n cen tratio n (i.e. a t
am b ien t concentrations, V < Vmax)T able 12. A verage u p tak e (V, h " l) a n d slopes (IT I nM~l) o f
lin e a r reg ressio n s of su b strate kinetics experim ents ( + I sta n d a rd
d ev iatio n ), 1993 experim ents. All v alues m u ltip lied b y IOzK * indicates a value n o t significantly d ifferen t from 0 (p > 0.05). ND
in d icates u p ta k e was n o t detectable. ID labels in clu d e d ep th , lobe (E
o r W, a n d n u trie n t analyzed.
r
Slope
17E N H 4+
0 .1 3
1 .2 5
Vm ax
*-0.16
5.47
-0.19
-0.44
6.02
9.94
*0.03
3.37
0.04
0.04
4.50
5.60
. *0.03
2.97
1 .4 0
0.03
3.33
2 5 W N H zj+
0.08
3.60
1 3 .4 1
1.20
5.67
20W N H 4+
5 .7 2
1 1 .7 7
25E N H 4+
83
T able 12, cont. E stim ated Vm ax ( h 'l) a n d slopes (h - ^ nM"*) of
lin e a r regressions of su b strate kinetics experim ents ( ± I sta n d a rd
dev iatio n ), 1993 experim ents. * - indicates a value n o t significantly
d iffe re n t th a n 0 (p > 0.05). AU values m uldpH ed b y IOzK
ID
Slope
Vmax
5ENO3-
ND
ND
SWNOg-
ND
ND
ISENOg-
-7.67
*27.14
13WNOg"
*-0.05
. *0.02
17ENOg-
*0.02
*0.00
20WNOg-
ND
ND
25ENOg"
*-0.01
25WNOg-
ND
*1.70
ND
5EN02"
0.08
0.45
5WN02"
*0.01
*0.08
ISENO2'
*0.00
*0.10
At 17ENH4+ a n d 20WNH4+, th e average uptake ra te w as n o t
significantly in creased b y correction w ith th e average reg en e ra tio n
ra te . However, I7ENH4"1" a n d 2OWNH4+ u p tak e was significantly
in c re a se d w h en th e average reg en eratio n ra te plus o n e sta n d a rd
d ev iatio n was in se rte d in to Equation 10. Average u p ta k e o f NH4+
w as n o t significantly affected b y reg en eratio n a t 25 m in e ith e r lobe.
84
Since average reg en eratio n rates d id n o t significantly affect
averag e u p ta k e rates,
NH4+ u p tak e experim ents a t 17 m ea st lobe,
20 m w est lobe, 25 m ea st lobe, arid 25 m w est lobe d o n o t re q u ire
co rrectio n fo r reg en eratio n . It should b e n o ted , how ever, th a t if
re g e n e ra tio n ra te s d u rin g a given ex p erim en t are la rg e r th a n th e
re p o rte d 24 h average, NH4"1" u p tak e could b e significantly
u n d erestim a ted .
C om pared to 5 m east lobe, Ks fo r NH4+ u p tak e was relatively
low a t 5 m w est lobe, indicating relatively h igh affinity fo r NH4+ a t 5
m in th e w est lobe. Ks values fo r NH4+ u p tak e in c re ase d w ith d e p th
fro m 5 to 13 m in each lobe. This is p articu larly e v id e n t in th e w est
lobe w h ere Ks fo r NH4+ up tak e was m o re th a n 30 tim es g re a te r a t 13
m th a n a t 5 m. However, th e tre n d tow ards increasing Ks w ith
increasin g d e p th is dim inished w hen u p tak e rate s a re co rrected fo r
reg en eratio n . For instance, if reg en eratio n of NH4+ a t 13 m in th e
e a st lobe was occurring a t a ra te o f 0.70 m-M h " l (th e m e asu red 24 h
average plu s one sta n d a rd deviation), th e Ks fo r NH4+ u p tak e a t this
d e p th w ould b e low er th a n the Ks a t 5 m in th e ea st lobe.
N u trien t g rad ien ts (See P art I, Results, N utrients) w ere u se d to
estim ate th e ra te o f u p w ard diffusion o f n u trie n ts in to selected I m
th ick layers. Diffusive flux (DF) was d eterm in ed w ith th e following
equation:
85
DF = g * Kz * I
I m
w h e re
*
I m3
(H)
IO3 liter
DF = ra te o f upw ards diffusive flux ( fxM h 'l )
g= n u trie n t gradient( ^mole m"4)
Kz = m olecular diffusion coefficient (5.4 x ICT^
h “l)
I m = thickness of th e lay er
V lO 3 = conversion of m 3 to liters.
U pw ard diffusion rates of NFL}+ a n d NOg" in to I m thick layers
w ith m id p o in ts a t 5 a n d 13 m i n each lobe are su m m arized in Table
13 . In situ u p ta k e rate s of each n u trie n t are in c lu d ed fo r
com pariso n . NFL}+ u p tak e rates w ere calculated w ith M ichaelisM enten p ara m e te rs. To o b tain V ( h 'l ) , S was assu m ed to eq u al th e
co n cen tratio n of NH4+ a t the b o tto m of th e layer. Because NHq+ was
n o t m e a su re d a t 5 m d u ring ro u tin e d a ta collection, th e average o f
NHq+ con cen tratio n s a t 4 m an d 6 m was used in calculation o f V.
M ultiplying V b y th e p articu late n itro g en a t 5 a n d 13 m p ro d u c e d
th e u p ta k e p a ra m e te r p (pM h 'l ) . A verage NOg' u p ta k e ra te s fro m
su b stra te kinetics experim ents w ere u se d fo r estim ates o f NOg"
u p ta k e. (V - DF) in d icates th e ra te a t w hich th e n u tr ie n t m u st be
su p p h e d fro m sources o th e r th a n u p w ard diffusive flux (i.e.,
reg en e ra tio n ) to su p p o rt in situ uptake.
86
T able 13. 1993 u p w ard diffusive flux (DF, |xM h "l) a n d u p tak e rate s
(p, fxM h _1) o f DIN a t 5 a n d 13 m i n each lobe. ID labels in clu d e
d e p th , n u trie n t, a n d lobe (either E o r W).
ID
Date
EF
(P -D F )
P
x 10-5
x 10"5
x 10" 5
5ENH4+
10 Nov
0
55.7
55.7
SENH4+
7 Dec
0
102.7
102.7
SWNH4 +
12 Nov
0
354.6
354.6
SWNH4+
9 Dec
0.0
175.0
175.0
ISENH4 +
10 Nov
1.4
17.2
15.8
ISENH4 +
7 Dec
0.2
19.7
19.5
ISWNH4 +
12 Nov
19.6
69.9
50.3
97.7
11.9
-22.0
0.2
0.0
-0.2
0.6
0.0
-0.6
ISWNH4 + 9 Dec
SENOgSENOg-
10 Nov
. 7 Dec
SWNOg-
12 Nov
0.0
0.0
0.0
SWNOg-
9 Dec
0.0
0.0
0.0
ISENOg-
10 Nov
0.0
0.8
0.8
ISENOg-
7 Dec
5.9
4.9
-0.9
ISW NOg-
12 Nov
3.2
5.3
2.1
ISW NOg-
9 Dec
5.3
5.0
-0.3
87
W ater Colum n U ptake
W ater co lum n profiles of DIN u p tak e (see Figure 12) show
several tre n d s. W ith th e exception of a single h ig h v alu e fo r NO3"
u p ta k e a t 35 m east lobe (1993), NH4"1" is th e p referen tia lly
tra n sp o rte d fo rm o f DIN a t all depths. M aximum NFLj.+ u p tak e occurs
co n sisten tly w ith in th e first chem ocline (betw een 17 a n d 10 m ). In
co n tra st, th e h ig h est u p tak e rates o f NO3" o ccu rred a t 5 m, w ith th e
exception o f th e east lobe 1993 experim ent. NO2" u p ta k e ra te s w ere
low a n d o ften below detection.
D ata fo r NO3" u p tak e east lobe (1992) a n d N O 2' u p ta k e w est
lo b e (1992) a re unavailable; th e sealed tu b es fro m th ese ex p erim en ts
inexplicably b u rs t d u rin g Dumas com bustion o f th e filters.
Time Course
Tim e course results are illu strated in Figure 13. R apid in itial
u p ta k e w as always a p p a re n t fo r NH4"1" u p tak e a t 5 m in ea ch lobe a n d
13 m ea st lobe. In b o th 1992 a n d 1993, ra p id sh o rt te rm NH4+
u p ta k e a t 17 m e a st lobe was n o t ev id en t after 5 m in b u t was
e v id e n t a fte r 4 h . If th ese u p tak e resp o n ses are typical, estim ates o f
m axim um u p tak e rate s of NH4+ b ased o n 24 h in cu b atio n s w ould b e
significantly u n d erestim a ted a t 5 m i n each lobe, 17 m east lobe, a n d
13 m e a st lobe.
U ptake o f NO3" displays a dow nw ard tre n d o v er tim e a t 5 m in
th e w est lobe a n d a t 13 m in each lobe. However, th e se tre n d s
88
NH4 + U p t a k e (h X) x 10
0.0
1.0
2.0
NH4
3.0
0.0
Dec 19 19 92
\w est lobe
U p t a k e (h ! ) x 10
1.0
2.0
3.0
Dec 12 1992
ea st lobe
25
3 0 o35 o-
1.0
2.0
0
^
CO
I
X
0.0
1
NO 3
NO2
3.0
0. 0
U p t a k e (h
1.0
2.0
3.0
Dec 2 2 1 9 9 2
5 o-
i o i-
e a s t lobe
15
I 20
si
a 25
0)
Q
30 635 o40
Figure 12 . Uptake of dissolved inorganic nitrogen throughout the
water column, 1992 season. Error bars indicate range of uptake
measurements at each depth. Note some depths are described by a
single datum point.
89
NOg U p t a k e (h *) x 10 3
NH
U p t a k e (h
) x 10
0,0 0 .2 0 .4 0.6 0.8
0 .0 0.2 0 . 4 0.6 0.8
1.0
—i.
0 .0 0.2 0 .4 0.6 0.8
1.0
-3
1.0
Nov 14 1 9 9 3 e a s t lob e
25 030 o35 o
Figure 12, cont. Uptake of dissolved inorganic nitrogen throughout
the water column, 1993 season. Error bars indicate range of uptake
measurements at each depth.Note some depths are described by a
single datum point.
90
fH4
U p t a k e (h
) x 10
NOg
0 .0 0.2 0 .4 0 .6 0 .8 1.0
0
5
I0
?
U p t a k e (h 1J x 10
0 .0 0.2 0 .4 0 .6 0 .8 1.0
Nov 19 1 9 9 3 _
w est lobe
Nov 19 1 9 9 3 w est lobe
15
5 20
a
Q)
Q 25
Q 25
30
30 o
35
40
NO2
U p t a k e (h
) x 10
0.0 0 .2 0 .4 0 .6 0 .8 I .0
0
5
Nov 2 4 19 93
w e st lobe
10
?
15
5
20
O h
(LI
Q 25
30
35
40
Figure 12 , cont. Uptake of dissolved inorganic nitrogen throughout
the water column, 1993 season. Error bars indicate range of uptake
measurements at each depth. Note some depths are described by a
single datum point.
91
sh o u ld b e in te rp re te d w ith caution. In each o f th e 5 m w est lobe
a n d 13 m w est lo b e tim e course experim ents, u p tak e ra te s quickly
d ec lin e d fro m m axim al to below detection, hi each case, th e -^N
ato m -p e rc en t o f p articu lates actually d ecreased d u rin g th e
incubatio n , a p h en o m en o n w hich has n o sim ple biochem ical
explanatio n . As fo r 13 m east lobe, th e decline in th e r a te of NO3"
u p ta k e o v er tim e ap p e ars to be significant.
NO3™ u p ta k e a t 5 m ea st
lo b e d isp lay ed a slow resp o n se to n u trie n t ad d itio n as u p tak e
in c re a se d w ith tim e.
G oldm an a n d G hbert (1982) n o te th a t su b strate d ep letio n can
le a d to u n d erestim a tes o f Vm ax m easu red over a tim e p erio d g re a te r
th a n surge, p a rtic u larly if th e pool size is small a n d in itial u p tak e is
ra p id . Because tim e course experim ents suggested in itial ra p id
u p tak e o f NH4+ a t 5 m i n b o th lobes, w here [NH4+] is generally less
th a n I jiM, th e re a p p e a re d to be p o ten tial fo r su b stra te d epletion.
D isregarding 5 m in u p tak e rates, th e m axim um NH4+ u p ta k e ra te
su stain a b le o n a n h o u rly scale a t 5 m was 0.003 |xM h~l, m e asu red
a fte r 5 h o f in cu b atio n (east lobe tim e course, see below ). If this
ra te re m a in e d co n stan t, it w ould take approxim ately 85 h to ex h au st
a n in itial NH4+ pool o f 0.50 \M. Because the u p tak e experim ents
re p o rte d h e re w ere in c u b ate d fo r approxim ately 24 h a n d a h in itial
NH4 + pools exceed 0.50 ^M, u p tak e ra te s re p o rte d h e re w ere n o t
co rre c te d fo r su b strate depletion.
92
30.0
5 m
Jan
13 m
e a s t lobe
8
1993
Jan
e a st lobe
8
1993
NHil
U p t a k e (h
20.0
5 . 0 [J=&=-Q— 1D
10
D1
O
20
25
15
0. 0 B30
T im e (h)
T im e (h)
17 m e a s t l o b e
Jan 8 1993
T im e (h)
Figure 13. Time course of NH4 + uptake (h"1), 1992 season.
93
5 m
e a s t lobe
Oct 25
13 m
1993
e a st lobe
Oct 25
Time (h)
1993
Tim e (h)
17 m
Oct 25
e a st lobe
1993
Time (h)
Figure 13, c o n t . T im e course o f NH4 + uptake, e a st lo b e 1 9 9 3 . U ptake
at
5
m a n d 13 m w as m easu red for b o th h igh a n d lo w e n r ich m en ts.
UPTAKE (h
94
O
5
1 0 1 5
20
25
30
TIME ( h
TIME ( h _ 1 )
l)
20.0
17 m e a s t l o b e
Oct 27
1993
■£>—ei—e10
15
20
TIME ( h
25
^
Figure 13, continued . Time course of NOg" uptake, east lobe 1993.
95
CO
x
25
Nov 5 1993
5 m w e s t lob-e
-Q
>
18
10
rg -^ -B
15 20
Time (h)
CO
Nov 5 1993
13 m w e s t lobe-
10
15
20
Time (h)
Figure 13, cont . Time course of NH4 + uptake, west lobe 1993.
Uptake at 5 m and was measured for both high and low enrichments.
96
CO
1O
0.6
x
0.5
7
I
5
0,
=
0,
CO
§
l
l
Nov 7 1993
5 m
0 4
M 0.3
l
-
w e s t lobe_
-A
/ L x ..;
0 .0 (
C)
5
10
15
T im e
(h)
20
25
CO
X 0.5
Nov 7 1993 13 m w e s t l o b e
H 0.3
%
0.0
T im e (h)
Figure 13, cont . Time course of NO3 " uptake, west lobe 1993
97
DISCUSSION
Regeneration
Results fro m m o st reg en eratio n experim ents w ere in co n sisten t.
How ever, th e re was a g en eral tre n d fo r increasing re g e n e ra tio n w ith
d e p th . R egeneration a p p e a re d to b e low est a t 5 m w est lobe.
Interestin g ly , th e p lan k to n ic com m unity a t this d e p th d isp lay ed th e
low est Ks a n d th e h ig h est Vm ax fo r NH4+ u p tak e (see P art II, Results,
S ub strate Kinetics).
Increases in reg en e ra tio n w ith d e p th could re s u lt fro m a
n u m b e r of factors including decreased C:N ratios of organic
su b stra te s a n d in c re ase d b acterial activity w ith d ep th .
U nfortunately, th e dynam ics of of DON a n d DOC in Lake Bonney h av e
y e t to b e fully exam ined.
Results fro m th e NH4+ p ro d u ctio n from serine ex perim ent
in d ic a te th a t free am ino acids below 13 m in th e e a st lo b e w ere
quickly consum ed, p resu m ab ly as a carb o n source, w hich results in
th e p ro d u ctio n of NHq+. The fact th a t rem oval of p h y to p lan k to n h a d
n o d iscern ib le effect o n reg en eratio n provides fu rth e r evidence th a t
b acterial d eam in atio n of DON is th e p rim ary source o f reg en e ra ted
NHq+. F u rth er investigation of the dynam ics of DON a n d DOC is
n e e d e d b efo re th e exact role of dissolved organic su b strates o n
re g e n e ra tio n can be determ ined.
98
S u b strate Kinetics
S u b strate kinetics d a ta show th a t u p tak e of DIN in a m ajo rity
o f experim en ts was su b strate satu rated . Only u p tak e o f NH4+ in th e
u p p e r (5 m i n each lobe) an d m iddle (13 m in each lobe) trophogenic
zones was significantly en h an ced b y in creased co n cen tratio n s o f
su b stra te. L aboratory-studies w ith cu ltu re d m icroalgae in stead y
sta te grow th in d icate th a t th e u p tak e o f NO3" is o ften s a tu ra te d b y
NOg" co n cen tratio n s o f approxim ately 5 pM (Dugdale a n d W ilkerspn
1992). T his is co n sisten t w ith th e finding th a t NOg" u p ta k e in Lake
Bonney, w h ere surface NOg- concen tratio n s typically exceed 5 yM,
c a n n o t b e d escrib ed b y M ichaelis-M enten kinetics.
Vm ax o f DIN u p tak e is a n in d icato r of th e physiological state of
th e cell; it increases w ith increasing n itro g en deficiency, given
co n stan t te m p eratu re a n d light (G oldm an an d G hbert 1982, H arrison
e t al. 1989). Vm ax was h ig h est a t 5 m w est lobe. Vm ax o f NIfy+
u p ta k e d id n o t change fro m 5 m east lobe to 13 m ea st lobe,
indicatin g th a t, d espite its proxim ity to th e chem ocline, th e
p la n k to n ic’com m unity a t 13 m east lobe was n o t significantly m o re
n itro g e n rep le te th a n th e 5 m east lobe com m unity. This a p p a re n t
co n tra d ic tio n can b e p artially explained b y th e relativ ely low affinity
(large Ks) fo r NH^+ a t 13 m in th e east lobe, w hich is discussed in
d e ta il below.
. 99
Vm ax as re p o rte d in this stu d y does n o t re p re s e n t th e ab so lu te
m axim um physiological po ten tial fo r DIN uptake^ Tim e course o f DIN
u p ta k e ex p erim en ts show ed th at, in m o st cases, specific u p ta k e o f
DIN a t 24 h is low er th a n specific u p tak e a t 5 m in. Also, b ecause
inorganic n itro g en u p tak e can be lim ited in a low irra d ian c e
e n v iro n m e n t (see Priscu 1989, Priscu e t al. 1991, P art III o f this
re p o rt), Vm ax a t in situ irrad ian ce m ay n o t re p re se n t th e ab so lu te
m ax im u m u p ta k e rate .
Ks is a n in d icato r o f th e n u tritio n al status of a p h y to p la n k to n
com m unity. P hytoplankton in n itro g en rep lete en v iro n m en ts often
exhibit h ig h e r Ks values th a n identical species growing in n itro g en
d e p le te d en v iro n m en ts (Owens a n d Esaias, 1976). Since Ks is
in d e p e n d e n t of CHL con cen tratio n a n d detritus, it is a sensitive
m e asu re of a com m unity's response to DIN (M urphy, 1980). Ks fo r
NH4+ u p ta k e is low est a t 5 m w est lobe. As p rev io u sly m en tio n ed ,
th e low est m e asu red NH4+ reg en eratio n rates a n d th e h ig h est Vm ax
of NH4+ u p tak e also occu rred a t this d epth. Elevated NH4+ up tak e
ra te s in resp o n se to increases in NH4+ con cen tratio n a n d h ig h affinity
fo r NH4+ m a y th erefo re allow p h y to p lan k to n a t 5 m w est lobe to
co m pensate fo r low er NH4+ supply rates.
T he d ecrease in affinity fo r NH4+ w ith d e p th in each lobe h as
two p o te n tia l explanations. Firstly, th e h igh co n cen tratio n s of
ch lo rid e salts (p articu larly NaCl) in th e deep w aters o f Lake Bonney
co u ld d ecrease th e stability of glutam ine synthetase, a n enzym e
too
n ecessary fo r assim ilation of inorganic nitrogen. T he in te rferen c e of
salin ity w ith g lu tam in e sy n th etase h as b ee n show n to d ecrease th e
a ffin ity fo r NH4+ in halophytic angiosperm s (A hm ad e t al. 1982).
Secondly, h ig h am b ien t concentrations of DIN could allow
p h y to p la n k to n in th e d eep w aters o f Lake Bonney to satisfy
n u tritio n a l req u irem e n ts w ithout h igh affinity fo r n u trie n ts. Owing
to in creases in reg en e ra tio n rates w ith d e p th a n d u p w a rd diffusive
flux of n u trie n ts fro m th e chem ocline, th e supply ra te s o f DIN are
g re a te r a t 13 m th a n a t 5 m. ..
C om parisons of diffusion rates a n d in situ u p tak e rate s (Table
13 ) in d icate th a t reg en eratio n is th e p red o m in a n t source o f NH4"1" a t
5 m a n d 13 m east lobe a n d occasionally a t 13 m w est lobe. In
co n trast, "regenerated" NOg", p ro d u ced b y nitrification, is ap p a re n tly
n o t a significant source o f DIN a t these depths.
W ater Colum n U ptake
U ptake o f NH4+ exceeded u p tak e of NO2" th ro u g h o u t th e w ater
colum n of each lobe during each season. Uptake o f NHq+ also
g en erally exceeded NOg" uptake, alth o u g h h ig h er ra te s of NOgu p ta k e w ere m e a su re d a t 25 a n d 35 m in th e east lo b e (1993). In
p lan k to n ic com m unities w ith actively photosynthesizing
p h y to p la n k to n (5 m to 17 m i n each lobe), th e ratio o f m easu red
NHq+ u p ta k e ( I r 1) to to tal DIN u p tak e ( I r 1) ra n g e d fro m 0.50 (5 m
w est lo b e 1993) to 0.80 (5 m east lobe). At th e tim e o f th e 1993
w est lo b e NHq+ u p tak e experim ent, NHq+ was u n d etectab le a t 5 a n d
101
13 m ; th e re fo re in situ NH4"1" up tak e rate s a t th ese d e p th s m ig h t h av e
b e e n insignificant. In contrast, in situ u p tak e of NH4"1" a t 5 a n d 13 m
e a st lobe 1993 (w here NH4+ co n cen tratio n was 0.68 a n d 1.49 nM
respectively) exceeded th e com bined u p tak e o f NO2" a n d N O 3".
A lthough Lake Bonney p h y to p lan k to n p referen tially u tilized NH4+
w h en it w as available, periodic scarcity o f NH4+ in c re ase d th e
re la tiv e im p o rta n ce o f NOg- a n d NO2" u p tak e in th e shallow w aters
o f th e w est lobe.
T he resu lts of th is stu d y are co n sisten t w ith p rev io u s studies
th a t in d icated a strong preference fo r NH4+ b y p h y to p lan k to n in
p e re n n ia lly ice-covered A ntarctic lakes (Priscu e t al. 1989) a n d in
off-shore A ntarctic w aters (Koike e t al. 1986). U ptake ra te s o f NH4+
p ro v id e in d ire c t evidence th a t reg en eratio n in large p a r t su p p o rts
p h y to p la n k to n p ro te in synthesis, a n d ultim ately p ro d u ctiv ity ,
th ro u g h o u t th e trophogenic zone (Goldm an an d G ilbert 1982, Priscu
e t al. 1989).
P h y to p lan k to n p ro te in synthesis a t 5 m w est lobe, w here NH4+
re g e n e ra tio n is a p p a re n tly lowest, m ay occasionally b e p rim arily
s u p p o rte d b y NO2- a n d NOg- uptak e. It is u n clear if th e relativ ely
low su p p ly ra te s o f NH4+ tran slate in to low er grow th ra te s fo r this
com m un ity . Levasseur e t al. (1993) estim ated th a t m a rin e algal
assim ilation o f n itro g en a n d carb o n a t subsaturating irrad ian ce
102
re q u ire s 50% m o re en erg y w hen th e algae are grow n o n NO3"
co m p ared to NHq+. Interestingly, d esp ite the u n fav o rab le energetics
o f NO3" assim ilation, grow th rates o f m arin e algae a t su b satu ratin g
irra d ia n c e w ere sim ilar w hen grown o n NO3" o r NHq+ (Levasseur
1993). A lthough som e investigators re p o rt declining grow th rate s
associated w ith NO3" assim ilation (Rhee a n d L ederm an, 1983),
p h y to p la n k to n a re a p p a re n tly capable o f growing o n NO3" th ro u g h
physiological ad ap tatio n s o th e r th a n red u ctio n o f grow th rates, h i
conclusion, p h y to p la n k to n grow th rate s in th e shallow w aters o f th e
w est lo b e a re n o t necessarily im p aired b y th e low in situ u p tak e
rate s of NHq+ .
T h ere w ere g en erally two peaks of NHq+ u p ta k e in th e w ater
colum n of each lobe: a t 5 in a n d w ithin th e first chem ocline
(betw een 10 a n d 17 in). Peaks of NHq+ u p tak e w ithin th e chem ocline
c o rre sp o n d ro u g h ly w ith peaks of CHL a n d p rim a ry p ro d u ctiv ity (see
Figure 2).
In co n trast, m axim um levels o f NO3- a n d NO2" u p ta k e o cc u rred
a t 5 m (w ith th e exception of e ast lobe NO3" u p tak e 1993), w here
NHq+ co n c e n tra tio n was lowest. It is well estab lish ed th a t th e u p ta k e
o f NO3", th e n u trie n t m ost associated w ith "new" p ro d u ctio n , is
p artia lly reg u la ted b y th e ab u n d an ce of NHq+. However, th e
sim ultan eo u s u p tak e o f d ifferen t form s of n itro g en p o se difficulties
103
in calculation of uptake. Collos (1987) suggests th a t re p o rts o f NH4+
re p re ssio n o f NO3" u p tak e m ay be artefacts of calculation.
Specifically, in experim ents w here m o re th a n o n e n itro g en o u s
su b stra te is p re se n t a n d p articu late n itro g en changes significantly
ov er th e course of incubation, dilution o f the p articu late
label
could re su lt fro m th e u p tak e of u n lab eled n itro g en (Collos 1987). The
tru e rela tio n sh ip betw een NH4+ a n d NO3" u p tak e co uld b est be
o b se rv e d in experim ents w here p articu late n itro g en does n o t change
significantly d uring th e incubation.
D ata fro m th e 1993 w ater colum n u p tak e in d ic ate com bined
u p ta k e of NH4+, NO3", NO2" at 5 m in th e east lobe («0.0025 ixM h -1)
w ould le a d to a 0.07% increase in PN (3.77
a t tim e zero) o ver a 24
h incu b atio n . For com parison, th e increase in PN a t 17 m fo r th e sam e
ex p erim en t w ould b e approxim ately 0.20%. T herefore, calculation o f
DIN u p ta k e sh o u ld n o t b e affected b y changes in PN o v er 24 h of
incubation.
Tim e Course
Some rese arch ers have suggested th a t th e p o te n tia l fo r sh o rt
te rm elev ated ("surge") up tak e rate s of NH4+ b y p h y to p la n k to n could
b e a physiological ad ap tatio n to m icropatches of NH4+ p ro d u ced b y
zooplankton excretion (McCarthy a n d Goldm an 1979, G ilbert an d
G oldm an 1981, G oldm an an d G ilbert 1982, Priscu 1987). A lthough
th e d istrib u tio n o f n u trie n ts in Lake Bonney on a m icroscale basis
h a s n o t b e e n d eterm in ed , th e absence of zooplankton sh o u ld
104
d ecrease th e p o ten tial fo r significant m icropatches. Rapid sh o rt te rm
u p ta k e o f N H zf1" in Lake Bonney could th erefo re in d icate th a t this
stra te g y m a y h av e com petitive value b ey o n d utilizatio n of
m icro p atch es (See C urrie 1984). Rapid initial u p tak e o f N O g ", a
n u trie n t w hich is o ften assum ed to n o t p ersist in m icro p atch es
(D ortch e t al. 1982, Priscu e t al. 1989), provides fu rth e r evidence
th a t ra p id u p tak e o f DIN m ay n o t necessarily be a resp o n se to a
p a tc h y en v iro n m en t.
A n a ltern ativ e explanation is th a t h y d rau lic stab ility allows fo r
p ersisten ce o f m icropatches of DIN p ro d u ced b y m icrobial excretion.
A lthough n u trie n t m icropatches are generally associated w ith
zooplank to n excretion a n d n o t bacterial rem in eralizatio n (M cCarthy
a n d G oldm an 1979, G oldm an an d G ilbert 1982, Suttle a n d H arrison
1988), NH4+ re g e n e ra te d b y m icroplankton in Lake Bonney m ig h t
p e rsist lo n g er th a n in o th er, m ore tu rb u len t, system s (see C urrie
1984, G oldm an a n d GUbert 1982, Priscu et al. 1989). T hese p atches
could, in tu rn , be co n v erted to NO2" a n d NOg- b y n itrificatio n (Priscu
e t al. 1989). If m icroscale patch es of DIN do p ersist in Lake Bonney,
sh o rt te rm ra p id u p tak e of DIN b y th e planktonic com m unity m ay b e
a n a d a p ta tio n fo r u tilization of m icropatches.
R apid N H zfi" u p tak e is generaU y associated w ith system s w h ere
p ro d u ctiv ity is Umited b y n itro g en (GUbert a n d G oldm an 1981,
D ortch e t al. 1982, Suttle an d H arrison 1988), b u t som e
p h y to p la n k to n a re capable of ra p id NHzt+ uptake ev en w hen grow th
ra te s a re n e a r m axim al (G oldm an a n d GUbert 1982). S hort-term
105
ra p id u p tak e of N O g' is also a n am biguous in d icato r o f n itro g en
deficiency (Priscu 1987). D ortch et al. (1982) re p o rte d th a t several
species o f m a rin e p h y to p lan k to n exhibited a d ecrease in NO3" u p tak e
cap ab ility following n itro g en deprivation. Conversely, Priscu (1987)
m e a su re d ra p id sh o rt te rm NO3" u p tak e b y p h y to p la n k to n in a
n itro g e n d eficient lake.
U ptake o f N 03"-in Lake Bonney was ex trem ely ra p id a fte r 5
m in a t 5 m ea st lobe, 17 m east lobe a n d 13 m w est lo b e relativ e to
ra te s a t I h . T he 15N ato m -p ercen t o f p articu lates a fte r 5 m in fo r 17
m e a st lo b e N O 3' u p ta k e is w ithin I sta n d a rd d ev iatio n o f th e
estim ate fo r abiotic 15N ato m -p ercen t en rich m en t o f th e filter;
e lev ated NO3" u p ta k e rate s a t tim e zero a t 17 m ea st lo b e m ay
th e re fo re be a n experim ental artefact. F u rth er in v estig atio n o f
p ersisten ce of m icropatches o f NO3” in a n o n -tu rb u le n t e n v iro n m e n t
is n ecessary to in te rp re t th e ecological significance o f ra p id sh o rt
te rm u p ta k e o f NO3".
106
CONCLUSION AND AREAS OF FURTHER RESEARCH
T he p lan k to n ic com m unities o f Lake Bonney show a strong
p referen ce fo r NH4+ as a source of inorganic nitrogen. It can
th e re fo re b e concluded th a t reg en eratio n of NH4+ p rim arily fuels
productivity, a conclusion su p p o rted b y com parisons o f NH4+
diffusion ra te s a n d u p tak e rates in th e trophogenic zone. In areas
w h ere re g e n e ra tio n is low, e.g. 5 m w est lobe, p h y to p la n k to n h av e
ad a p ted /acc lim ate d to maximize utilization of NELj+ th ro u g h high
affin ity fo r NH4+, surge uptake of NILf1" pulses, a n d h ig h sustained
NILj.+ u p tak e rates.
It is ev id en t th a t NH4+ reg en eratio n is a n im p o rta n t ecological
pro cess in Lake Bonney, b u t th e rate s a n d sources h av e n o t b ee n
a d e q u a te ly d e te rm in e d b y this study. Any fu rth e r in v estig atio n in to
n itro g e n tran sfo rm atio n s in th e w ater colum n of Lake Bonney sh o u ld
in c lu d e a n a tte m p t to clarify this issue.
A lthough u p ta k e rate s of NH4+ w ere n o t su b stra te sa tu ra te d in
th e shallow w ater of e ith e r lobe, u p tak e kinetics o f NO3" a n d NO2"
in d ic ate th a t p ro d u ctiv ity is generally n o t n itro g en lim ited. The
a p p a re n t co n trad ictio n betw een this conclusion a n d p articu late C:N
ratios, w hich in d icate n itro g en deficiency, m a y b e p artia lly explained
th ro u g h analysis o f th e control o f irrad ian ce o n n u trie n t u p tak e (See
P a rt III). It is possible th a t th e u p ta k e resp o n se o f in o rg an ic c a rb o n
107
a n d inorganic n itro g en to irrad ian ce could create diel a n d seasonal
p erio d ic ity of C:N u p tak e ratio s even w hen DIN is available in
satu ratin g am ounts.
Several prev io u s research ers h a d concluded th a t p ro d u ctiv ity
a n d p h y to p la n k to n grow th d u rin g th e la tte r p a rt o f th e su m m er was
lim ite d b y n u trie n t su p p ly in Lake B onney (Iizo tte a n d Priscu 1993,
S harp 1993). If p h y to p lan k to n are in d e e d n o t lim ited b y n itro g e n
supply, it is likely th a t p h o sp h o ru s lim its productivity.
T ransform atio n s of p h o sp h o ru s should b e studied fo r a th o ro u g h
u n d e rsta n d in g of controls of p ro d u ctiv ity in Lake B onney.
PART III
Irradiance Requirements of DIN Uptake
109
INTRODUCTION
P hy to p lan k to n cellular grow th rate, a n d th u s p rim a ry
p ro d u ctiv ity , is a fu n ctio n of light, tem p eratu re, a n d n u trie n t supply
(Hecky a n d K ilham 1988). These p aram eters are n o t en tirely
separable. For exam ple, p h y to p lan k to n n itro g en assim ilation into
p ro te in in n itro g e n rep le te system s utilizes th e en e rg y a n d th e
c a rb o n skeletons p ro d u ced b y photosynthesis a n d h a s b e e n show n to
b e d irec tly co rrelated w ith irrad ian ce (T urpin 1991, K anda e t al.
1989). U ptake o f inorganic n itro g en (in this study, "uptake" refers to
tra n s p o rt p lu s assim ilation) is also o ften strongly c o rre la te d w ith
irra d ian c e, alth o u g h th e m echanism fo r this rem ains u n clear (Piiscu
1989, COchlan et al. 1990).
T he ratio o f u p tak e rates of carb o n a n d n itro g e n h av e
fre q u e n tly b e e n u sed as a m easu re o f n u tritio n a l statu s of
p h y to p la n k to n (DiTulho a n d Laws 1986, Priscu e t al 1989).
However, n itro g en u p tak e a n d carb o n u p tak e can b e d ifferentially
d e p e n d e n t o n irrad ian ce. Photosynthesis is a first o rd e r process w ith
re sp e c t to irrad ian ce; n itro g en up tak e is a second o rd e r process th a t
utilizes p h o to ch em ical en erg y as well as en erg y d e riv e d from
in te rm e d ia ry m etabolism (Priscu e t al. 1991, M iyazaki e t al. 1987).
As a resu lt, n itro g e n u p tak e v ersus irra d ian c e (NI) curv es can d iffer
significantly fro m ph o to sy n th esis v ersu s irra d ian c e (PI) curves in th e
sam e com m unities. Specifically, inorganic n itro g en u p tak e in th e
d a rk can b e significant, w hereas biosynthetic d ark in o rg an ic carb o n
u p ta k e is negligible fo r m o st p h y to p lan k to n . Piiscu e t al. (1991)
d e m o n stra te d th a t dissim ilarities in th e response o f c a rb o n a n d
110
n itro g e n u p tak e to irrad ian ce can lead to strong diel p erio d icity o f
c a rb o n a n d n itro g en up tak e ratios (see also M iyazaki e t al. 1987).
Thus, th e effects o f irra d ian c e on n itro g en u p tak e m u st b e know n fo r
d e te rm in a tio n o f cellular carbon a n d n itro g en u p tak e ratios, and,
u ltim ately , p h y to p la n k to n growth.
A lthough n itro g en a n d carbon u p tak e do n o t resp o n d
id en tically to irrad ian ce, the response of each process to irrad ian ce
ca n p ro v id e a n in d icatio n of ad ap tatio n (long-term ev o lu tio n ary
response) a n d acclim ation (sh o rt-term physiological response) to
a m b ie n t irra d ian c e.
Both NI a n d PI curves p ro v id e in sig h t in to
aspects o f p h y to p la n k to n physiology including th e efficiency o f lig h t
utilizatio n , th e ability to quickly m axim ize u p tak e u n d e r optim al
conditions, a n d th e ability to sustain u p tak e u n d e r lim iting
conditions.
C ontroversy cu rren tly exists reg ard in g th e g en eral effect of
irra d ia n c e o n n itro g en u p tak e in p o la r aquatic system s. Previous
studies in d icate n itro g en u p tak e is generally less d e p e n d e n t on
irra d ia n c e in p o la r system s th a n in tem p erate o r tro p ical system s
(Sm ith a n d H arrison 1991). However, Muggli a n d Sm ith (1993)
n o te d th a t irra d ian c e was th e m ost im p o rta n t facto r regulating NH4+
u p ta k e in th e G reen lan d Sea. In addition, K anda e t al. (1989)
re p o rte d a strong relatio n sh ip betw een p h o tosynthesis a n d NO3"
u p ta k e in A uk Bay, Alaska. A pparently, th e d eg ree to w hich
n itro g e n u p tak e d ep en d s on irrad ian ce is a function o f n u trie n t
supplies, th e stability Of th e light field, a n d th e physiological state o f
th e p h y to p la n k to n (Sm ith a n d H arrison 1991, K anda e t al. 1989).
111
This stu d y investigates inorganic n itro g en u p tak e in resp o n se
to irra d ia n c e b y planktonic com m unities in stable, low lig h t
en v iro n m en ts. It consists of m easu rem en ts o f e ig h t d iffe re n t
p lan k to n ic com m unities (sam pled a t 4 d ep th s in each lo b e o f Lake
Bonney) o v er two years. Lake B onney's g rad ien ts o f n u trie n ts,
irra d ian c e, a n d p h y to p lan k to n will allow fo r co m parison o f NI
resp o n se s in m a rk e d ly d ifferen t enviro n m en ts.
V ariability o f NI
curves will clarify th e in teractiv e roles o f n u trie n t su p p ly a n d
irra d ia n c e in reg u latio n of inorganic n itro g en uptake.
112
H ypotheses
This study investigated the following hypotheses:
1.
Nitrogen uptake is extremely "shade adapted," Le., quickly
saturated with respect to irradiance, particularly in nitrogen
depleted surface waters.
2.
Photoinhibition of nitrogen uptake, when present,
occurs at an
irradiance substantially above ambient levels.
3.
Compared to NH4+ uptake, NO3" uptake is more dependent on
irradiance, i.e. uptake of NO3" in the dark will be a lower proportion
of maximum uptake.
113
METHODS
E xperim ental P rocedure
As w ith su b stra te kinetics, w ater fo r th e n itro g e n u p ta k e
v ersu s irra d ia n c e experim ents was collected in a 20 lite r carb o y a n d
tra n sfe rre d to a series o f I liter p o ly carb o n ate bottles. A dditional
w ater was rese rv e d fo r analysis of DIN, CHN, a n d CHL. The
p ro c e d u re fo r collection a n d m easu rem en t of DIN, CHN, a n d CHL was
id e n tic al to m eth o d s d escrib ed in P art I. Each b o ttle in a given
ex p erim en t received identical enrichm ents of ^ N H ^C l, N a^ N O g , o r
N a^^N 0 2 . U ptake of NO2" was n o t analyzed a t 20 m w est lobe, 25 m
e a st lobe, o r 25 m w est lobe d u rin g th e 1992 season. U ptake of NO3"
w as n o t an aly zed a t 25 m w est lobe d u rin g th e 1992 season.
A fter ino cu latio n , bottles w ere placed in sleeves o f n e u tra l
d e n sity screening. V ariations in thickness a n d color (eith e r black,
g ray o r w hite) of th e sleeves crea ted a ran g e of lig h t tran sm ittan ce.
T h ree b o ttles fo r each ex p erim en t w ere n o t placed in sleeves: o n e
clear b o ttle, one d a rk b o ttle (w rapped in alum inum foil a n d several
lay ers o f d u c t tap e), a n d th e kill bottle.
T he b o ttles w ere in cu b ated in a te m p eratu re a n d light
co n tro lled in c u b ato r fo r approxim ately 24 h. C alibration tests w ith a
Li-Cor 4jt q u a n tu m sen so r a tta c h e d to a Li-Cor m o d el LI-1000 d a ta
logger in d ic ated th a t irrad ian ce was n o t consistent th ro u g h o u t th e
in c u b ato r. Irrad ia n ce m easurem ents have b een ad ju ste d fo r each
b o ttle's p o sitio n w ithin th e incu b ato r.
Irradiance th ro u g h o u t th e
114
in c u b a tio n p e rio d was reco rd ed b y a Ii-C or 4% q u a n tu m sen so r
a tta c h e d to a Ii-C or m odel Ii-IOOO d a ta logger. W ide flu ctu atio n s in
irra d ia n c e o ccu rred d uring 1992 incubations owing to fluctuations in
o u r field g e n e ra to r’s pow er output. For 1992 experim ents, n itro g en
u p ta k e is co m p ared to th e average irrad ian ce d u rin g th e incubation.
A n u n in te rru p te d pow er supply u n it elim in ated this p ro b le m fo r
1993 exp erim en ts.
Sam ples w ere in c u b ate d fo r ap p roxim ately 24 h th e n filtered
o n to p reco m b u sted 47 m m W hatm an GF/F glass fib er filters u n d e r
low v ac u u m (<0.6 atm ). A fter filtratio n , filters w ere rin se d w ith
ap p ro x im ately 20 m l DIW to rem ove inorganic n itro g en . Filters w ere
a ir d rie d in alu m in u m w eigh-boats fo r several days b efo re being
tra n s p o rte d to C rary L aboratory w here filters w ere fro z en (-45° C)
b efo re tra n s p o rt to MSU fo r final analysis.
Calculations
U ptake rate s w ere calculated as d escrib ed in P art I. D ata w hich
ap p ro x im ated a hyperbolic curve w ith n o evidence o f p h o to in h ib itio n
w ere fitte d to th e following eq u atio n (Platt e t al. 1980) :
115
( 12)
cd
V = Vjyi * tcUlb-C Vjjj ) + D
W h ere
V= specific u p tak e ra te (h"l)
a = th e slope of th e light lim ited p o rtio n o f th e curve
( h '1 (nm ole-1 m -2 s-1 )-1 )
I = Irra d ia n c e (nm ole m -^ 1S"1)
D = specific u p tak e of th e su b stra te (h -1) w h en I=O
Vii1 = ra te of lig h t sa tu ra te d u p tak e (h-1). Note th a t D,
w h en p resen t, m u st b e a d d e d to th is term .
D ata th a t exhibited a hyperbolic increase coupled w ith
p h o to in h ib itio n w ere m o deled w ith th e following E quation (Platt et
al. 1980):
V = Vs ( I - e -a ) e-b + D
w h e re
(!3 )
Vs = m axim um DIN u p tak e (h-1) in th e ab sen ce of
photoinhibition.
a = cd
Vs
b= J L ,
Vs
w here p = th e slope of th e p h o to in h ib ite d
p o rtio n of th e curve
Vjj1 o f ex p erim en ts d escrib ed b y E quation 13 was d e te rm in e d w ith
th e following equation: -
Vju = Vs *( a / a + P)*(p/a + P ^ a
W here all te rm s a re d efin ed as in Equations 12 a n d 13.
L inear fits w ere ap p lied to d a ta th a t d id n o t conform to e ith e r
( 1^
116
E quation 12 o r 13. Vm of lin e ar m odels was d e te rm in e d to be th e
m ax im u m y-value o f th e regression w ithin th e ex p erim en tal ra n g e of
irrad ian ce.
117
RESULTS
NH4+ u p tak e a t 5 m an d 13 m in b o th lobes could alm ost
alw ays b e d escrib ed b y e ith e r Equations 12 o r 13 (Figure 14). The
excep tio n was th e 5 m ea st lobe sam ple (1992) w h ere u p tak e
in c re ase d approxim ately linearly w ith irrad ian ce. NH4"1" a n d NO3™
u p ta k e a t 17 m east lobe (1993) could b e m o d eled b y E quation 12.
NO2" u p ta k e a t 5 m east lobe (1993) a n d 5 m w est lo b e (1993) co uld
also b e ap p ro x im ated b y Equation 12, b u t th e d a ta a re relativ ely
scattered , possibly d u e to th e e rro r in h e re n t in m e asu rem e n t o f such
low u p ta k e rate s. All o th e r d a ta sets w ere fit lin e a rly w ith a least
sq u ares regression. R eported sta n d a rd deviations fo r all d a ta sets
a re eq u iv alen t to th e sta n d a rd e rro r o f th e estim ate as d e te rm in e d
b y M a rq u a rd t’s algorithm .
Results fro m b o th seasons in d icate a strong positive co rrelatio n
b etw een NOg" a n d NO2" uptake a n d irrad ian ce a t 5 m in b o th lobes.
W eaker positive co rrelatio n s betw een NO2" u p tak e a n d irra d ian c e
o c c u rre d a t 17 m east lobe (b o th seasons). NO3" u p ta k e was w eakly
c o rre la te d w ith irra d ia n c e a t 13 m ea st lobe (1992), 13 m w est lobe
(b o th seasons), a n d 20 m w est lobe (1992). Slopes o f lin ear
reg ressio n s o f NHq+ u p tak e w ere significantly positive a t 5 m east
lobe (1992), 20 m w est lobe (1992) a n d 25 m w est lo b e (1993).
O nly o ne ex p erim en t could be m o deled w ith a significantly negative
slope : NHq+ 25 m east lobe (1993).
118
5. 0
4. 0
Dec 5 1 9 9 2
13 m e a s t l o b e -
3. 0
'/
Dec 10 1 9 9 2
13 m w e s t l o b e
2.0
1.0
0.0
10
0
20
10
30
40
50
I r r a d i a h c e (yu.mol m 2 s 1J
I r r a d i a n c e (yumol m
5. 0
■
4. 0
Nov 29
20
1992
I
'
I
'
I
'
I
'
Dec I 1992
5 m w e st lobe
/
3. 0
5 m e a s t lobe
2. 0 b
CL.
I .0
-C l
n
^
O
' f
0.0
0
Irradiance(yLtm ol m
10
20
30
I r r a d i a n c e (/Ltmol m
40
50
s )
Figure 14 . NH4"1" u p tak e vs irradiance,1992. D ata fro m 13 m east
lobe fitte d w ith in h ib itio n equ atio n a n d d a ta from 5 m east lobe
fitte d w ith lin e a r regression. O thers fitted w ith h y p erb o lic ta n g en t
equation. D ashed line describes function norm alized fo r chlorophyll a
co n c e n tra tio n (^iM N ng CHL I '1 h '1).
119
De c 13 1 9 9 2
De c 21 1992
20 m w e s t l o b e
2 5 m e a s t lo b e
I r r a d i a n c e (yumol m
I r r a d i a n c e (yumol m
Dec 15 1 9 9 2
2 5 m w e s t lo b e
0.4 -
I r r a d i a n c e ( / xmo l m
Figure 14, com . NH4 + uptake vs irradiance, 1992. Data from 25 m
west lobe fitted with inhibition equation, others fitted with linear
regression.
120
2 .0
D ec I 1 9 9 2
1.5
5 m west lobe
1. 0
0. 5
- V l-
0 .0
0
10
20
30
40
50
I r r a d i a n c e (/Ltmol m 2 s ^
I .0
Cl
1
'
'
'
'
'
I .0
1 "1
I
-
I
i
-
I
-
O
%
7
13 m w e s t l o b e
13 m e a s t l o b e
0. 6
De c 10 1992
0. 8
Dec 6 1 9 9 2
°-8
0. 6
.d
0. 4
% 0. 4
cd
D
0. 2
O
0. 2
---------------------cyO
C
O
o
O.o
0. 0 '»— -------1-----------------1---------------d-------'--------1--------i------]
10
20
30
40
50
-------*--------1
--------1 -n b— .------- 1--------,--------1____,____
)
10
20
30
40
I rr ad ia nc e (//mol m
Figure
- v
14, com.
50
-2
s
- 1
)
I r r a d i a n c e (yumol m
NOg' uptake vs irradiance, 1992.
—
2
Data fitted with
linear regression. Dashed line describes function normalized for
chlorophyll a concentration (pM N pg CHL L1 I r 1).
s
- 1
)
121
4.0
I
I
I
Dec 13 1 9 9 3
3.0
5 m w est lobe
2.0
1.0
r-e-e-
0.0
0
20
40
60
I r r a d a i n c e (yitmol m
80
-2
-I
s )
Dec 5 1 9 9 3
13 m e a s t l o b e
De c 8 1993
13 m w e s t l o b e
I r r a d i a n c e ( / / mo l m
I r r a d i a n c e ( / / mo l m
Figure 14, c o n t . NH4+ uptake vs irrad ian ce, 1993. D ata from 13 m
e a st lobe fitte d w ith in h ib itio n equation; o th ers fitte d w ith
hyperbolic ta n g en t equation. D ashed line describes function
n o rm alized fo r chlorophyll a con cen tratio n (^M N ng CHL I 'I h 'l ) .
122
3.0
CO
I
i
O
X
I
i
3.0
r
2.5
I
T
I
2.5
Dec I 1993
2.0
Dec 3 1993
2 .0
1 .5
17 m e a s t l o b e
1.5* L n
OJ
«d 1 .0
1 .0
■4-J
20
(?
—
m
w e s t
lobe
O
°°
-
PL,
D 0 .5
+
X 0.0
0.5
O_______ I_ _
0
20
80
0
1
60
80
s )
I r r a d i a n c e ( / / mo l m
s )
0 .0
40
60
I r r a d i a n c e (//.mol m
_______ I_______ L
20
40
1.0
Nov 29 1993
25 m e a s t l o b e
0.8
Nov 27 1993
0.6
25 m w e s t lobeCHL U n d e c t a b l e
0.4
0.2
0.0
0
I r r a d i a n c e ( / / mo l m
20
40
60
I r r a d i a i n c e ( / / mo l m
80
s ^)
Figure 14, cont. NH4+ uptake vs irradiance, 1993. D ata from 17 m
e a st lobe fitte d w ith hyperbolic ta n g en t equations. O thers fitted w ith
lin e a r regressions. D ashed line describes function n o rm alized for
chlorophy ll a co n cen tratio n (nM N ng CHL I'* h '1).
123
CO
1
2
x
0.20
0 . 1 5 -O'
N0
U p t a k e (h
^
I
0.1 0
I
0.20
I
0.05
0
O
— O-------------0 Dec 6 1993
5 m e a s t lobe_1_____ 1_____ 1_
20
40
60
0
I
I
0
s )
O
20
40
60
80
I r r a d i a n c e (//mol m
1 .0
I
1
1
s
)
I
Dec 12 1993
0.8
0.8
5 m w e st lobe
U p t a k e (h
) x 10
1.0
0
__ I_ ______ 1_______ 1__
80
0.6
I
O
,//O 5 m e a s t lobe
3 1
1
1
0.0
0
20
40
60
80
C
O
P
0.05
0.00
I r r a d i a n c e (//m ol m
I
-------------------0
()
0.00
I
Dec 12 1993
5 m w e s t lobe
Cji
0.1 0 lO
O
"
/
;
0 . 15
tf°
Cf
I
/
0.4 -
O
/'z
0.6
/O
0 .4 —
Dec 6 1993
0.2
0.2
O
d
I r r a d i a n c e (//m ol m
—
s
)
(
k
0
-------- 1-------- 1-------20
40
60
80
I r r a d i a n c e ( / /m o l m
Figure 14, c o n t . NO3" an d NO2" uptake vs irrad ian ce, 1993. D ata fo r
NO2 " uptake at 5 m in each lobe fitted with hyperbolic tangent
equation. Others fitted with linear regression. Dashed line describes
function normalized for chlorophyll a concentration (p,M N ng CHL I"I
h-1).
s
-i
)
124
4 Dec 1 9 9 3
13 m
u 7 Dec 1 9 9 3
13 m w e s t l o b e
e a s t lobe
0 . 0 Q&
I r r a d i a n c e ( / / mo l m
-2
—I
s )
I r r a d i a n c e ( / / mo l m
0 .3 0
U p t a k e (h
0.25
0.20
7 Dec 1 9 9 3
13 m w e s t lo b e
0.1 5
0. 10
NO
0.05
0 . 0 0 OO O 1
0
20
40
60
I r r a d i a n c e ( / / mo l m
80
-2
-I
s )
Figure 14, co n t. NOg" and NO2 " uptake vs irradiance, 1993. Data
fitted with linear regression. Dashed line describes function
normalized for chlorophyll a concentration (pM N pg CHL I"1 h"1).
125
CO
I
8 Dec 1993
3.2
20 m e a s t l obe
3 Dec 1993
17 m
e a s t lobe
te 1 .6
I r r a d i a n c e (/Ltmol m
O 4.0
I
0.5
I
0.4
«
4-)
a,
^
%
20 m w e s t lo b e
0 . 2
"
0.1
0 . 8
0.0
I
"
1.6 -
CM
I
m
0.3
2 .4 -
6
I
(/L tm o l
8 Dec 1993
3 Dec 1993
17 m e a s t l o b e
x 3 .2
o
I
Irradiance
C
O
20
.
.
40
60
I r r a d i a n c e (/Ltmol m
m ^ CX - Q . J 0 ____i______ j
80
—2
—1
s )
0
20
40
60
80
I r r a d i a n c e (/Ltmol m 2 s ^
Figure 14,cont. NOg" and NO]" uptake vs irradiance, 1993. Data
fitted with linear regression. Dashed line describes function
normalized for chlorophyll a concentration (nM N ^g CHL I"1 h '1).
126
OT
I
O
SX
I
1 .0
0.8
Xi
0.6
<D
/X
id
0 .4
O1
D
0.2
I
O
r
I
I
Nov 3 0 1 9 9 3
25 m e a s t lobe
^
OT
0.0
60
I r r a d i a n c e ( / / mo l m
o .i o
o
U p t a k e (h
NO
80
-2
-I
s )
I r r a d a i n c e ( / / mo l m
—|— —I
Dec 3 1 9 9 3
X 0.08
"
Nov 2 7 1993
25 m w e s t l o b e
-
_________ O
40
7
0 .8 0
Dec 2 1993
0.08
25 m e a s t l o b e
25 m w e s t l o b e
0.06
0.06
0.04
0.04
0.02
0.02 00
o.oo cfc) o d
0.00
60
I r r a d i a n c e ( / / mo l m
— 6
80
-2
-I
s )
I r r a d i a n c e ( / / mo l m
Figure 14, c o n t . NO3" a n d NO2" uptake vs irrad ian ce, 1993. D ata
fitte d w ith lin e a r regression.
127
T he slopes o f lin e a r regressions, as well as a values fro m h y p erb o lic
curves, w ere g en erally h ig h est fo r NH4+ uptake. L inear reg ressio n
slopes a n d a values te n d e d to d ecrease w ith d ep th . T he ra tio of d a rk
u p ta k e to m axim um u p tak e (D:Vm ) indicates a d istin ctio n betw een
p h y to p la n k to n com m unities.
Ik valu es of sam ples d escrib ed b y h y p erb o lic eq u a tio n s w ere
g en erally sim ilar, ran g in g from 2.72 to 3.95, w ith th re e significant
exceptions. 5 m east lobe (1993) a n d 13 m w est lo b e (1993)
d isp la y e d rela tiv ely h ig h Ik values (6.59 a n d 29.42, resp ectiv ely ). Ik
a t 17 m e a st lo b e (1993) was relativ ely low (0.44). It sh o u ld be
n o te d th a t a in this ex p erim en t was n o t significantly d iffe re n t fro m
zero (p > 0.05); Ik a t this d e p th could th erefo re b e m u c h larger.
P h o toinhibition was a p p a re n t fo r 13 m east lobe NHq+ u p tak e
d u rin g b o th seasons a n d b o th d a ta sets w ere fitted to E quation 13;
how ever, p was n o t significantly d ifferen t fro m zero (p > 0.05) in
e ith e r case, indicating th a t p h o to in h ib itio n was n o t significant.
U ptake o f NHq+ was negatively co rrelated w ith irra d ia n c e a t 25 m
e a st lo b e (1992), b u t th e slope was n o t significantly d iffe re n t fro m
zero (p > 0.05). It a p p e ars th a t u p tak e of inorganic n itro g e n b y Lake
B onney p h y to p la n k to n is n o t subject to significant p h o to in h ib itio n a t
th e irra d ian c es u se d in these experim ents w hich encom pass those
fo u n d in th e lake.
N ote th a t NI g rap h s d epict m e asu red u p tak e ra te s, th eo retical
u p ta k e functions, an d , in some cases, functions n o rm alized to th e
co n c en tratio n of CHL a t tim e zero in th e sam ple. E xperim ents in
w hich n itro g e n u p ta k e was n o t significantly c o rre la te d w ith
128
irra d ia n c e w ere n o t no rm alized to CHL P aram eters o f th e NI curves,
specific to nitro g en , are sum m arized in Table 14 .
T able 14 . N itrogen up tak e vs irra d ian c e p aram ete rs fo r hyp erb o lic
curves. AU v alu es (except Ijc a n d D:Vm ) m uldpH ed b y IO4 . *indicates a v alue n o t significantly d ifferen t from 0 (p > 0.05). ID
labels inclu d e d ep th , lobe (E o r W) a n d n u trie n t analyzed.
ID
Vm
a
D
D: Vm
1K
x 100
1993 E xperim ents
SENtLj.+
20.05 ± 2.56
2.16 ±1.08
5.80 ±1.40
5ENC>2"
1.11 ± 0.33 *0.26 ± 0 .2 4 *0.30 ±0.21
SWNHzj.+
9.50 ± 0.76
5WNC>2~
6.13
29 ± 8
*4.27 ±
3.68
*27 ± 2 1
1.12 ±0.46
5.70 ±0.50
8.48 ±
3.85
60 ± 7
*0.78 ± 0.40 *0.18 ± 0.51
*0.37 ± 0 .2 6
*4.33 ±
6.65
*47 ± 4 1
*47.44 ± 1 3 2 .5 2
*17 ± 4 7
ISENFLj.+ * 9.81 ± 32.55
0.25 ±0.05
2.05 ± 0.13
13WNH4+ 12.60 ± 0.73 0.33 ± 0.03
2.89 ±0.02
17ENH4+
IYEN O g-
*9.28 ±
5.58 ± 0.11 *5.12 ±5.81
10.65 ± 1.02
2.47 ±0.09
38.18 ±
3.23
23 ± I
3.30 ±0.07 .
1.08 ±
0.03
59 ± 2
2.73 ±0.08
4.31 ±
0.44
26 ± 3
1992 E xperim ents
SWNtLj.+ *10.43 ± 11.90
13ENH4+
8.28 ± 0.88
13WNH4+ 8.67 ± 0.74
*1.86 ± 1.03 •3.82 ±1.14
1.00 ±0.52
1.16 ±0.38
*5.61 ± 7.26
*37 ± 4 3 .
2.62 ± 0 .6 9
*10.90 ± 7.57
24 ± 7
4.09 ±0.49
7.47 ± 3.2 2
47 ± 7
129
T able 15 . Slope (ITI nmole q u an ta"*
s), d a rk u p ta k e (y-
in te rc e p t, h ' 1), a n d m axim um u p tak e (h"1) fo r u p ta k e vs irra d ia n c e
ex p erim en ts d escrib ed b y lin e ar regression. AU values m uldpU ed
b y IO4 . *- indicates a value n o t significantly d ifferen t fro m 0 (p >
0.05). ID labels include depth, lobe (E o r W), an d n u trien t.
ID
SLOPE
D
Vm
D:Vm x 100
1993 Experiments
5ENOg"
0.11 ±0.02 *-0.02 +0.30
5.45
*0 ±
SWNOg"
*0.01 ±0.01
0.41 ±0.20
0.93
44 ± 22
lSENOg"
*-0.02 ± 0.02
2.66 ±0.42
266
100 ± 16
13EN02"
*0.00 ±0.01
0.19 ±0.11
0.19
100 ± 58
ISWNOg-
*0.05 ±0.03
*0.74 ±0.58
3.43
*22 ± 17
13WN02"
*0.00 ±0.01
*0.15 ±0.19
0.15
*100 ±127
0.09 ±0.06
1.96 ±0.85
4.18
47 ± 20
17EN02"
6
20WNH4+
*-0.01 ± 0.04
15.11 ±0.76 15.11
20WNOg"
*0.00 ±0.02
*0.66 ±0.55
0.66
100 ± 83
20WN02'
*0.00 ±0.01
0.02 ±0:01
0.02
. 100 ± 50
25ENH4+
. *0.02 ±0.03
1.36 ±0.47
2.38
57 ± 20
25ENOg-
*0.00 ±0.02
5.65 ±0.45
5.65
25EN02"
*0.00 ± 0.00
0.07 ±0.07
0.07
25WNH4+
0.08 ±0.02
7.00 ±0.27
8.80
25WNOg"
*-0.04 ±0.04
4.16 ±0.91
4.16
100 ± 22
25WN02"
*0.00 ±0.00
0.12 ±0.05
0.12
100 ± 42
100 ±
100 ±
5
8 -
*100 ± 100
80 ±
3
130
Table 15, cont. Slope (h'1 pinole quanta'1 m2 s), dark uptake (yintercept, h"1), and maximum uptake (h"1) for uptake vs irradiance
experiments described by linear regression. AU values muldpHed by
10^. *- indicates a value not significantly different than 0 (p > 0.05).
ID labels include depth, lobe (E or W) and nutrient.
ID
SLOPE
D
Vm
D: V;
1992 Experiments
5ENH4+
0.27 ±0.14 11.19 ±0.26 21.99
0.51
SENOg-
0.06 ±0.01 *0.11 ±0.32
235
0.05
SENO2'
0.02 ± 0.01
0.31 ±0.14
1.24
0.25
SWNOg-
0.03 ±0.01
1.70 ±0.38
3.19
0.53
SWNO2-
. 0.01 ±0.00 *0.03 ±0.11
0.56
0.05
ISENOg-
0.02 ±0.02 *0.22 ±0.43
0.92
0.24
ISENO2-
*0.01 ±0.02
0.53 ±0.22
0.93
0.57
ISWNOg-
0.02 ±0.02 *0.32 ±0.42
1.05
0.30
ISWNO2-
*0.00 ±0.02
0.96 ±0.34
0.96
1.0
17ENH4+
*-0.01 ±0.02
2.68 ±0.36
2.68
1.0
17ENOg-
*0.00 ±0.38
3.23 ±0.74
3.23
1.0
0.02 ±Q.01 *0.25 ±0.93
1.22
0.20
20WNH4+
0.07 ± 0.04
4.96 ± 0.56
6.69
0.74
20WNOg~
0.09 ±0.05
5.35 ±0.76
8.05
0.66
25ENH4+
-0.02 ±0.02
2.33 ±0.37
2.33
1.0
25ENOg"
*0.02 ±0.03
3.53 ±0.74
4.33
0.82
17EN02-
*
131
In som e situations, correction fo r reg en eratio n could
significantly affect m easu red relationships betw een NH4+ u p tak e a n d
irra d ia n c e . It was previously d eterm in ed th a t a re g e n e ra tio n ra te o f
0.08 |aM NH4 + f f l a t 5 m east lobe could lead to a 10%
u n d e re stim a tio n of u p tak e at su b strate co n cen tratio n s u se d for
u p ta k e vs irra d ia n c e ex p erim en t (see P art II, R esults,Substrate
Kinetics). A sim ilar re su lt is fo u n d if a reg en e ra tio n ra te o f 0.10 m-M
NH4+ h"1 is assum ed fo r 13 m east lobe. U nfortunately, attem p ts to
m e a su re reg en e ra tio n ra te s in Lake Bonney d id n o t p ro d u ce
defin itiv e results; it is th erefo re im possible to p recisely co rrect
u p ta k e ra te s fo r reg en eratio n . To u n d e rsta n d th e p o te n tia l effect of
re g e n e ra tio n o n m easurem ents re p o rte d h ere, fu n ctio n s fro m
selected ex p erim en ts w ere ad ju sted using re g e n e ra tio n rate s
e stim a te d fro m 1993 isotope d ilu tio n experim ents (see p a r t II).
Specifically, th e functions describing NH4+ u p tak e ra te s a t 5 a n d 13
m e a st lo b e a n d 13 m w est lobe(1993 data) w ere co rre c te d fo r
re g e n e ra tio n using Equation 10. P aram eters fro m co rrected curves
a re sum m arized in Table 1 6 .
C orrections fo r reg en eratio n increased V^, a, a n d D; how ever,
th e co rre c te d p aram ete rs w ere n o t significantly d iffe re n t th a n th e
o riginal estim ates. .
132
T able 16. U ptake p a ra m e te rs fo r selected 1993 u p ta k e vs irra d ia n c e
ex p erim en ts co rrected fo r reg en eratio n . AU p aram eters, except fo r r
(ra te o f reg en eratio n , nmoles NH4+ h 'l ) , m ultipU ed b y 10^.
JD
r
SENHq+
0.08
ISENHq+
ISWNHq+
Vl
a
D
15.79
2.40
6.46
0.10
9.01
0.29
2.31
0.26
10.18
' 0.35
3.02
T he m in im u m irrad ian ce fo r sa tu ra te d u p tak e w as graphicaU y
estim a ted fo r all experim ents d escrib ed b y Equations 13 a n d 14.
T his th re sh o ld irra d ian c e (It ) was co m p ared to a re p re se n ta tiv e diel
ra n g e o f in situ irra d ia n c e ( Nov 17 1993, see P art I, Results,
Irrad ia n ce ). Nov 17 was co n sid ered rep rese n tativ e o f th e sam pling
p e rio d becau se th e average irrad ian ce o n th a t d a te ap p ro x im ated
th e averag e daUy irrad ian ces from Oct-Dec 1993. T able 17
sum m arizes It a n d th e p ercen tag e o f Nov 17 1993 w h en I<If
133
Table 17. Threshold irradiance (It, fxmol
s'1) and estimated
percentage of time irradiance was subsaturating diiring Nov 17,
1993.
ID
%of day I<It
It
1992 experiments
5WNH4+
9
0
13ENH4+
11 _
83
13WNH4+
6
38
1993 experiments
33
5ENH4+
15
5VVNH4+
5EN02"
7
10
0
0
5WN02™
4
0
13ENH4+
21
100
13WNH4+
50
100
17ENH4+
4
38
134
DISCUSSION
Following th e m eth o d s of MacIsaac a n d D ugdale (1972), several
in v estig ato rs h av e ap p lied M ichaelis-M enten kinetics to d escrib e th e
effect o f irra d ian c e o n n itro g en up tak e (Nelson a n d Conway 1979,
W halen a n d A lexander 1984, Cochlan e t al. 1990). This ap p ro ach h as
b e e n p o p u la r largely because th e p aram ete rs of th e M ichaelisM enten Equation, Vm ax a n d Ks> pro v id e useful in sig h t in to algal
physiology a n d th e ecology of th e system. The m ag n itu d e o f Ks, th e
h a lf-satu ra tio n constant, can be used to gauge th e efficiency of
p h y to p la n k to n lig h t harvesting a n d n itro g en assim ilation. A nalogous
to th e kinetics o f su b strate concentration, a sm all Ks w ould in d icate
"affinity specialists" (Kilham a n d Hecky 1988), w hereas a large Vm ax
w ould in d icate "velocity specialists" (Kilham a n d H ecky, 1988). In
su b stra te kinetics, affinity specialists a re m ore com m on in
en v iro n m e n ts w here su b strate co n cen tratio n is low a n d relativ ely
stab le (K ilham a n d Hecky, 1988). A low Ks fo r a n itro g en u p tak e vs
irra d ia n c e (NI) curve w ould th u s b e expected in a stable, low -light
system .
D espite its fre q u e n t application, th e M ichaelis-M enten
E quation is in ad eq u ate fo r describing th e d ep en d en ce o f n itro g en
u p ta k e o n irra d ia n c e in m o st system s. It assum es th a t th e re is n o
d a rk u p ta k e o f n itro g en a n d n o p h o to in h ib itio n (Priscu 1989). In
m a n y env iro n m en ts, p articu larly th o se ch aracterized b y low n itro g en
a n d low light, th ese assum ptions are likely to be invalid. Priscu
(1989) m o d ified th e p h otosynthesis equations of P latt e t al. (1980) to
135
d escrib e th e effects o f irrad ian ce o n n itro g en uptake. The
p h o to sy n th esis eq u atio n s a re able to describ e system s w ith
significant d a rk u p tak e a n d photoinhibition.
Since p h o to sy n th esis a n d n itro g en uptake m a y n o t b e d irectly
coupled, th e p aram ete rs o f th e p h o tosynthesis eq u atio n s (e.g. a, p,
Pm , a n d Ik) m u st b e re in te rp re te d w h en ap p lied to n itro g e n u p ta k e
(Priscu 1989). W hen describing photosynthesis, a, th e slope o f th e
lig h t lim ited p o rtio n o f th e curve, is a m easu re of lig h t h arv estin g
ab ility a n d th e efficiency of energy conversion (H enley 1993).
W hen describing n itro g en uptake, a is m o re am biguous; light is m o re
stro n g ly co u p led to p ro te in synthesis th a n to u p tak e (T u rp in 1991).
Vm , th e ra te of u p tak e a t optim al irrad ian ce; Ik, a s ta n d a rd index o f
p h o to a d a p ta tio n th a t m arks th e irrad ian ce a t w hich ex trap o latio n s o f
a a n d Vm intersect; a n d p, th e slope of th e p h o to in h ib ite d p o rtio n of
th e cu rv e a re also m ad e am biguous b y th e often in d ire c t
physiological relatio n sh ip betw een irrad ian ce a n d n itro g e n u p tak e
(Priscu 1989).
W hile n o t d irectly equ iv alen t to th e results o f p h o to sy n th esis
vs irra d ia n c e (PI) curves, th e p a ra m e te rs of NI curv es ca n b e u se d to
d escrib e n itro g en u p tak e in term s o f p h y to p lan k to n physiological
re sp o n se to irra d ian c e. A large a a n d low Ik in d icate a h ig h affinity
fo r lig h t a n d a n efficient, if indirect, tran sfer of p h o to sy n th etic
en e rg y to n itro g e n uptak e. The Platt equations fo r p h o to sy n th esis
h a v e b e e n successfully fitte d to NI curves for sev eral lakes a n d p o la r
m a rin e en v iro n m en ts (Priscu 1989, D odds an d Priscu 1989, Sm ith
a n d H arrison 1991).
136
T he relatio n sh ip betw een inorganic n itro g en u p tak e a n d
irra d ia n c e varies w ith th e source a n d ab u n d an ce o f inorganic
n itro g e n as well as th e level of p h y to p lan k to n a d a p ta tio n a n d
acclim ation to am b ien t irrad ian ce a n d n u trie n t concentrations.
Previous re se a rc h e rs h av e n o te d th a t NO3" u p tak e is g en erally
stro n g ly d e p e n d e n t u p o n irrad ian ce. P hytoplankton typically exhibit
negligible d a rk u p tak e o f N O g conversely, d ark NH4+ uptake,
p a rtic u la rly in n itro g en d ep leted w aters, m ay ran g e fro m 40% to
100% o f m axim um NH4+ uptake (MacIsaac a n d D ugdale 1972, N elson
a n d Conway 1979, Koike e t al. 1986, Sm ith a n d H arrison 1991).
Because NOg- m u st b e red u ced to NH4+ before assim ilation, th e
d e p e n d e n c y of NOg- u p tak e on irrad ian ce m ay be a re su lt o f a
re q u ire m e n t of ph o tosynthetic energy fo r th e p ro d u c tio n of
re d u c ta n ts (M iyazaki e t al. 1987).
Because th e d ep en d en ce of inorganic n itro g en u p tak e o n
irra d ia n c e h as b e e n show n to increase w ith increasing n u trie n t
su p p ly (T u rp in 1991), it is im p o rta n t to n o te th a t ex p erim en ts
re p o rte d h e re w ere b ro u g h t to satu ratin g levels o f DIN.
C onsequently, th e in situ d ep en d en ce o n irrad ian ce m a y be less th a n
re p o rte d h ere, p articu larly in th e n itro g en d ep leted shallow w aters.
However, since light a n d n u trien ts can p o ten tially co-lim it n u trie n t
u p ta k e (H ealy 1985), it is difficult to m ake a n ac cu rate su b stra te
co rrectio n fo r u p tak e. Before an y estim ate of NI rela tio n sh ip s a t
a m b ie n t n u trie n t co ncentrations can b e m ade, th e d eg ree to w hich
tig h t com p en sates fo r n u trie n t supply, a n d vice versa, m u st be
137
d eterm in ed . Once again, this issue could be clarified th ro u g h g reater
u n d e rsta n d in g of th e m echanism fo r n itro g en uptake.
D ark up tak e o f b o th NOg "and NH4+ tends to becom e
increasin g ly significant in n itro g en d ep leted system s (Cochlan et al.
1990). In a n itro g e n d ep leted en v iro n m en t, starch reserv es p ro v id e
th e ca rb o n a n d en erg y fo r am ino acid synthesis a n d n itro g en
assim ilation. In such cases, n itro g en assim ilation, an d , to a sm aller
extent, n itro g e n tran sp o rt, is relatively in d e p e n d e n t o f irra d ian c e
(T u rp in 1991). T he ratio of d a rk u p tak e to m axim um u p tak e (D:Vm )
fo r a p a rtic u la r fo rm o f DIN m ay th erefo re b e u sed to assess th e
n u tritio n a l status o f th e com m unity.
At 5 m in each lobe of Lake Bonney, D:Vm is sim ilar fo r NH4"1",
NOg", a n d NO2". F urtherm ore, D:Vm ten d s to in crease w ith d e p th
fo r all n u trie n ts. Indeed, in m ost d eep p opulations (below 13 m ),
d a rk in o rg an ic n itro g e n u p tak e was essentially eq u iv alen t to th e
m axim um level o f uptak e.
This final re su lt indicates th a t D:Vm should o n ly cautiously be
u se d as a n in d ic ato r fo r n u trie n t deficiency. It is a p p a re n t th a t
p la n k to n fro m th e d eep w aters o f Lake Bonney a re n o t lim ited b y
th e availability o f inorganic nitrogen; am b ien t DIN co n cen tratio n s a re
ex trem ely h ig h a n d am b ien t u p tak e velocity h as b e e n show n to
eq u a l Vm ax (see P art II, Results, S u b strate Kinetics). T he lack of
co rrela tio n betw een irrad ian ce a n d n itro g en u p tak e in th e d eep
lay ers im plies th a t bacteria, n o t phy to p lan k to n , a re th e m ain
co n su m ers of inorganic n itro g en a t th ese depths. As previously
discussed, n itro g en assim ilation b y algae m ay n o t b e d irectly fu eled
138
b y irra d ian c e; it is p rim arily d riv en b y th e en erg y a n d ca rb o n
skeletons o f ph o to sy n th esis (T urpin 1991). Since p h o to sy n th etic
cap acity does n o t exist a t 25 m in b o th lobes a n d 20 m in th e w est
lo b e (S harp 1993, Priscu u n p u b lish ed data) it is n o t surprising th a t
n itro g e n u p tak e is n o t stim ulated b y in creased irra d ia n c e a t these
dep th s.
D:Vm was consistently h ig h er fo r all inorganic n itro g en o u s
n u trie n ts a t 5 m w est lobe co m p ared to 5 m east lobe. This tre n d , in
c o n c e rt w ith low er 1%values a t 5 m in th e w est lobe, in d ic ates th at,
co m p a re d to 5 m east lobe, DIN u p tak e a t 5 m in th e w est lobe is less
likely to b e lim ited b y irrad ian ce.
D ata fro m 1993 a n d 1992 show th a t d a rk u p tak e o f NOg" a n d
N 02" a t 5 m, p articu larly in th e w est lobe, is a significant p o rtio n of
to ta l NOg' a n d NO2"" uptake. Low a a n d
values a t 5 m in b o th
lobes p ro v id es fu rth e r evidence th a t NOg" a n d NO2' u p ta k e a t th ese
d e p th s is n o t strongly reg u lated b y irrad ian ce. This o b serv atio n
co n tra d icts m a n y previous rep o rts th a t suggest u p ta k e o f NOg' a n d
NO2" is strongly d e p e n d e n t on irrad ian ce (MacIsaac a n d D ugdale
1972, N elson a n d Conway 1979, Koike e t al. 1986, Sm ith a n d
H arrison 1991). Two p o ten tial explanations fo r this co n trad ictio n
follow.
(I) D ue to chronic nitro g en depletion, p h y to p lan k to n a t 5 m in each
lobe re ly o n reserves of red u ced carbon, n o t re c e n t p h o to sy n th ate, to
p ro v id e th e en erg y a n d carb o n skeletons fo r assim ilation o f inorganic
n itro g en . P h y toplankton typically ad ju st NOg" a n d NO2" u p tak e to
139
c o rresp o n d w ith th e p o ten tial fo r assim ilation. For in stan ce, if th e
reso u rces fo r n itro g en assim ilation in to p ro tein are n o t available,
tra n s p o rt o f NO3" a n d NO2" ten d s to be lim ited (T urpin 1991).
T herefore, u p tak e o f NO3" a n d NO2™ in a system w h ere n itro g en
assim ilation is co n tro lled b y starch reserves w ould te n d to be
relativ ely in d e p e n d e n t o f sh o rt te rm changes in irrad ian ce.
(2) T he stab ility o f th e lig h t regim e in Lake Bonney h a s allow ed
p h y to p la n k to n a t 5 m in each lobe to en h an ce u p ta k e o f NO3" an d
NO2" a t low levels o f irrad iance.
T he second of these two explanations ap p ears m o re plausible.
U ptake o f NO3" a n d NO2™ was su b strate sa tu ra te d th ro u g h o u t th e
w ate r co lu m n in b o th lobes (See P art n, S u b strate Kinetics, Results),
indicatin g th a t m icrobial com m unities w ere n o t n itro g e n starved.
F u rth erm o re, th e d eg ree of n itro g en lim itation n ecessary fo r
d e p le tio n o f reserv es of red u ce d carb o n typically lim its u p tak e of
in o rg an ic carb o n . P rim ary pro d u ctiv ity m easu rem en ts in d icate th a t
th e 5 m com m unities w ere actively pho to sy n th esizin g a t relativ ely
h ig h ra te s (see Figure 2) d u rin g th e p erio d o f NI experim ents.
T he second explanation is su p p o rted b y com parisons o f
It v alu es w ith a m b ie n t irrad ian ces.
and
values a t 5 m in b o th lobes
a re consisten tly low er th a n values re p o rte d fo r p h y to p la n k to n in
o th e r system s, indicating extrem e ad a p ta tio n to low irrad ian ces. In
fact, th e Ik v alu es re p o rte d h e re a re m u ch low er th a n Ik v alues
re p o rte d fo r n e a r surface w aters o f o th e r p eren n ially ice-covered
A ntarctic lakes w ith sim ilar irra d ia n c e regim es (Priscu 1989).
th e
140
m in im u m u p tak e satu ratin g irrad ian ce, fo r NO3™ a n d NO2' u p tak e a t
5 m in b o th lobes exceeded th e m inim um irra d ian c e a t th ese d ep th s
d u rin g th e p e rio d o f collection (approxim ately 12 ^m ol m"2 s ' s e e
Figure 5). P resum ably owing to p h y to p lan k to n a d a p tio n /acc lim a tio n
to low irrad ian ces, u p tak e o f NO3" a n d NO2" a t th ese d ep th s is n o t
lim ited b y irra d ia n c e d u rin g late a u stra l spring a n d au stra l sum m er.
D:Vm values fo r NH4+ uptake a t 5 m in each lobe w ere
co n siste n t w ith D:Vm values re p o rte d fo r o th e r lake a n d m arin e
en v iro n m en ts (MacIsaac a n d D ugdale 1972, Nelson a n d Conway
1979, Koike e t al. 1986, Smith a n d H arrison 1991). However,
v alu es fo r NH4"1" u p ta k e w ere extrem ely low a t th ese d e p th s relativ e
to p rev io u sly re p o rte d values fo r o th e r system s. T he h ig h e st 5 m
fo r NH4+ u p ta k e (9.28, ea st lobe 1993) is well below Ijc values
re p o rte d fo r com m unities in o th e r p erm a n en tly ice-covered
A ntarctic lakes a n d A ntarctic sea ice (Priscu 1989, Priscu e t al. 1991).
D espite a low
relativ e to o th e r system s, NH4"1" u p ta k e a t 5 m
e a st lo b e was tig h t lim ited approxim ately 33 % o f ea ch d a y d u rin g
th e 1993 collection p erio d (see Table 19). F urtherm ore, NH4+ u p tak e
a t 5 m e a st lo b e d u rin g th e 1992 season was always irra d ia n c e
lim ited d u rin g th e p erio d of collection (see Figure 5). C om pared to 5
m w est lobe, th e m icrobial com m unity a t 5 m ea st lo b e was
a p p a re n tly less ad a p ted /acc lim ate d fo r NH4+ u p tak e a t low
irradiances.
141
T he in crease o f Ijc o f DIN u p ta k e w ith d ep th , as well as
re la tiv e ly h ig h 1%v alues a t 5 m east lobe co m p ared to 5 m w est lobe,
p a ra lle l tre n d s o f Ks o f DIN u p tak e (See P art II, Results, S u b strate
K inetics). Both Ijc a n d Ks increase w ith increasing DIN su p p ly rates.
As w ith Ks, th e in crease in % in d icates a relativ e d eclin e in affinity,
in this case fo r irrad ian ce. However, th e m echanism s fo r a decline in
DIN u p ta k e affinity fo r irrad ian ce a re u n clear (Priscu 1989).
A lthough p h o to in h ib itio n was n ev e r significant, th e re was
som e in d icatio n th a t NH4+ up tak e was susceptible to p h o to in h ib itio n ,
p a rtic u la rly a t d ep th s below 5 m. This is co n sisten t w ith Priscu e t
al.'s (1991) finding th a t th e irrad ian ce necessary fo r p h o to in h ib itio n
in a stable com m unity (e.g., ice algae) is a function of am b ien t
irra d ian c es. Specifically, n itro g en up tak e b y p h y to p la n k to n growing
a t low irra d ia n c e s was p h o to in h ib ite d a t relativ ely low irra d ian c es
(Priscu e t al. 1991).
Lake B onney p h y to p lan k to n show ed n o in d icatio n o f
p h o to in h ib itio n o f NOg" o r NO2" u p tak e a t an y d ep th . Previous
studies h av e show n NOg- a n d NH4+ u p tak e rate s to b e differentially
susceptible to p h otoinhibition, alth o u g h studies in p o la r system s
h av e p ro d u c e d conflicting results. Muggli a n d Sm ith (1993) re p o rte d
th a t NH4+ u p ta k e in th e G reenland Sea was p h o to in h ib ite d a t low er
irra d ia n c e s th a n was NOg- uptake. However, Sm ith a n d H arrison
(1991) re p o rte d th e opposite effect a t several sites in th e Arctic a n d
A ntarctic. Priscu (1989), studying p erm a n en tly ice-covered
A ntarctic lakes, fo u n d th a t NH4"1" u p tak e was p h o to in h ib ite d a t
142
relatively low irradiances. Interestingly, (3 was relatively large for
NO3" uptake. All of these researchers reported initiation of
p h o to in h ib itio n a t irrad ian ces m uch h ig h er th a n th e m axim um
irra d ian c es in this c u rre n t study. Clearly, th e m ech an ism of
p h o to in h ib itio n o f n itro g en u p tak e m u st be b e tte r u n d e rsto o d fo r
in te rp re ta tio n o f th ese conflicting results.
NI p a ra m e te rs fro m th e c u rre n t stu d y w ere co m p a re d to
p rev io u sly re p o rte d PI p aram ete rs to investigate th e ro le of
irra d ia n c e in d eterm in in g C:N u p tak e ratios. Iizo tte a n d Priscu
r e p o r t PI p a ra m e te rs a t two east lobe d ep th s th a t c o rre sp o n d to th e
c u rre n t study: 5 m a n d 17 m . N itrogen u p tak e p a ra m e te rs
n o rm alized to CHL a t 17 m are a n o rd e r o f m ag n itu d e h ig h e r th a n
sim ilar p a ra m e te rs a t 13 m, th u s suggesting n itro g en u p ta k e a t 17 m
is n o t e n tire ly d u e to phy to p lan k to n . T herefore, th e 5 m ea st lobe
d a ta a re m o re suitable fo r investigation o f up tak e resp o n se b y
p h y to p la n k to n . A ran g e o f irrad ian ces w ere in se rted in to Equation
12, a n d th e resu ltin g NI p aram ete rs w ere co m p ared to PI
p a ra m e te rs. T he C:N ratio of daily in teg rated u p tak e (n-MipM) was
18.35, w ell above th e ra tio (6.6) th a t is assum ed to b e typical of
b ala n c e d grow th (Redfield 1958). However, C:N u p tak e ratio s
in d ic ate b a la n c e d grow th a t approxim ately 10 nm ole q u a n ta m “2 s '
T he resp o n se o f C:N up tak e ratios to irrad ian ce is illu strate d in Figure
24.
T he resp o n se of C:N uptake ratio s to irrad ian ce illustrates th e
ro le o f NI relatio n sh ip s in regulating b alan ced grow th. In early
spring, w h en irrad ian ces are low a n d reg en e ra ted n u trie n ts h av e
accu m u lated (Sharp 1993), C:N u p tak e ratio s can b e expected to be
143
R edfield R atio
I r r a d i a n c e (yumol m
Figure 15. Estimated Carbon : Nitrogen uptake ratios vs irradiance at
5 m east lobe.
144
significantly less than 6.6.
Nitrogen deficiency is likely to occur only
when irradiances are above 10 nmole quanta m~2 S-1 for a majority
of the day.
In conclusion,
mid-summer C:N uptake ratios are likely
to be inadequate indicators of phytoplankton nutrient history or
physiological state.
145
CONCLUSION AND AREAS FOR FURTHER RESEARCH
Phytoplankton in Lake Bonney have adapted/acclimated to
enhance utilization of irradiance for DIN uptake. This is particularly
evident at 5 m in the west lobe where dark uptake of NOg"and NO2'
is significant, Ijc of NH4"1"uptake is much lower than values reported
from other systems, and It of NH4"1"uptake is significantly lower than
the average in situ daily irradiance during late austral spring and
early austral summer.
However, microplankton at 5 m east lobe and 13 m in each lobe
are relatively poorly adapted to utilize ambient irradiance for NLLi+
uptake. The lack of adaptation may be a reflection of the nutritional
status of the communities. Relatively high supply rates of NH4+ via
regeneration and diffusion at 5 m east lobe and 13 m in each lobe
may compensate for inefficient utilization of irradiance. The exact
mechanism for irradiance control of DIN uptake must be understood
for interpretation of NI trends.
146
GENERAL CONCLUSION
NH4+ is the preferred source of DIN for microplankton in Lake
Bonney. Regeneration of NH4+ is the primary source of DIN in the
upper trophogenic zone, and microplankton in this zone have
developed several strategies to enhance NH4+ uptake. Strategies
include high affinity for NH41", "surge" uptake of NH4+ pulses, and
efficient utilization of irradiance for NH4+ uptake.
NO3" and N02“ are minor sources of DIN in Lake Bonney, but
microplankton are still well acclimated/adapted for NOg" and NG2'
uptake. Dark uptake of NOg"and NO2" was significant in the shallow
waters of Lake Bonney, and "surge" uptake of NOg' and NO2" was
also observed.
Perhaps as a result of extreme acclimation/adaptation to in situ
supplies of DIN and irradiance, Lake Bonney microbial communities
do not appear to be extremely nitrogen deficient. Since several lines
of evidence suggest nutrient control of productivity in the upper
trophogenic zone, phosphorus should be considered a likely limiting
nutrient in this zone.
147
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148
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