NITROGEN
FIXATION
IN CLEAR LAKE, CALIFORNIA.
II.
SYNOPTIC
STUDIES ON THE AUTUMN
ANABAENA
BLOOM’
Clear
Lake Algal
Alexander
J. Home”, J. E. Dillard,
Research
Institute
Unit,
of Ecology,
University
of California,
Davis
D. I<. Fujita, and C. R. Goldman
Division
of Environmental
Studies,
University
of California,
Davis
ABSTRACT
Nitrogen fixation at three stages of an autumnal bloom of Anabaena circinulis was measured after almost simultaneous
collection
at up to 32 stations in Clear Lake and algal
hetcrocysts,
phytoplankton
cell numbers, NO,-N, NIL-N,
dissolved organic-N,
POh-P, Fe,
primary production,
particulate
carbon, and chlorophyll
a were also measured.
Nitrogen
fixation was significantly
and positively
correlated to Anabaena heterocyst numbers (P >
correlated to dissolved
0.001 ), negatively
correlated to NO,-N (P > 0.01 ), and positively
A negative correlation
with NH4 is proborganic-N
(P > 0.01) and PO,-P (I’ > 0.05).
able; no significant relationship
was found with the other variables mcasurcd. An cxplanation of the apparent restriction
of substantial cyanophyccan
Nz fixation to nonoligotrophic
waters is proposed.
The results arc consistent with the theory that heterocysts are responsible for Nz fixation in situ under oxic conditions.
INTRODUCTION
Combined inorganic nitrogen is the major
limiting nutrient in many aquatic ecosystems (Thomas 1969; Ryther and Dunstan
1971; Home and Vincr 1971). Nitrogen
fixation
is the only method by which
plants can provide new combined nitrogen
an d overcome this dcficicncy.
In oligotrophic lakes Ng fixation can usually be
dismissed as negligible, but it frequently
plays a major role in the nitrogen mctabolism of blue-green algae (Horne and Fogg
1970) and that of whole lakes and marshy
areas (Horne and Vincr 1971; Hornc 1971;
Hornc and Goldman 1972; IIorne 1972).
The contribution
of biological Nz fixation to lake nitrogen budgets has been
measured in several lakes including Windermcrc (Hornc and Fogg 1970), Lake
Gcorgc ( Horne and Viner 1971)) and
1 This study was supported by the County of
Lake, the California
Department
of Water Resources, the Institute
of Ecology
(University
of
California,
Davis ), and the Soap & Detergent
Association.
2 Present address:
Division
of Sanitary Engincering, University
of California,
Bcrkclcy
94720.
LIMNOLOGY
AND
OCEANOGRAPIIY
Clear Lake (Horne and Goldman 1972).
However, the synoptic technique, measuring many stations virtually simultaneously,
has not previously been successful for Ng
fixation. The high seasonally and spatially
variable rates of N2 fixation in Clear Lake
(IIornc 1971; Horne and Goldman 1972)
provide an ideal opportunity to study NZ
fixation by the synoptic technique.
The object of these studies is to understand Nz fixation in relation to blue-green
algal blooms, nitrogen compounds in the
lake, and the role of the hctcrocyst. Some
of thcsc can best bc shown by seasonal
analysis (Horne and Goldman 1972) but
because many measurements of blooms
will be irregular and made only during
the bloom period some method of interpreting incomplete data is nccdcd. The
synoptic tcchniquc cnablcs instantaneous
relationships to bc intcrpretcd in the light
of seasonally gathered information.
WC wish to thank E. de Amezaga for
the computer analysis, C. Carmiggclt and
B. Jones for drawing the figures, and the
many others who assisted in the collection
and processing of the samples.
693
SEPTEMBER
1972,
V. 17(5)
694
HORNE,
TABLE
1.
Significant
DILLAW,
relationships
between
d f = 6; Het = heterocysts;
FUJITA,
algae’
key:
>Om
Vol.
GOLDMAN
and N2 fixation
on 26 August
synoptic
>0.9?_49 = P?O.OOl;
‘,0.8343
=: P>O.Ol;
= P:,O.O5;
.,0.6215
----
Anabaena
N2
fixed
AND
No. Het
% Het
(IO
stations).
= P:aO.lO
Aphani-
Micro-
Oscil-
Melo-
Crypto-
zomenon
cystis
latoria
s ira
monads
Vol.
oss!5
Het
0.8754
LZ.4
0.9621
II-.
5-E
% Het
0.4308
0.3916
0.6121
Aphanizomenon
-0.0192
0.1463
0.0326
-0.3548
Microcystis
-0.2461
-0.2459
-0.3756
-0.7293
.-
0.4734
Oscillatoria
0.7446
0.8652
~
0.8951
-1z-
0.4356
-0.0125
-0.1909
Melosira
0.6352
0.3862
0.3294
0.0671
0.0092
0.3012
0.1731
Cryptomonads
0.7310
0.5714
0.5539
0.1770
-0.1399
0.1344
&69&O
--0.6251
Ciliates
0_h 6482
0.5901
0.4626
-0.0813
-0.3176
-0.2127
0.4,502
0.2906
0.4385
0.9321
--
0 9406
+,-
0.I.- 8930
0.3657
0.0634
-0.1377
0.7855
Lv-
!I,6384
0.7256
--
Anabaena
Total
No.
biomass
Ciliates
0.5705
--
tNo
significant
flagellates,
relationship
Scenedesmus,
was
found
between
N2 fixation
and
the
following
algae:
Chroococcus,
Oocystis,
Cyclotella,
Pandorina.
METHODS
Collections were made at up to 32 stations from two or three boats so that all
samples were collected within 2 hr. This
synoptic technique has been described by
Goldman and Carter (1965). Samples were
collected on 8 and 21 Scptcmber using
O-3-m tube samplers with a 4-cm diameter. On 26 August samples were taken
at 0 to 0.6 m with a Van Dorn bottle.
The method of assay for acctylcne rcduction, based on that of Stewart et al.
( 1967)) has been described previously
(Hornc and Goldman 1972). The only difference for the synoptic work is that all
samples were incubated at the standard
depth of 0.5 m. The methods used for the
mcasurcmcnt of the biological, chemical,
and physical parameters wcrc given by
IIornc and Goldman ( 1972).
For each of the synoptics the simple
correlation cocfficicnt “8’ has been computcd for all variables measured taken by
pairs. Computation of r was done using
the maximum available data for each pair
of variables. This gave r 6 dE for the synoptic of 26 August, 14 df for 8 September,
and 28 df for 21 September, with the cxccption of 27 dff for all r computed for
carbon fixed on 21 September, since station 32 was not assayed for carbon fixation
on that day.
Studies using pair correlations with large
amounts of data collected without refcrcncc to the final analysis occasionally
show biologically
meaningless but statistically significant
results. We cxcludcd
most parameters known from culture studies not to affect Nz fixation in any direct
biological sense, or do not show them in
de tail. This in troduccs some improvcmcnt
to this rclativcly
simple analysis, along
with a certain clement of subjectivity.
Bccause it was impossible to measure all the
variables that may affect Na fixation (c.g.
MO) we feel this approach justified.
The major rcscrvation with regard to the
USCof a simple pair analysis is that a high
degree of direct correlation bctwccn two
parameters (e.g. Nz fixation and hctcro-
NITROGEN
TABLE
2. Significant
Het = heterocysts;
CLEAR
LAKE
695
2.
relationships
between
algae+
and N2 fixation
for 8 September
synoptic
(17 stations).
;O 7420 = P>O.OOl;
bO.6226
= P>O.Ol;
>0.4973
= P>0.05;
d f = 14; key:
*-__L_
>0.4259
Chl
N2
fixed
Chl
IN
FIXATION
= P>O.iO
Aphanizomenon
Anabaena
a
Vol.
No. Het
No. Het
Vol.
% Het
% Het
Micro-
Oscilla-
cystis
toria
0.9337
a
Vol.
0.9319
----- __
0.9606
:m
Het
0.9499
_-z
0.9681
-_zYzzzzz
% Het
-0.2828
-0.3882
-0.3913
-0.3270
Vol.
0.0741
-0.0508
-0.0059
-0.0252
-0.3648
Het
-0.0540
-0.1699
-0.0652
-0.0895
-0.3782
0.6116
% Het
-0.1189
-0.1695
0.0894
-0.1002
-0.1825
0.3918
O.J893
Le.--
Microcystis
o.9200
0.9753
----
0.9765
E-
-0.9697
-0.3913
-0.0975
-0.1934
-0.1700
Oscillatoria
0.4914
0.3558
0.1967
0.2504
-0.0300
0.0343
-0.2019
-0.1782
0.2929
Cryptomonads
0.9137
~-
0.8535
-__
__-
0.8552
-_
-__
0.8488
z=z
-0.3428
0.0303
-0.1645
-0.1425
0.8958
Anabaena
No.
Aphanizomenon
+No
significant
Pandorina,
No.
relationship
Cyclotella,
was
flagellates,
found
between
Peridinitrm,
0.9943
--
N2 fixation
Navicula,
and the following
Planktococcus,
cysts) dots not, in itself, mean that a dircct relationship actually exists. But when
such a relationship is already known to
exist from laboratory culture studies, then
the level of probability
of the correlation
is useful to define the relationship more
precisely than could be done using the
raw data. The validity of our choice of
revclant paramctcrs is partially
dcmonstratcd in Tables l-3 and their respcctivc
footnotes, which show that most of the
significant
relationships
involve paramcters already known to be rclatcd to blucgreen algal N2 fixation,
In view of the
occasional and sometimes valid claims for
Nz fixation involving nonhctcrocys tous Cyanophyta or cpiphytic
bacteria, we felt
that some negative correlations dcscrvcd
brief mention in the table footnotes.
The level of significance of the correlation coefficients was dctcrmincd with the
table of Fisher and Yates ( 1953). Results
and level of significance
arc shown in
Tables l-3.
Nitrogen fixation (acctylenc reduction)
algae
Scenedesmus,
tested:
Oocystis,
ciliates,
Melosira,
0.5466
Coelastrum,
Chroococcus.
is expressed as nmolcs cthylcnc
liter-l hr-I.
produced
RESULTS
Small synoptic: 26 August 1970
(10 stations)
Nitrogen fixation in Oaks Arm was very
high and almost zero clsewherc (Fig. IA).
Values in Oaks Arm ranged from 102-750
( mean = 400) t-moles liter-l hr-I. Average values in Lower Arm and Upper Arm
wcrc 40 and 5. Nitrogen fixation was dircctly related to the amount of Anabaena
prcscnt ( P > 0.01) and also to the number
of Anuhaenu hctcrocysts (P > 0.01) (Tablc 1). IIcterocysts
varied from 1,012
4,148 (mean = 2,800) ml-l (Fig. 1B) and
pcrccnt hcterocysts from 3.85-6.7, but percent hcterocysts wcrc not significantly
rclatcd to Na fixation. Aphanixomenon flosaquae, the other main potential N,-fixing
alga, showed no significant
relationship
with Nz fixation (Table l), presumably
because thcrc were no Aphanizomenon
hctcrocysts at this time.
0.1083
0.0066
No. Het
% Het
ZOllle”OIl
-0.3881
?r\/\-\/
-0.3727
7,,c\-
P
Total
a
Achnanthes.
for carbon
biomass
-Values
Total
Chl
Dlatomata,
-0.2786
0.1330
-0.3186
Mollomonas,
-0.0207
0.0337
-0.0500
-0.2255
-0.1328
0.3620
\,\,\ L
0.0669
0.2472
-0.3223
-0.1875
-0.1777
-0.3488
-0.0680
-0.0694
0.2456
0.3862
,‘,‘\1\1
-0.2099
-0.0040
-0.0316
0.0888
-0.2803
-0.2660
Vol.
0.8947
-
-0.1994
-0.0125
-0.0651
-0.0246
-0.0658
0.1473
0.1346
-0.1603
-0.2113
0.5316
-__
-0.1933
-0.1079
-0.1842
Cryptomonas
for C fixed.
0.3288
-0.0441
-0.2616
O.i438
-0.0018
-0.3056
-0.2952
0.0233
0.2129
-0.2034
~0 3363
-0.1374
0.2900
No. Het
Aphaniromenon
-b.lOSS
-0.1137
given
between
% Het
Key:
algae*.
‘No
marssonii,
-0.5541
-0.2266
0.2975
-0.2665
-0.0112
relationship
-0.1809
0.0728
-0.0006
-0.2314
-0.0743
0.2721
C. reflexa,
0.5281
-0.4593
-0.3230
0.4305
-ml\-\,
-0.4751
-0.6360
-
-0.64820.2587
0.2243
0.3917
T-i.‘,-2
-0.3824
z--i\.
0.3039
-0.2198
C. curvata,
was found
0.2075
0.0586
0.2028
o.:e3!
--
0.2317
-0
L- 4641
-0.4955
-
0.0637
0.3740
,* “J
-0.2717
Schr.
-0.2028
0.6159
E
-0.6113
--
-0.0093
-0.5407
___
Coelastrum.
between
-0.4512
-
-0.0880
0.2806
-E
parameters
SOI.
0.0752
-0.2430
-0.2384
Oocystis,
-0.2129
0.5650
-
Pediastrum,
algae:
-0.3521
-0.3316
h,. , / *
-0.2064
m
0.5478
-0.7276
__
Scenedesmus,
0.1027
0.1784
-0.2980
-
N2 fixationand
-0.4241
y,,-,
I
-0.2520
-0.3657
-1
-0.3762
?-w-Y-/z-u
-n
- 7i73
w-w
Tota I
P
0.6311
Partic.
C
C
fixed
Fragilaria,
-0.1884
-0.1139
Total
Fe
Rhiocosphaerium.
Melosira,
0.1718
0.3152
-0.2977
Closterium,
Cyc/ofe/fa,
0.0631
0.8571
-
-0.0718
= P-O.10
-0.3646
Yryu-.z
0-z
SOI.
P04-P
the following
Org-N
20.2960
September
- P. 0.05;
for 21
“.3!85 0.1444
-0.1629
=zztzzZ
-0
6528
-0.6753
--
0.3291
N03-N
-= P.%O.Ol; ‘0.34~4
and chemical
:0.4487
-
cost.
physical
- P>O.OOl;
Oxill.
N2 fixation,
Micro.
significant
-0.2915
0.0121
-0.1291
-0.0031
-0.0070
0.0307
0.0239
-0.1677
-0.1813
0 3401
L
-0.2025
-0.1131
-0.1936
Het = heterocysts;
relationships
=28:
Rhodomonas,
to those
Gymnodrnium,
are identical
-0.0695
-0.1670
efficiency
-0.0717
0.1415
-0.3712
IV\A/X?*
-0.2457
O.s01_7
-
0.0812
-0.0071
Total
-0.0890
~14;089
-0.2308
Fe
1
0.4896-0.2771 --0.4969
0.4344
0.5097-i-^
0.3843
.- / I_, __
“”
C fixed-
Particulate
P04-P
Sol.
Sol. organic-N
NO34
0.2553
0.3698
-vu
Schroederia
$364
-0.3256
Coscinodiscus
0.5257
0.2739
0.5540
-0.1777
0.1536
-o.T?5J
oscr//atorra
0.0196
-0.1388
% Wet
d f
Significant
No. Het
Anabaena
-0;49
OUvS83
0.1584
0.3226
__
-0.1077
-0.1223
0,3682
Vol.
MKrocystis
fixation
-0.1930
Vol.
Aphani.
C
0.8990
-0.1067
% Het
Anabaena
No. Het
N2
fixed
3.
(32 stations).
TABLE
synoptic
a
0.1024
Chl
-
---_.
-----
NI’I‘IIUGJAN l!lXA’I‘lUN
IN
CLEAR
LAKE,.
7
697
2.
4. Nutrient
concentrations
(yg liter-‘) for
26 August and 8 September at representative stations in Cleaf Lalce. (u = <lO pg liter-‘)
TABLE
upper
26
N114-N
surface
bottom
Aug
Arm
Lower
Q Scp
26
Aug
Oaks
mm
Q sep
26
Aug
Arm
Q sep
u
u
Ll
"
40
Go
I.7
14
-
4:"
2:o
20
N03-N
(O-3
m)
Soluble
orqanic-N
N2
FIXATION
n moles
(O-3
AS C2H4
liter-’
519
m)
Soluble
P04-P
hr-’
(O-3
Total
17
533
-
-
-
340
360
-
160
17
-
410
895
m)
Pe
to-3
160
m)
1,430
1,485
B
u HETEROCYSTS
ml”
C
PHYTOPLANKTON
[black
VOLUME
area
is
740
pm3
x
lo6
ml-’
Anabaena)
FIG. 1. Nitrogen
fixation (A), hetcrocysts
(B),
and phytoplankton
(C) on the 26 August synoptic. Diameters of circles indicate quantities measured.
Oaks Arm is enlarged to clarify
results
(synoptic station letters are given in B).
Thcrc was a statistically significant rclationship of Nz fixation with Oscillatoria
(P > 0.01) and also cryptomonads
(P >
0.05) but not with any other algae counted
(Table 1). This is the result of an indircct relationship
because the abundance
of OsciL!atoria and Anabaena are significantly related to each other (P > O,Ol) and
explains the apparent relationship of OScillatoria and Na fixation.
Cryptomonads
arc rclatcd to OsciZZatoria (P > 0.1) and
the relationship between them and NB fixation is again indirect.
Nitrogen fixation was significantly
rclated to total biomass ( P > 0.001) mainly
because Anabaena was the codominant
spccics (with Microcystis)
at this time,
but Anabaena’s larger size gives it much
in biomass measurcmore importance
ments. Measured as a pcrccnt of total phytoplankton biomass, Anabaena accounted
.for 24-51 (mean = 40%) in Oaks Arm.
Mcasurcmcnts at one central station from
Upper and Lower Arms gave 4 and 30%
Anabaena (Fig. 1C). Bccausc Nz fixation
was known from routinc measurements to
be appreciably high only in Oaks Arm at
this time, only the 8 stations in Oaks Arm
wcrc trcatcd separately and avcragc valucs for the other two basins wcrc obtained
using samples from 5 offshore stations in
each.
Table 4 shows the major nutrients:
At
this time the concentration of Non-N was
low,
there was little intcrbasin variation,
and N&-N
was undetcctablc
(<lo
pg
liter-’ ) . Phosphate-P was high, especially
in Upper Arm,
Nitrogen fixation rates in the 26 August
698
IIORNE,
Nq
FIXATION
FUJITA,
A
n moles
liter
HETEROCYSTS
PHYTOPLANKTON
(black
DILLARD,
-1
hr”
ml-’
VOLUME
area
pm3
x
lo6
nli"i
is % Anabaena]
FIG. 2. Nitrogen fixation
(A), heterocysts
(B),
and phytoplankton
(C) on the 8 September synoptic.
Diameters
of circles indicate
quantities
measured.
synoptic
(0-0.6-m samples) are higher
than the later synoptics (O-3-m samples )
but are not directly comparable bccausc
of the sampling diEfcrenccs. Surface samples gcncrally fix the most NB ( Horne and
Goldman 1972).
Minor
synoptic: 8 September 1970
(16 stations)
Nitrogen fixation occurred towards the
castcrn end of every basin (Fig. 2A) but
high rates were confined to Oaks and
Lower Arms, especially the latter whcrc
values of O-209 (mean = 105) nmoles liter-l hr-1 were recorded. This compares
with rates of O-82 (mean = 25) for Oaks
and 17-60 ( mean = 30) for Upper Arm.
AND
GOLDMAN
That this pattern of fixation was due to
Anabaena and its hctcrocysts is clearly
shown in Fig. 218 and Table 2. Fixation
was highly significantly
directly related to
Anabaena hcterocyst numbers (P > 0.001).
The concentration of hcterocysts per milliliter ranged from 400-850 (mean = 600)
in Upper Arm, 40-1,780 (mean = 600) in
Oaks Arm, and 180-5,600 (mean = 2,190)
in Lower Arm. The other phytoplanktonic
algae showed no significant
relationship
with fixation, with the exception of Microcystis (P > 0.001) and cryptomonads (P >
0.001). This is a similar situation to that
found For the 26 August synoptic and is
presumably again due to indirect relationships of these algae with the main Nzfixing species. Anabaena was related to
Microcystis ( P > 0.001) and to cryptomonads ( P > 0.001).
There was a significant correlation with
N2 fixation and chlorophyll
a, owing to
the dominance of Anabaena biomass in the
phytoplankton.
Measured as a percentage
of total phytoplankton
volume ( Fig. 2C),
Anabaena accounted for 7.9-15.6010 (mean
= 10.4%) in Upper Arm, 0516.3% (mean
= 7.7%) in Oaks Arm, and 9.2-81% (mean
= 43%) in Lower Arm.
In this synoptic, few chemical mcasuremcnts were made but rcprcscntative samples indicate the general trend of each
basin (Table 4),, In the surface water of
the central area of Lower Arm, NOS-N was
low but still considerably higher than that
in cquivalcnt areas of the other two arms.
Not unexpectedly ammonia was gcncrally
very low (5-17 pg liter’)
and showed a
pattern opposite to that found for NOR-N.
Total-P and Pod-P were very low in Oaks
Arm where Na fjixation was low, but much
higher in the two basins where high NZ
fixation was occurring, suggesting the possibility of a lasting N and P limitation in
Oaks Arm following the peak of the bloom
at the end oE August. Dissolved organicN and total iron showed no significant
Howcvcr,
bccausc
intcrbasin
variation.
thcsc values rcflcct only the gcncral trend
of each basin more reliance should bc
NITROGEN
2 FIXATION
n moles
PHYTOPLANKTON
(black
VOLUME
area
pm3
FIXATION
AS
liter”
x lo6
IN
CLEAR
LAKE.
2.
699
C2 H4
hr”
r-l
ml”
is % Anabaena)
FIG, 3. Nitrogen fixation
tember synoptic.
Diameters
(A), hcterocysts (B), phytoplankton
(C), and Non-N
of circles indicate quantities mcasurcd.
placed in the final synoptic whcrc
cnts wcrc measured at all stations.
nutri-
Major synoptic: 21 September 1970
(32 stations)
Nitrogen fixation was confined to Upper
Arm and its connection with the other
basins at the Narrows (see Fig. 3A). Rates
of N2 fixation in Upper Arm varied from
32425
( mean = 92) nmoles liter-l hr-l
and were highest at the nearshore sites 9,
10, and 11, reflecting the abundance of
the potential Na-fixing algae and heterocysts at these places (Fig. 3B).
There was a highly significant positive
correlation between Ns fixation and Anahnena heterocyst numbers (P > 0.001) and
a significant
relationship
with Anabaena
volume (P > 0.05) (Table 3). There was
no significant
relationship
between NB
fixation and Aphanixomenon
hctcrocysts.
Nitrogen
fixation
was significantly
and
negatively correlated to the concentration
of NOs-N (P > 0.05), directly corrclatcd
to dissolved organic-N (P > 0.01 ), total-l?
(D) for the 21 Sep-
(P > O.OS), and P04-P (P > 0.05) (Table
3, Figs. 3 and 4). There was no significant
relationship bctwecn Na fixation and chlorophyll a, particulate-C, carbon fixed, total
iron, total biomass, or efficiency of C fixation (Table 3).
As opposed to the earlier synoptics there
was a significant
negative (rather than
positive) correlation bctwcen Nz fixation
and Microcystis (P > 0.05). Thcrc was a
significant
positive relationship
between
Nz fixation and Oscillatoria (P > 0.01) and
Schroederia ( P > 0.05). Once again this
was probably due to an indirect relationship bccausc these two species were both
highly correlated to total-l? and PO,+-P
( P > O.OOl), which is itself significantly
corrclatcd to N2 fixation (P > 0.05). No
other of the 23 algal species counted were
significantly
rclatcd to N2 fixation.
Measured as a pcrccnt of total phytoplankton
volume Anabaena accounted for 1.3-32%
(mean = 16%) in Upper Arm, 1.4-11%
(mean = 4%) in Oaks Arm, and O-69%
(mean = 12%) in Lower Arm.
An interesting effect shown by this syn-
700
pg
IIORNE,
DILLARD,
N liter-’
(O-3m)
FUJITA,
AND
GOLDMAN
(light
area
is % POT-P)
D
IMARY
mg
PRODUCTIVITY
C mw3
day’]
mg
mm2
(O-3
m)
FIG 4. Dissolved organic-N
(A), total-P and PO,-P (B), p rimary productivity
(C), and chlorophyll
a
(D) for the 21 September synoptic.
Diameters of circles indicate quantities measured.
Synoptic station
numlxrs
are
given in D.
optic is the biologically
scparatc behavior
in IJppcr Arm and higher concentrations
of the basins as demonstrated by the con- of 0.1-0.93 mg literl in Oaks and Lower
Arms rcspcctively.
centration of NOS-N and P04-I?. In Upper
Arm, where Na fixation was high, NO:%-N
The data for C fixed show what appears
concentrations
were very low, ranging
to be an cxamplc of limitation of photosynthesis by lack of nitrogen in Upper
from lo-83 (mean = 21) PI; N liter-l; but
Arm relative to the other two arms (Figs.
in Lower and Oaks Arms whcrc fixation
3 and 4). In Lower and Oaks Arms where
was insignificant, N03-N was much higher,
ranging from 129-192 (mean = 162) in nitrogen was more concentrated than in
Upper Arm, C fixation ranged from 279Oaks Arm and from 15-130 (mean = 79)
in Lower Arm. Phosphate-P showed the 677 (mean = 370) mg C m-3 day-” in
opposite picture, being low where NO.?-N Lower Arm and from 142-163 (mean =
was high and Nz fixation low. In Upper
210) in Oaks Arm; in Upper Arm values
Arm, Pod-P ranged from 260404 (mean
of only 49-191 ( mean = 140) wcrc found
= 352) pg liter-l, and in Oaks and Lower
(Fig. 4C). This suggests that Nz fixation
Arms values ranged from 160-190 (mean
alone was not able fully to supply the
= 165) and .from 50-100 (mean = 70)
algal demand for nitrogen.
rcspcctively.
Chlorophyll a and particulate-C showed
Ammonia concentrations wcrc not mcaa relationship to N2 fixation in Upper Arm
sured at all synoptic stations, but mcasurcbut mainly bccausc Anabaena dominated
mcnts taken at the time of the synoptics
the phytoplankton
there. High standing
indicate that there was almost no ammonia
crops
of
Chl
a
and
particulate-C
were
in the surface water layers of any basin
produced
in
the
other
arms
without
high
On 21
on 26 August or 8 Scptembcr.
N2
fixation
or
C
fixation
at
this
stage
September, however, there was a surface
although active Nz fixation did occur in
ammonia concentration of 0.06 mg liter-’
NITROGL;.N
FTXATION
these basins earlier in autumn (Figs. 1
and 2).
Micronutricnts
are suspected as a possiblc limiting factor for the growth of blucgreen algal blooms in Clear Lake (Horne
and Goldman 1972). The only “micronutricnt”
measured, total iron, showed a
trend towards lower values of 1.28-2.43
(mean = 1.60) pg liter-” and 1.58-3.08
( mean = 2.05) in Lower and Oaks Arms
respectively, rclativc to the higher values
of 1.54-3.72 (mean = 2.29) found for Upper Arm. Our total nutrient results show
that a scvcrc N limitation in Upper Arm
occurred during a time when Pod-P was
in cxccss and that Na fixation was unable
to meet the need for added nitrogen. At
this time there seemed to bc a phosphorus
or iron ( or other trace element) limitation
or possibly an inhibitor in the other basins.
However, this limitation or inhibition
occurrcd a&r a large peak of Na fixation in
thcsc b‘asins.
DISCUSSION
In Clear Lake the autumnal bloom of
Anabaena circinulis fixes N2 where there
arc large numbers of hcterocysts, relatively
low concentrations of nitrate and ammonia, high concentrations of phosphate, and
moderately
high concentrations
of dissolved organic nitrogen. Only when thcsc
five conditions wcrc satisfied clid very
much N2 fixation occur.
Perhaps the most important of our findings is that of the role of the hcterocyst
bccausc there has been sonic controversy
over its function
(Stewart ct al. 1969;
Smith and Evans 1970). This study supports previous claims that in oxic lake
cnvironmcnts hcterocysts give a good mcasure of in situ rates of N2 fixation by Anuhaena (Hornc and Fogg 1970; IIornc and
Goldman 1972).
Our data rcprcscnt a step forward in
the understanding of in situ N2 fixation by
planktonic blue-green algae bccausc until
now there has been no clear relationship
shown between Na fixation
and o thcr
chemical parameters. Early studies have
shown various types of rcsponsc to inor-
IN
CLEAR
LAKE.
2.
701
ganic-N added tither from natural sources
or added artificially
(Dugdale and Dugdale 1962; Gocring and Necss 1964).
Our study of an Anabamu bloom, showing a clear invcrsc relationship
between
Nz fixation and the instantaneous conccntrations of N03-N, now clarifies this uncertainty, at least for N03-N for which
turnover rates arc slow. Blue-green algae
appear to need a certain amount of combincd nitrogen as a “starter” if they are to
fix appreciable quantities of Na. Seasonal
studies (Horne and Fogg 1970; IIornc and
Goldman 1972) show various slightly positivc, or uncertain relationships
between
N03-N and Nz fixation. Because high N2
fixation by cyanophyceans occurs in situ
only when other inorganic nitrogen sources
arc used (Dugdalc
and Dugdale 1965;
Billaud 1967, 1968)) whole-season studies
can bc cxpectcd to show occasional indircct positive relationships between Nz fixation and N03-N, NHJ-N, or both, at least
in cutrophic lakes. When measurements
are made at the height of Nz fixation of
the bloom most inorganic-N will already
have been dcplcted to product the high
N2 fixation. This situation would not ncccssarily hold for NHd-N, where regeneration rates may be sufficiently
rapid to
supply much of the nitrogen used in
growth but insufficiently
fast to build up
a significant standing concentration.
The relationship between the growth oE
blue-green algae in lake waters and organic-N has been much discussed (Pearsall 1932; Hutchinson
1957, 1967; Fogg
1965; Lund 1965). According
to Fogg
(1965) thcrc is probably some general rclationship bctwecn the two but the basis
for this is still obscure. Home and Fogg
(1970) showed a noncausativc relationship
bctwccn N2 fixation by blue-green algae
and dissolved organic-N (which was rclatcd to phytoplankton
biomass) in Windcrmcrc and Esthwaite Water. The results
of our study confirm this result correlating
Nz fixation and dissolved organic-N ( P >
0.01). In particular on 21 Scptcmbcr N2
fixation. was highest at site 9 (421 nmolcs
liter-1 hr-I) and was corrclatcd to the high-
702
HORNE,
DILLARD,
FUJITA,
est dissolved organic-N value (5,600 ,ug N
liter-l).
The mean for Upper Arm was
only 910 lug liter l. Our studies show that
Nn-fixing Cyanophyta also arc associated
with quite high levels of dissolved organicN (0.35 mg litcr’l ) ,
Our data could provide a rcasonablc
answer as to why high NP fixation is only
found at certain times and only in nonoligotrophic lakes. In oligotrophic lakes thcsc
conditions of some combined inorganic-N
plus sufficient
dissolved organic-N may
never occur. In stratified nonoligotrophic
lakes these conditions are normally only
provided toward the end of a spring bloom
and during autumn overturn, In nonstratificd, nonoligotrophic
lakes these conditions
may occur at irregular intervals throughout the year, depending on nitrogen turnover rates.
It is thus obvious that the rate of supply
and uptake, as well as just the standing
concentration, of nitrogen should bc considered, at least in the metabolism of Nzfixing cyanophycean blooms. We disagree
with IIutchinson
(1967) and believe it is
to be expected, not paradoxical, that standing concentrations
of the limiting nutricnt (s ) will be low during the development
of a healthy blue-green algal bloom, bccause uptake rates may exceed supply
rates. The probable controlling factor in
these fast, large blooms is the rate of
turnover of limiting nutrient ( s ) , and the
standing
concentration
is a secondary
factor.
The methods of mathematical
analysis
used in these studies will not show any
significance for a variable that fluctuates
above the critical level for participation
in
For example photosynthesis
Nz fixation.
in Clear Lake has no significant rclationship with NP fixation bccausc it is probably ,always adcquatc to power Na fixation,
despite the fact that the large amounts of
energy needed to break the NsN
bond
may be biologically
disadvantageous
in
seasonal competition with other algae that
can use existing low levels of NIL-N. Thus
in Clear Lake the alga Oocystis which
does not fix Nz dominates the summer
AND
GOLDMAN
phytoplankton
after the collapse of the
spring Aphanixomenon bloom which does
fix Nz.
The seasonal variations
of P04-P in
Clear Lake arc now well defined (Hornc
1971) but because of the huge relcasc of
this compound in summer there is ncvcr
any real prolonged shortage. The results
of our study are thus not readily applicable to other lakes whcrc P04-P concentration and rate of turnover may bc low
in summer-autumn.
We do not know how these conclusions
will apply to other lakes whcrc Nz fixation
plays a significant role. Certainly Nz fixation occurs usually when N03-N and
NHd-N levels are much less than maximum in most lakes (Dugdalc and Dugdalc
1962; Goering and Neess 1964; Horne and
Fogg 1970 ) , although this is not always
true (Horne and Fogg, 1970). The data
obtained for Clear Lake suggest that when
the demand for inorganic-N
exceeds the
supply (ic. nitrogen stress) then Nz fixation will occur regardless of the ambient
concentration
of inorganic-N-unless
this
is toxic.
Most algal species not known to fix N:!
showed no significant relationship with Na
fixation, hetcrocysts, or Ns-fixing algae.
The few which did show a significant relationship are all obviously related to some
third factor ( Tables l-3).
There is thus
good reason to state that the autumnal
Anabaena bloom is responsible for all of
Clear Lake’s planktonic N2 fixation.
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