APPLIED AND ENVIRONMENTAL MICROBIOLOGY. June 1989. p. 160(5-1611
0099-2"40/89/061605-07S02.00/0
Copyright t 1989. American Society for Microbiology
Vol. 55. No. 6
Experimental Tests of Nutrient Limitation in
Freshwater Picoplankton
JOHN D. WEHR
Ca/lder E(ology Center. Ford/han Unih-ersitv. Bo.x 273, Armslomik. Newt- Yo,-k 10504
Received 27 Januac-ry 1989/Accepted 20 March 1989
On the basis of correlative studies, picoplankton in Calder Lake, New York, are apparently unaffected by
seasonal fluxes in nutrient (N and P) levels. In this small eutrophic lake, picoplankton (<2.0- to 0.2-,um size)
and nanoplankton (<20 to >2 ,um) predominate. Microplankton (>20 ,um) are typically least important.
Experiments were conducted in situ to test whether N, P or N/P ratios affect the predominance of these smaller
organisms. Manipulations were run in 4-liter microcosms during June, July, and August 1988, corresponding
to periods of increasing stratification and nutrient depletion. Following nutrient additions, phytoplankton were
harvested and fractionated into three size classes. Microplankton and nanoplankton were significantly
stimulated by both N (2.5 to 50 ,uM) and P (I to 20 ,uM) additions. The severity of nutrient limitation was
greatest during July. Picoplankton responded less strongly to N additions and were never P limited. These field
data support laboratory studies which indicate that bacterium-sized phytoplankton use nutrients more
efficiently and are superior competitors within mixed commtinities.
Bacterium-sized phytoplankton are widespread and abundant in the world's oceans and in many lakes (40. 43). This
has caused a major reevaluation of microbial ecology in
aquatic ecosystems. These microorganisms, termed picoplankton, include autotrophic cells from 0.2 to 2 p.m in size
(38). Algal picoplankton may predominate in certain North
American Great Lakes (6, 13) and in Lake Zurich. Switzerland (7). Studies of smaller eutrophic. oligotrophic. and
meromictic lakes indicate the importance of these microorganisms more widely (8, 10, 24. 37). Remarkably. both
marine and freshwater studies report that the predominant
taxon is the cyanobacterial genus Svnechlococcis. The role
that picoplankton play in aquatic primary production and
within the microbial loop is now thought to be considerable
(26, 39).
Less has been said in this expanding literature about what
role these tiny autotrophs may play in nutrient cycles.
Marine picoplankton apparently dominate N uptake in south
Atlantic and Antarctic waters (27, 28). These organisms
were found to have marked preferences for NH4-N over
N03-N. In contrast, a picoplankter from Lake 302-S (a
Nannochloris sp.) was equally efficient with both forms.
while a co-occurring nanoplankter (a Cliir .sochronuiilina sp.)
may be unable to metabolize NO-N (47). Laboratory studies suggest that some strains of Sv'zllChococcits may be quite
adept at metabolizing a variety of N forms (23) and that
uptake rates may be quite rapid (30).
Previous studies suggest that freshwater algae are unable
to compete with bacteria for inorganic phosphate (9. 32). yet
smaller picoplankton may still prove to be equal competitors. Short-term. very rapid P uptake r-ates by nano- plus
picoplankton in Lake Michigan support this idea (19). In
times of nutrient surplus. picoplankton may not. however.
be superior competitors. For example. there is strong evidence which suggests that smaller-sized cells lose their
competitive advantage when P levels increase (18). Another
study found that smaller cells are relatively more abundant
in lakes with lower levels of total P (44). although nano- and
picoplankton were not differentiated.
Recently. I completed a 2-year study of picoplankton
abundance in a small eutrophic system. Calder Lake. N.Y.
(J. D. Wehr. Hydrobiologia. in press). The phytoplankton
community consisted predominantly of nanoplankton and
picoplankton, about 74% of the total biomass as measured
by chlorophyll ai (Chl ti). Despite surface blooms of colonial
cyanobacteria (e.g.. Microcvstis and Anabaiernai spp.) and
subsurface blooms of large dinoflagellates (Ceraditiiiii hirii dielali). picoplankton represented the largest (P < 0.001)
proportion of phytoplankton biomass in 1987. In 1988.
nanoplankton predominated (P < 0.001). This contradicts
somewhat the findings of other studies (e.g., reference 44).
Past Calder Lake data suggest that midsummer nutrient
limitation, when N/P ratios are either >30 (= P limitation),
or <15 (= N limitation, 32. 35). may affect only the largest
species and favor picoplankton dominance. For the present
study, it was decided that direct experiments would be
conducted in situ to test these observations. The following
four hypotheses were tested. (i) Nutrient loading favors
shift from picoplankton- to nanoplankton- and microplankton-dominated communities. (ii) A specific N/P ratio (or
range) can be identified which may be used to predict a
predominant phytoplankton size class. (iii) Microplankton
(large cyanobacteria in particular) are primarily limited by P.
while nanoplankton will be both N and P limited. (iv)
Picoplankton will never be P limited, despite temporally
changing nutrient conditions over the course of a summer.
a
MATERIALS AND METHODS
Study site. Experiments were conducted in Calder Lake.
which is located in southeastern New York. Chemical and
physical features were detailed previously (Wehr, in press).
but brief description is provided here. Calder is 3.9-ha
dimictic lake with an average depth of 2.8 m and a maximum
depth of 6.7 m. The lake stratifies in late spring (usuallv late
May). and autumn turnover usually occurs in early October.
Typically. ice cover forms on the lake in December and
breaks up in mid-March. The lake is spring fed and is located
within a relatively undisturbed deciduous forest. The Fordham University Limnology Laboratory is located on the
southern shore of Calder Lake. Samples are processed and
analyzed there.
Field procedures. Experiments were run on 3 June. 7 July.
a
1605
a
1606
APPL. ENVIRON. MICROBIOL.
WEHR
NITROGEN ADDITIONS
(,amol/L)
2.5
5.0
12.5
25
50
2.5
5.0
12.5
25
50
I
i-
12.5
1
-u
I
0
n
25
2
_s
r_0c
3
12.5
=
N: P RATIO
5.0
mu
0 D
-
10
LI
0
z
2.5
U)
20
FIG. 1. Experimental design of nutrient additions for microcosm
experiments in Callder Lake. Concentriations are levels added to
experimental bags (each treatment wals
run in
triplicate).
and 26 August 1988, each for 4 days. On each starting date,
a complete set of water chemistry and phytoplankton samples were collected for comparison with the microcosm
study. For comparative samples and experiments, water was
collected by peristaltic pump, fitted with prewashed silicone
tubing. Experimental microcosms were 4-liter Cubitainers
attached to floats which were anchored to a location near the
center of the lake. Water was collected from a 1-m depth,
prefiltered with 250-p.m-pore-size Nitex mesh (with in-line
filter units) to remove large grazers, and directly added to the
microcosms. Smaller mesh sizes proved to remove larger
phytoplankton species. Each treatment was replicated three
times. The overall design for each date was the same (Fig. 1).
This provided several total loading regimens where [N], [P],
or the N/P ratio was held constant. Experimental bags were
fertilized by additions of NH4CI (stock [N] = 10 pM),
KH2PO4 (stock [P] = 4 p.M), or both by using syringes fitted
with 0.2-p.m-pore-size sterile filter units (Gelman Acrodisc).
KCI was added to bags to equalize K additions to all
treatments. They were mixed and attached to the flotation
anchors. Bags were positioned so that light penetration
approximated levels measured in situ at a 1-m depth. The
lake water is fairly well buffered (ca. 500 p.eq/liter), and
measurements at the end of each experiment indicated that
algal and microbial activity did not appreciably shift HCOor CO, levels in the bags. At the end of each experiment,
microplankton were collected in situ by using a peristaltic
pump and 25-mm in-line filter units fitted with 20-p.mpore-size Nitex mesh disks. Water was then taken to the
laboratory for further fractionation (<30 min later).
Laboratory procedures. Phytoplankton fractions follow
the definitions of Sieburth et al. (38); microplankton cells
or colonies of <200 to >20 p.m: nanoplankton
<20 to >2
pLm; and picoplankton
<2 to >0.2 p.m. Following microplankton removal, filtrate was filtered twice more under
low vacuum pressure (<25 to 100 mm Hg; 20) in the
laboratory. A series of 1,000-ml Nalgene polysulfone filter
units were linked in series to process several sample depths
simultaneously. Nanoplankton were collected on 2.0-p.mpore-size Nuclepore filters (ca. 0.1 to 0.3 liter per sample or
replicate); picoplankton passed through and were later col=
=
=
lected on 0.2-pLm-pore-size Nuclepore filters (0.1 to 0.2 liter
per sample, 2-p.m-pore-size filters retained <1% of cells of
Svnellcococciis sp. laboratory cultures). Filters were placed
in extraction vials containing 3 to 4-ml of neutral (with
MgCO3) 90% acetone (1), ground by using a glass pestle and
extracted cold (4°C) in darkness for 18 h. Extracts were
made to 5 ml and centrifuged, and Chl a was measured
spectrophotometrically (Shimadzu UV-160) and corrected
for pheophytin a (21). Water chemistry samples for comparative collections were filtered (0.45-p.m pore size) prior to
analysis to remove organisms and approximate dissolved
fractions only. Soluble reactive phosphate was analyzed by
the antimony-molybdate method; the same procedures were
followed for total dissolved P after persulfate-H,S04 digestion (11). NH44-N was determined via phenol-hypochlorite,
and NO3- was determined by using the sulfanilamideNNED method, following Cd reduction to NO,- (22).
Phytoplankton from the water column and experimental
bags were routinely identified by using an inverted microscope, after Lugol iodine for preservation and sedimentation
(42). Not all picoplankton cells were counted by this method,
and cell numbers were occasionally verified after being
filtered (unpreserved) onto 0.2-pLm-pore-size prestained
black Nuclepore filters and examined by epifluorescence
microscopy (43). Biomass is presented strictly as Chl a,
primarily because many treatments and replicates and several sampling dates could be processed much more efficiently. However, spot comparisons found a reasonable
correspondence between cell numbers and Chl a concentrations. Data were compiled and analyzed by using the
SYSTAT statistical program (47). Formal hypothesis tests
were performed by using regression models with nutrient
levels or N/P ratios as independent variables; the significance of the fit was determined by least-squares analysis
(mathematical solutions via analysis of variance [ANOVA]).
RESULTS
Experiment 1. In June, Calder Lake had begun to stratify
and water clarity was near its maximum (Fig. 2). Soluble
reactive and total soluble P levels declined slightly since
spring turnover (cf. March results: 3 pLg of soluble reactive
phosphate per liter, 7 pg of total P per liter in epilimnion).
Inorganic N in the epilimnion was essentially unchanged
since turnover. Chlorophyll densities were similar among the
three size fractions, with little difference with respect to
depth. Control microcosms and (surface) reference lake
water samples contained phytoplankton in the following
proportions: picoplankton > nanoplankton > microplankton
(Fig. 3). Total phytoplankton biomass increased in response
to nutrient additions, both N and P loading caused a shift to
nanoplankton-dominated communities. Microplankton re-
mained the size fraction with the lowest biomass levels,
regardless of nutrient regimen. ANOVA tests indicate that
microplankton, while present at lower levels, did not respond significantly to either nutrient (Table 1). Nanoplankton were very strongly N limited and also significantly P
limited. Picoplankton responded significantly only to N
additions. No significant effects of N/P ratio were observed.
Communities in unfertilized bags and in surface waters in
the lake in early June consisted primarily of a Synechococ(ciS sp. and also CvNptoinonas erosa, Ci-yptoinonas oVata,
and the chrysophyte Diniobrvon cylindricuin. Following N
additions (N/P ratios .25), many phytoplankton species
increased, including a Mailloniona.s sp. 2, C. ovata, Eiinotia
cf. c(Irl'nt(i, and 1). cylill1r icitn. Synechococcus abundance
Vol. SE. 1989
NUTRIENT LIMITATION IN PICOPLANKTON
PHOSPHORUS (sg / L)
5
1.
o
E
3.
0Q
4. _
ci)
15
*-S
/
NITROGEN (Cg / L)
100
150
200
50
0
20
0-0 SRP
*
/
2
10
1- .
1607
0-0N03-N
o o
q
0
2
-t
* -S NH4-N
7
TOT-P
2c-
i
4.
/0
\
5
6
TEMPERATURE ( °C )
_Z
0
0~
10
5
20
Chlorophyll a (,t.g/L)
25
34
o
2
3
0-0_0
0
2
c4i)
0-
15
4
Micro
* *0= Nano
1
.
rp
*
2
4-\A = Pico
o
3
0'
:1
4-
0
5
0
A
/
6
FIG. 2. Physical and chemical conditions in Calder Lake on 3 June 1988. when the first microcosm experiment was run. Water clarity is
expressed as Secchi depth (m).
-11"
V.u
-
N11
CP
2
-
am
MICRO
NANO
PICO,
oo
-i~~~~~~~~~~~~~~~~~~~~>-
4.0-
X~~~~~~~~~~~~~~~~
Ofn
FI
2.0
C.
0
2.5
5.0
12.5
25
NITROGEN ADDED (jAM)
50
12.0-J
r2.0- MICRO
0'
9.0-
PICO
Lez-- NANO
cline (Fig. 4). Surface temperatures increased, and stratification was more obvious. Also, a noticeable decline in
nitrogen and, to some extent, phosphorus occurred in the
epilimnion. Nitrogen additions resulted in increases in total
phytoplankton biomass (Fig. 5). P additions also stimulated
phytoplankton growth, but concentrations >2 F.M had little
or no effect on biomass. As in June, picoplankton predominated without fertilization (micrograms of Chl a per liter:
picoplankton, 0.52; microplankton, 0.29; nanoplankton,
0.16). but nanoplankton became the largest proportion of the
total biomass with either N or P addition. At this time,
microplankton also increased with both N (ca. fourfold
greater) and P (ca. sevenfold greater) manipulations. These
large biomass differences were very highly significant, especially in response to N fertilization (Table 1). Unlike the
larger two size classes, picoplankton was not P limited.
Phytoplankton species composition in July within the
0
-J
-J
C"
I
a.
0
0
-j
3.0-
2:
C.
TABLE 1. Tests of nutrient limitation of phytoplankton
biomass in three size fractions within experimental
microcosms in Calder Lake"
6.0
0.0
rl
0
nn
1.0
2.0
4.0
(date)
N addition
P addition
Microplankton
Nanoplankton
Picoplankton
NS"'
+"d
NS
+"
NS
NS
NS
NS
2 (Julv 1988)
Microplankton
Nanoplankton
Picoplankton
+'
+'
+
'
+'
+"
NS
NS
+"
NS
3 (August 1988)
Microplankton
Nanoplankton
Picoplankton
+'
+-
+"
+
1 (June 1988)
10
20
PHOSPHORUS ADDED (IM)
FIG. 3. Effects of N aind P additions on phvtoplankton biomaiss
in microplankton. nainoplankton. and picoplankton size classes
within Calder Lake. June 1988. Error bars indicate ±1 standard
error (/ = 3).
increased slightly. P additions resulted in massive increases
cryptomonads (C. ero.sa plus C. ovatti were >50% of total
cell numbers). which were much less abundant in control
bags. There were also increases in the diatom Fa gilarii
(r-iotonlesis (rare in control bags).
Experiment 2. By July. water clarity had declined and
phytoplankton biomass increased greatly near the thermo-
riatio
Size fraction
Expt
in
Indiependent
+C
+d
NS
N/P
NS
+d
+'
variables were classified on the basis of N and P addition aind
N/P r-aitio aind analyzed by ANOVA.
NS. Not significant.
)
.())1.
d P < 0.)5
P
().()1.
AppI ENVIRON. MICROBIOL.
WEHR
1608
PHOSPHORUS
5
10
(Cg / L)
NITROGEN (jg / L)
15
1
_E
100
50
0
**- NH4-N
/
2- T
2-
1200
0-0 N03-N
0
.
150
0
3.
Q)a)
45.
_
5
Chlorophyll a (,ug/L)
TEMPERATURE ( °C )
10
20
25
15
0
°0
E
1
1k
1
T
2
Q
2
1
3
4
o- - Micro
0 -*= Nano
*
^-A=
Pico
Q)
0
FIG. 4. Physical and chemical conditions in Calder Lake on 7 July 1988. when the second microcosm experiment was run. Water clarity
is expressed as Secchi depth (m). Chi a value at 6-m depth for microplankton. 30.5 jig/liter.
epilimnion and in control bags was fairly diverse. It was
numerically dominated by a Svinechococcus sp. and included
several greens (especially Pedfiastrum borvinl/n, Oocv,stis
bo)rgei, Stichococcis bacillaris, Scenedesmnus quadlricaudla
Anikistr-odes nniis.ficlkatis), the chrysophytes MAillonwnoaitis sp.
8 (7.5 to 9 rim) and D. cvlindricuim, and the cryptomonad C.
la
8.0 -
Ea
Cm
I
IJ~MICRO
N.ANO
O
6.0
0
-J
-J
I
a0
a::
4.02.0-
0.0--
0
0-1
0
5.0 12.5
2.5
NITROGEN ADDED
25
(/hM)
4.0
10
50
12.0
-J
-J
I
-
9.0
MICRO
NANO
PICO
0
-4
00
0
-4
I
6.0
3.0-
00
1.0
2.0
20
PHOSPHORUS ADDED (,uM)
FIG. 5. Effects of N and P additions on phytoplankton biomass
in microplankton. nanoplankton. and picoplankton size classes
within Calder Lake. July 1988. Error bar s indicate + 1 standard error
(n = 3).
erosva. In contrast to results of the June experiment. N
additions resulted in large increases (more than 10-fold
greater) in Svnechococcus numbers and also in the numbers
of several chlorophytes observed in control treatments,
especially A. fii1catits and Scenedesinus quadricalda, P
fertilization had a very different response, because of the
greater stimulation of larger phytoplankton, and little effect
on Svneclhococcis abundance. Greens dominated, particularly A. JalcatitIs and P. boryalinimn. Another taxon seen
rarely in control and N treatments, Coelastritm mnicropormn,
was also abundant. Chrysophytes were less abundant in
P-fertilizing treatments.
Experiment 3. In August, chlorophyll densities were at
their greatest levels in Calder Lake, both in surface and
subsurface communities (Fig. 6). The former consisted of
colonial cyanobacteria and the deepwater community of
Ceradinlltira
Indlltiditiella and several smaller taxa (especially
C. erosa). Secchi depth was shallower, and inorganic N and
P were near their summer minima. Experiments revealed
that the entire community (all size classes) responded positively to N additions, but there was an apparent threshold
near 12.5 pLmol/liter (Fig. 7). There were also very large
increases in nanoplankton and microplankton with only
1-[.mol/liter additions of P, with no further enhancement at
greater levels. These were the largest biomass levels measured in any experiment and were more than an order of
magnitude greater than controls. No consistent response
was seen in the picoplankton. ANOVA indicates that nitrogen alone stimulated all fractions most strongly and that
picoplankton was again unaffected by P fertilization (Table
1). Even though the lake most strongly stratified in August,
P limitation was apparently less severe for nanoplankton and
microplankton than in July.
Control communities were considerably different from
those observed earlier, consisting of colonial cyanobacteria
A t(iabenai linnetiuca, Microcvstis (lrl-lginoSoI, and a Synechococcus sp., as well as Malloinonatis cf. caudlata. This
result was quite similar to that observed in the epilimnion
NUTRIENT LIMITATION IN PICOPLANKTON
VOL. 55. 1989
PHOSPHORUS
5
10
(Cg / L)
1609
NITROGEN (.g / L)
100
150
50
15
e-c
_C
0.-
Q)
0
Chlorophyll a
TEMPERATURE ( °C )
5
10
15
25
20
0
30
f%
1
2-
I
~-45-a
64-
5
1
2
(Ag/L)
3
.
i\\/
'~~~~~~ ~~
o0-0- Micro
0~-0= Nano
'
__
FIG. 6. Physical and chemical conditions in Calder Lake on 26 August 1988. when the third microcosm experiment was run. Water clarity
is expressed as Secchi depth (m). Chi a values at 0.1- and 6-m depths for nanoplankton. 5.4 and 27.8 p.g/liter. respectively: for microplankton
at 6 m, 18.2 ,Lg/liter.
during August. In response to nitrogen addition, species
composition shifted slightly, with increases in numbers of
most taxa but especially of the colonial green Gonizuii
pect'orale, which was not common in control treatments.
There was however, a very noticeable decline in heterocyst
production by Anabaenia spp., from an average of 1 hetero-
cyst per 26 vegetative cells in control bags to 1 in 130 after N
fertilization. P-enriched treatments resulted in greater numbers of most taxa, although Aniabaenba and Microcvstis spp.
had only modest (<20%) increases. An unidentified Pyrainiinonas species, rare in other treatments, increased by more
than 10-fold following P additions.
DISCUSSION
-J
-J
0
0
tL
I
-J
I
-C
0
0
25
12.5
5.0
2.5
NITROGEN ADDED
(/iM)
4.0
10
50
a.
0
0
-J
la
Is
I
0
0
1.0
2.0
20
PHOSPHORUS ADDED (14M)
FIG. 7. Effects of N and P additions on phytoplankton biomass
in microplankton, nanoplankton, and picoplankton size classes
within Calder Lake, August 1988. Error bars indicate ±1 standard
error (n = 3).
Calder Lake is a small, eutrophic lake, typical of many
which are found in north temperate regions; its phytoplankton composition and biomass are comparable with those of
well-studied eutrophic systems elsewhere in the world (e.g.,
Lunzer Untersee, Windermere, Mendota; cf. references 16,
31. and 46). As such, it may be seen as a fair place to test the
idea that smaller cells, and picoplankton in particular, are
relatively unimportant in eutrophic systems (15, 40, 44).
Biomass estimates over a period of 2 years in Calder Lake
indicate that picoplankton may predominate over nanoplankton and microplankton despite the presence of large.
colonial cyanobacteria (45).
Direct experiments reported here support the first stated
hypothesis that nutrient enrichment favors a nanoplanktondominated community. This only partly supports earlier
hypotheses, however, as no enrichment treatments or N/P
ratios favored microplankton dominance. In June, there
were large numbers of the large colonial chrysophyte Diniobr-yon sp., and in August. colonial cyanobacteria Anabaenia
and Microcystis spp. were common, yet none of these
microplankters surpassed smaller taxa as the largest proportion of the total phytoplankton biomass. This is somewhat
surprising, as Microcystis spp. have a much lower critical
N/P ratio (ratio = 9) than many chlorophytes, such as
Scenedesinuis spp. (ratio = 30) or Antkistrodesitulis spp. (ratio
= 21: 33). One would expect that under increasing P loading,
N-fixing cyanobacteria should predominate (34). Thus, no
strong evidence was found to identify N/P ratios that can
predict a predominant phytoplankton size class. Greater
1610
WEHR
experimental N/P ratios in Calder Lake did not favor the
dominance of Synechococucis spp., as has been shown in
oligotrophic lakes (41). An artificial reduction in phosphorus
loading within eutrophic Lake Trummen, Sweden, caused a
shift from a "net plankton" (>20 pm) to a nanoplankton
(<20 p.m) community (14); however, picoplankton were not
differentiated from nanoplankton. In the absence of further
nutrient increases in Calder Lake, picoplankton frequently
represent the largest proportion of phytoplankton biomass.
The present study also indicated that nanoplankton and
picoplankton do not respond similarty to nutrient limitation
and should be considered separately.
An alternative explanation for these results would be that
rotifer and protozoan grazing during the 4-day experiments
would likely have been most intense on the smallest phytoplankton size fractions, thus causing a microplankton-dominated community. Preliminary tests of grazer removal from
Calder Lake assemblages by using various pore sizes of
Nitex mesh found that pore sizes finer than 250 pLm collect
significant numbers of algal cells, which were the principal
focus of this study. Some of these, such as D. cvlindricu(m
and Cryptornonas ovata, which were abundant during the
spring, may also act as phagotrophs on smaller algal and
bacterial cells (3, 25). Planktonic protozoans consume significant quantities of heterotrophic and autotrophic picoplankton (26, 36), which may favor larger size fractions in
these experiments. Their activity may also have resulted in
nutrient regeneration within the bags (4, 5). Some protozoans, such as tintinnids, may have been active in consuming
nanoplankton (29), which nonetheless experienced marked
increases in biomass in response to both N and P additions.
The hypothesis that specific nutrients limit different size
classes seems clear. Experiments indicate that microplankton and nanoplankton were limited both by N and P, with P
limitation becoming more intense by midsummer (July). This
dual limitation is known to occur in oligotrophic systems
(41). In contrast, Calder Lake picoplankton were N limited,
although apparently less severely than larger size fractions.
Finally, experiments support the hypothesis that picoplankton were never P limited, even though concentrations of
soluble reactive phosphate in the epilimnion were usually
<0.05 p.M during the summer. This would be expected, as
greater surface-area-to-volume ratios can affect nutrient
absorption rates and result in intrinsically thinner unstirred
layers at the cell surface (30). Protozoans may have increased the regeneration of P (2, 4) within the bags, although
this apparently did not alleviate P limitation in larger size
fractions. From this study, one may conclude that picoplankton may still dominate phytoplankton communities in
eutrophic lakes, particularly as seasonal periods of nutrient
depletion may limit the growth of larger species.
Phytoplankton have considerable effects on nutrient cycles through utilization and storage of key elements (17, 35).
The relative importance of heterotrophic and autotrophic
picoplankton within the microbial loop as sinks or sources of
N and P has become better known. Rapidly growing rotifers,
microflagellates, and other protozoans which graze on these
cells can accelerate the cycling of nutrients within the loop,
which reduces losses of nutrients from the pelagic zone (2, 4,
12). This study suggests that algal picoplankton may play a
central role not only in microbial food webs but also as
nutrient recyclers, through superior competitive abilities and
subsequent consumption by micrograzers.
APPL. ENVIRON. MICROBIOL.
ACKNOWLEDGMENTS
Financial support for this study was provided by an endowed
grant from the J. P. Routh Foundation and a Faculty Research Grant
from the Research Office of Fordham University.
Technical assistance was provided by T. Toolan.
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