simek, karel, jitka bobková, miroslav macek, jim nedoma

Oceanogr., 40(6), 1995, 1077-1090
0 1995, by the American Society of Limnology and Oceanography, Inc.
Limnol.
Ciliate grazing on picoplankton in a eutrophic reservoir
during the summer phytoplankton maximum: A study at the
species and community level
Karel ,%nek, Jitka Bobkovh, Mroslav Macek, and Jif’i Nedoma
Hydrobiological
Institute
of the Czech Academy of Sciences, Na sadkach 7, Ccske Budejovice
37005, Czech Republic
Roland Psenner
Institute
of Zoology,
Abstract
University
of Innsbruck,
Technikcrstrasse
25, A-6020 Innsbruck,
Austria
’
In late summer 1993 an intensive study was carried out on protozoan grazing in the epilimnion and
metalimnion of the eutrophic Rimov Reservoir in south Bohemia. On average, - 70% of bacterial production
was consumed by heterotrophic flagellates and -20% by ciliates. Ciliate numbers increased from 5 to 70
cells ml l over the 5-week study period. Ciliates ~30 pm in size were numerically dominant in both layers
and included Halteria grandinella and Strobilidium hexakinetum (Oligotrichida), Cyrtolophosis mucicola
(Cyrtolophosida), Cinetochilum margaritaceum (Scuticociliatida), Urotricha spp., and Coleps sp. (Prostomatida). Ciliate species-specific grazing rates on bacteria and picocyanobacteria were determined. The highest
individual cell grazing rates, 4,200 bacteria and 560 picocyanobacteria cell-’ h-l, were observed in Vorticella
aquadulcis-complex. Oligotrichs ingested on average 360-2,130 bacteria and 76-2 10 picocyanobacteria cell ’
h - I, with H. grandinella (1,560 bacteria cell I h-l), due to its high abundance, as the most important ciliate
bacterivore within the system. C. mucicola ingested on average 173 bacteria and 27 cyanobacteria cell- ’ h- l;
C. margaritaceum, 57 bacteria and 7 picocyanobacteria cell-’ h--l; and prostomatids, 23-100 bacteria and
2-14 picocyanobacteria cell-’ h- I. Although there was a tight relationship between grazing rates on bacteria
and picocyanobacteria (rs = 0.89, n = 12, P < 0.00 I), most of the ciliate species preferred larger picoplankton
(i.e. picocyanobacteria), as indicated by their clearance rates. According to our data, several oligotrichous
ciliate species and Cyclidium sp. can grow in pelagic conditions and exclusively on picoplankton food at
rates of one doubling every 24-75 h.
abundances are > 5 x lo6 ml-’ (Fenchcl 1980). To date,
very few reports are available in which bacterivory or
picoplanktivory
of freshwater pelagic ciliates have been
well documented in situ (Sherr ct al. 199 1; Simek and
Straikrabova
1992; Sommaruga and Psenner 1993). On
the other hand, increasing cvidcncc from marine systems
indicates that some ciliate taxa arc voracious consumers
of bacteria (Sherr and Sherr 1987, E. B. Sherr ct al. 1989;
B. F. Sherr et al. 1989). Most of these data were obtained
by direct microscopical inspection of the ciliate’s food
vacuole content after fluorescent prey labcling (Sherr ct
al. 1987). This approach is valuable for studies at the
community lcvcl but gives little information
about the
role of diffcrcnt ciliate taxa. That is why data of such
studies arc mostly interpreted in terms of the trophic role
of ciliates or dominant members of ciliate communities
in carbon flow through microbial food webs. Because the
Acknowledgments
methods used by ciliate taxonomists are time consuming
We thank John Stockner, Michael Landry, Helga Miiller, Ja(Montagncs and Lynn 1987; Finlay et al. 1988), there is
kob Pernthaler, Ruben Sommaruga, Vera Straskrabova, Jarosno report that combines fluorescent tracer techniques with
lav Vrba, and two anonymous referees for comments on earlier
the taxonomic survey of the whole ciliate community
versions of the manuscript and Wilhelm Foissner for help in
related to the trophic role of a single ciliate taxon.
identifying several ciliate species. We also thank Helga Miiller
We report an attempt to combine taxonomic approachfor supplying the reference cultures of Urotricha furcata and
es with approaches at the community level to assess the
Balanion planctonicum, and Angela Schwarzenbacher for sizing
significance of ciliate grazing on bacterial and picocyanoof picoplankton.
bacterial populations in a pelagic ecosystem. The 5-week
This study was supported by the Austrian Ministry of Eduprogram was conducted in a eutrophic reservoir during
cation (GZ 45.28 l/3-IV/6a/93) and by CAS GA research grant
6 17 102 awarded to K.S.
the summer phytoplankton
bloom. Sampling frequency
1077
The dominant role of heterotrophic flagellates as primary grazers of bacteria and picophytoplankton
in most
freshwater ecosystems has been well documented (e.g.
Porter et al. 1985; Sanders et al. 1989; Weisse et al. 1990).
However, ciliates also constitute an important component of freshwater pelagic ecosystems (e.g. Pace 1982;
Finlay et al. 1988). Recent work has recognized that generally small species (mostly ~30 pm) numerically dominate ciliate communities in most of the meso- to eutrophic lakes (Beaver and Crisman 1989; Miiller 1989)
and that they can exploit a variety of food resources ranging from picoplankton
(<2 pm) to nanophytoplankton
(~2 to ~20 pm; Sherr et al. 1991).
Bacterivory has been considered an important carbon
source for freshwater pelagic ciliates only when bacterial
1078
Simek et al.
was 3 times a week, which should bc close to the doubling
times of the dominant members of the reservoir microbial
food web, i.e. bacteria, flagellates, ciliates, and pica-, and
nanophytoplankton
(Vyhnalek 1989; Simek et al. 1990a;
StraSkrabova et al. 1993). Study objectives were to compare the dynamics of ciliate populations in the epilimnion
and metalimnion
in relation to the development of phytoplankton and zooplankton in the reservoir, to compare
the relative importance of ciliates and heterotrophic flagellates in planktonic bacterivory, to determine spcciesspecific grazing rates of ciliates on bacteria and picocyanobactcria, to determine whether ciliates are size-selcctive when feeding on picoplankton particles available in
the reservoir, and to determine whether reservoir picoplankton, as a sole food source, could meet all carbon
requirements of ciliates.
Materials and methods
Sampling- Water samples were collected from the Rimov Reservoir (south Bohemia, 470 m a.s.1.; area, 2.06
km2; volume, 34.5 x lo6 m3; max depth, 43 m; mean
depth, 16.5 m; mean retention time, 100 d; dimictic,
eutrophic) 3 times a week from 16 August-23 September
1993. The study site was located -250 m from the dam
of the reservoir.
Oxygen and temperature depth profiles were obtained
with an oxymeter type OXY 196 (WTW, Germany). Samples were taken with a 2-liter Friedinger sampler from
two layers: the epilimnion (a mixed sample from 1LO.5
m) and the metalimnion (a mixed sample from the thermocline +0.5 m) characterized by a concurrent strong
decrease of both temperature and oxygen. Seven samples
from each of the three depths in the epilimnion and metalimnion were mixed in a 50-liter plastic container to a
final volume of 42 liters. Two liters of the mixed sample
from both layers were used for further phytoplankton and
microzooplankton
analyses. The remaining 40 liters were
filtered through a lOO+m plankton net to concentrate
and remove zooplankton > 100 hrn. Within 30 min the
prescreened water (40 liters) was transported to the lab
for further processing.
Bacterial abundance and biomass- Subsamples were
fixed with Formalin (2% final concn), stained with DAPI
(final concn, 0.2O/0 wt/vol), and enumerated by epifluorescence microscopy (Olympus BH2). We sized between
400 and 600 bacteria by semiautomatic image analysis
(Lucia, Laboratory Imaging, Prague), carefully avoiding
interference from fluorescent detrital particles and autofluorescent cells. The system was calibrated with fluorescent microspheres, and volumes were calculated as dcscribed by Psenner (1993). Bacterial biomass was calculated according to the allometric relationship between cell
volume and carbon content described by Norland (1993).
Bacterial production - Bacterial production was measured via thymidine incorporation
with a method modificd from Riemann and Sendergaard ( 1986a). Duplicate
5-ml subsamples were incubated for 30 min at in situ
temperature with 10 nmol liter- l of [ methyZ-3H]thymidine
(Amersham) in the presence of different concentrations
(0, 10, 25, and 40 nmol liter- I) of unlabeled thymidine
(Sigma), then preserved with neutral buffered Formalin
(2% final concn), passed through 0.2~pm membrane filters
(Synpor, cellulose-nitrate),
and extracted 10 times with 1
ml of ice-cold 5% TCA. Replicate blanks prefixed by 2%
Formalin were processed in parallel. With the exception
of two experiments, no isotope dilution of [3H]thymidine
was found by the method of Moriarty and Pollard (198 1).
Empirical conversion factors (ECF) between thymidine
incorporation rate and bacterial cell production rate were
determined by incubating replicate 7 50-ml subsamples
of water passed through l-pm filters (Poretics) for 24 h
in the dark at in situ temperature. The cell production
rate was calculated from the slope of the increase of In
bacterial abundance over time (0, 12, 24 h). We used the
average ECF (2.252 1.38 and 2.74k2.27 x lo’* cells
mol-l, n = 16, for the epilimnion and metalimnion,
respectively) for calculations.
Protozoan grazing and abundance-Two kinds of prey
were used to measure protozoan grazing on picoplankton:
fluorescently labeled bacterioplankton
(FLB) and fluorescently labeled Synechococcus -like cyanobacteria
(FLC). Bacterioplankton
from the reservoir was concentrated on 0.2-pm pore-size filters after prefiltration through
2-pm pore-size filters (Poretics) (see Simek and StraSkrabova 1992). A Synechococcus-like species isolated from
the reservoir (mean cell volume -t SD, 0.48kO.23 pm3)
was grown in a culture and harvested by centrifugation.
Both kinds of prey were fluorescently labeled according
to the protocol of Sherr et al. (1987).
For grazing experiments, 500-ml samples without replicates were dispensed into acid-soaked and rinsed 1-liter
flasks and incubated at in situ temperature for 15 min (to
allow the protozoa to recover from the handling shock).
Flagellate and ciliate uptake rates were determined in the
same treatment, but separately for the two different tracers - FLB and FLC. Both tracers were added to constitute
5-l 5% of their natural abundances. However, the FLC
addition constituted up to 40% of picocyanobacteria
abundance at the beginning of September when abundances of picocyanobacteria
already dropped below
2.5 x lo4 ml-l. For the rest of the study, the use of the
tracer technique was impossible due to low picocyanobacteria abundance. Thirty-milliliter
subsamples for protozoan enumeration and tracer ingestion determinations
were taken 0,3,6, 10,20, and 30 min after tracer addition
and fixed by adding 0.5% of alkaline Lugol’s solution
immediately followed by 2% borate-buffered Formalin
(final concn) and several drops of 3% sodium thiosulfate
to clear the Lugol’s color (B. F. Sherr et al. 1989).
Previous measurements (Simek and Straskrabovi 1992)
have shown that grazing rates for bacterivorous ciliates
and flagellates in the reservoir were different by roughly
one order of magnitude. Therefore we determined ciliate
grazing rates in time series from 3- to lo-min subsamples
and flagellate grazing rates in subsamples from 10 to 30
min. Samples from zero time were also inspected to avoid
Ciliate picoplankton grazing
potential bias of our data due to attachment of noningested tracers on protozoan surfaces. Five-milliliter
(flagellates) or 20-30-ml (ciliates) subsamples were stained
with DAPI, passed through l-pm black Poretics filters,
and inspected via epifluorescence microscopy. All samples were inspected within 24 h after preservation, and
nonpigmentcd, heterotrophic nanoflagellates (HNF), and
plastidic flagellates were always differentiated. At least 30
ciliates and 50 flagellates were inspected for FLB and FLC
ingestion in each sample. Uptake rates of the tracers were
calculated from the changes of average number of tracers
per protozoan cell with time using linear regression. To
estimate total protozoan grazing, WC multiplied average
uptake rates of ciliates and flagellates by their in situ
abundances. Each ciliate individual
was also inspected
for uptake of natural phytoplankton;
ingested algae or
cyanobacteria were measured with an ocular micrometer
and sorted into two groups: pica-sized (< 2 pm) and nanosized (> 2-c 20 pm) phytoplankton
prey.
Because protargol staining (see below) and live sample
observation were applied in parallel with fluorescent microscopy for ciliates, we determined most of the ciliates
to the level of genus and, where possible, to species (Table
1). Usually between 5 and 15% of the ciliates could not
be identified. Ciliates observed with a fluorescent microscope are difficult to identify, so we used additional criteria, such as size and position of nuclei and the way in
which prey were arranged in the food vacuoles. We were
thus able to identify and distinguish the dominant taxa
determined with the protargol technique and live observation (Table 1). Moreover, most of the dominant ciliate
species in the reservoir have been isolated and identified,
so we used these reference cultures in both protargol and
fluorescent microscopy to confirm our determinations.
We based our identifications
on the publications of Foissner et al. (1991, 1992, 1994) and references therein.
Due to a low number of ciliate individuals of each taxon
inspected for grazing per sample, the grazing data of the
same taxa (only unambiguously
identified ciliates) from
the whole study period were pooled to calculate specific
grazing rates at the genus or species level (Table 2). To
estimate mean cell volumes, we measured all ciliates inspected for tracer uptake in preserved samples and calculated the volumes by approximation
to prolate spheroids. Ciliate cell C was estimated with a conversion factor of 140 fg C pm-3 recommended for Formalin-preserved samples (Putt and Stoeckcr 1989), because ciliates
were fixed with 0.5% alkaline Lugol’s solution followed
by 2% Formalin (final concn) 2-3 s later.
Protargol staining- Ciliate samples were fixed with
Bouin’s fixative (5%). Ten-forty milliliters
of a sample
were passed onto a nitrocellulosc
membrane filter (1.2pm pore-size, Millipore),
mounted in agar, fixed with
Formalin, and stained with protargol at 40-60°C according to the Skibbe’s (1994) modification of the method
of Montagncs and Lynn (1987).
Zooplankton-Zooplankton
> 100 pm were concentrated from the 40 liters of filtcrcd water (see above),
1079
Table 1. Mean cell volume of major ciliate taxa in the reservoir after their fixation with 0.5% alkaline Lugol’s solution
immcdiatcly followed by 2% Formalin (final concn).
Oligotrichida
Halteria grandinella
-Strobilidium hexakinetum
Strobilidium sp. (N 35 pm)
Oligotrichous ciliate on
Staurastrum
Peritrichida
Vorticella aquadulcis-complex
Cyrtolophosida
Cyrtolophosis mucicola
Scuticociliatida
Cyclidium sp.
Cinetochilum margaritaceum
Prostomatea
Urotricha spp. (> 90%
U. furcata)
Urotricha sp., small
Balanion-like ciliate
Coleps sp.
Litostomatea
Litonotus sp.
Lacrymaria sp.
Cell volume + SD
(m’)
?I
2,860+ 1,380
1,230+690
10,050-t-4,850
235
99
22
1,530-t- 1,130
45
8,940+4,940
33
2,050-t 1,590
146
1,070+510
3,290+ 1,400
46
106
93Ok690
234
72
46
87
450&210
1,16O-t630
2,850& 1,890
6,500+3,950
15,500_+17,600
5
5
preserved in 4% Formalin (final concn), and quantified
by direct microscopical counting of several subsamples
(McCauley 1984).
Picocyanobacteria abundance- Among picocyanobacteria, Synechococcus-like species (mean cell volume N 0.5
pm3) and Microcystis incerta of a similar mean cell volume dominated. Because the latter spccics is partly floc
forming, samples were sonicated for 1 min (ultrasonic
homogenizer 47 10, Cole Palmer Instr. Co., set at 1) before
counting picocyanobacteria
with fluorescent microscopy
and DAPI staining.
Phytoplankton -Chl a concentration in prescreened
water (< 100 pm) was determined after passage through
Whatman GF/C filters. Filters with retained seston were
ground, extracted in 90% acetone, and measured spectrophotometrically
according to Lorenzcn (1967). Samples for phytoplankton
counting were preserved with Lugol’s solution and abundances of respective species were
enumerated in Utcrmijhl
settling chambers on an inverted microscope. Ccl1 volumes of the species were calculated according to their linear dimensions measured on
live algae using appropriate geometric formulas corresponding to algal shapes. The only exception was cryptomonads that, due to their mobility, had to be preserved
with Lugol’s solution before being measured. Their volumes were corrected for shrinkage according to Mon-
1080
&mek et al.
Table 2. Per-cell grazing rates of ciliates (cells ciliate ’ h -I) on bacteria and picocyanobacteria calculated on the genus or species level, and the proportion of ciliate algivory on
phytoplankton > 2 pm observed within the ciliates inspected. Numbers are average values for
n individuals.
Grazing rates on picoplankton
Vorticella aquadulcis-complex
Strobilidium sp. (N 35 pm)
Halteria grandinella
Cyclidium sp.
Lacrymaria sp.
Oligotrichous ciliate on Staurastrum
Strobilidium hexakinetum
Litonotus sp.
Cyrtolophosis mucicola
Balanion-like species
Coleps sp.
Cinetochilum margaritaceum
Urotricha spp. (>90% U. furcata)
Urotricha sp., small
* Ingestio n 0f algae >2 pm.
t Total ciliates inspected.
Bacteria
n
Pcyano
n
4,200
2,130
1,580
470
470
440
380
365
173
100
63
57
23
61
23
12
118
30
5
29
72
5
115
34
65
76
151
49
560
120
210
80
90
76
27
3
8
10
10
117
16
0
16
27
0
30
12
22
30
83
23
tagnes ct al. (1994). Phytoplankton
biomass was expressed as algal fresh mass, where 1 mm3 = 1 mg.
Results
The water column of the Rimov Reservoir was thermally stratified, and temperatures slowly decreased in
both layers during the study period (data not shown).
Epilimnetic
(l-m depth) water temperatures were between 15 and 22°C and metalimnetic temperatures between 14 and 20°C. The metalimnion
was between 3.5and 5-m depth in the first days of the study, then it shifted
to between 5 and 7 m. Although epilimnetic waters were
always oxygen saturated, oxygen concentrations were from
0.6 to 7.3 mg liter-l in the metalimnion, with the strongest oxygen depletion (0.6-3.2 mg liter-‘) from 30 August
to 8 September (not shown).
Secchi disk transparency (data not shown) in the surface
water decreased from 3.1 to 0.9 m, with a corresponding
increase in Chl a concentrations from - 7 to 112 pg liter-l
in the epilimnion
and from 10 to 66 pg liter-’ in the
metalimnion,
reaching maxima in both layers on 15 September (Fig. 1). Phytoplankton
biomass was dominated
by diatoms and chlorophytes (Fig. 2). Conspicuous peaks
in phytoplankton biomass in both layers were mainly due
to the dominance of Staurastrum pingue and Fragilaria
crotonensis and to lesser degree by Eudorina elegans. The
proportion
of diatoms was usually much larger in the
metalimnion
than in the epilimnion (Fig. 2). At the beginning of the study, cyanobacteria accounted for a large
proportion of phytoplankton
biomass, being dominated
by pica-sized Synechococcus-like species and M. incerta,
and also partly by larger Microcystis aeruginosa. A drop
2
14
Algivory
Inges- Cilition* atest
VW
0-0
33
3
10
22
2
235
0
46
0
5
7
45
99
3
0
5
7
146
20
46
9
87
32
106
62
234
8
72
in picocyanobacteria
abundances (Fig. 1) was paralleled
by a decrease in the total cyanobacterial biomass in both
layers (Fig. 2). Ceratium hirundinella
(Dinophyceae)
formed a significant proportion
of phytoplankton
biomass only in the epilimnion
during the first days of the
study. Small, slowly increasing populations of Cryptophyta (especially Rhodomonas sp. and Cryptomonas sp.)
were in the epilimnion. Their percentage in the metalimnion was much lower, but relatively stable throughout
the study period.
Population densities of bacteria, cyanobacteria, HNF,
and ciliates showed distinct fluctuations, but we observed
a similar pattern and range of values in both layers (Figs.
1 and 3). Bacterial abundance (organisms ml-l) was relatively stable, ranging from 2.1 to 4.4 x 106. The number
of picocyanobacteria dropped sharply from - 3 to 4 x 1O5
at the beginning to <2 x lo3 by the end of the study (Fig.
1). HNF abundances ranged from 1.77 to 4.46 x lo3 in
the epilimnion and from 1.34 to 3.88 x lo3 in the metalimnion (Fig. 1); generally, the ratio between HNF abundance and bacterial abundance was - 1: 1,000. Ciliate
abundances increased throughout the study from 6 to 55
ml-l in the epilimnion
and from 13 to 70 ml-l in the
mctalimnion (Fig. 3). The ranges of bacterial cell volumes
were similar in the epilimnion
(0.039-0.066 pm”) and
metalimnion (0.036-0.049 pm3, Fig. 1). Mean ciliate cell
volumes were small in both layers, roughly between 1,400
and 4,500 pm3 (Fig. 3), because the ciliate community
was dominated by small species (Table 1; Fig. 4), usually
with > 95% of all ciliates < 30 pm and 75% ~20 pm long.
A comparison of estimated rates of bacterioplankton
production and protozoan grazing on bacteria for each
discrete sampling date showed frequent fluctuations of
these two parameters (Fig. 5). On average, however, they
1081
Ciliate picoplankton grazing
METALIMNION
EPILIMNION
5
Cu
4-
16
-o--
Picocyanobacteria
-o-
Chlorophyll
23
August
30
a
6
13
20
September
16
23
30
6
13
20
September
August
Fig. 1. Bacterial abundance, bacterial cell volume, and abundance of heterotrophic nanoflagcllates (HNF) (above) and picocyanobacteria abundance and Chl a concentration (below)
in the epilimnion and metalimnion, August-September 1993.
were roughly balanced such that 9 1% of the bacterial
production in the epilimnion and 92% in the metalimnion
were consumed by protozoa. Total protozoan bacterivory
was divided into HNF grazing and ciliate grazing (Fig.
5). Flagellate populations contributed on average 82 and
79% of the total protozoan grazing in the epilimnion and
metalimnion,
respectively. Ciliate grazing ranged from 8
to 42%, and averaged 18% of the total protozoan bacterivory in the epilimnion and 2 1% in the mctalimnion.
Total ciliate numbers, densities of the six most abundant species, and their proportion to the total ciliate abundance are shown in Figs. 3 and 4. These six species typically represented from 55 to > 80% of the total ciliate
assemblage. There was a significant shift in both abundance and species composition of ciliates throughout the
study. At the beginning of the study, the ciliate assemblages iti both layers were dominated by a species grazing
on picoplankton-Halteria
grandinella. Later, this species was partly replaced by another oligotrich-Strobilidium hexakinetum -in parallel with an increasing proportion of detritofagous species-Coleps
sp. and Cyrtolophosis mucicola (see Fenchel 1968; Foissncr et al. 199 1,
1994) -in September. Predominantly algivorous species,
Urotricha spp. (> 90% consisting of Urotricha furcata)
and partly detritofagous Cinetochilum margaritaceum,
created relatively stable proportions with minor fluctuations within both ciliate communities throughout the
study period.
In both layers, per-cell grazing rates decreased significantly when calculated as the mean values for the whole
ciliate community. The values dropped from 700-2,000
EPILIMNION
2016 -
c--l
Cyano
m
Bacil
L. 1 Dinoph
~~
Crypt0
m
Chloro
k%@
16
20 23
201
2730
3
6
Chryso
10 13
17
22
10 13
17
22
METALIMNION
16
2023
August
2730
3
6
September
Fig. 2. Taxonomic composition of phytoplankton in the epilimnion and metalimnion characterized as fresh mass, AugustSeptember 1993. Cyano - Cyanobacteria including picocyanobacteria; Bacil - Bacillariophyceae; Dinoph - Dinophyceae;
Crypt0 - Cryptophyceae; Chloro - Chlorophyceae; Chryso Chrysophyceae.
1082
simek et al.
EPILIMNION
70
60
--
7
Ciliate abundance
T
--
Cell volume
METALIMNION
1
5000
x
4000
3000
2000
m5
g
,=
P
3
a
1000 2
0
-t0
400
-
2000
Bacteria
grazing
---
Plcocyanobacteria
grazlng
:;<+A
i:;
O~~~,~,~,~,~~~~~~~I~,~~~~~,,~~~~~~~~~~~~~~
16
23
30
6
August
13
:
20
“‘...‘.‘.‘......l..‘.“..‘....‘...’....t
16
23
-A
0
30
6
August
September
13
20
September
Fig. 3. Ciliate abundance and mean cell volume measured on preserved samples (above)
and ciliate mean grazing rates on bacteria and picocyanobacteria (below), August-September
1993.
METALIMNION
EPILIMNION
16
23
August
3.0
6
13
September
22
16
23
August
30
6
13
22
September
Fig. 4. Abundances of the six most abundant species (above) and their proportions in total ciliate numbers (below), AugustSeptember1993. Note that >90% of Urotricha spp. is Urotricha furcata.
Ciliate picoplankton grazing
EPILIMNION
Bact. production
1083
0
2000-
n
Epilimnion
I
0
Ciliate grazing
1600- -r--l Metalimnion
n
I
Flag. grazing
0
I
I
I
800
0
0
I
0
n
P
METALIMNION
5
W
2
240
60
0
160
1
1
I
0
n
20
n
80
0
0
0.
16
23
August
30
6
13
bacteria cell-’ h- ’ and 130-395 picocyanobacteria cell-’
h-l in the beginning of the study to 80-280 bacteria cell-’
h-l and 18-38 picocyanobacteria
cell- ’ h- ’ during the
second part of the study (Fig. 3). This conspicuous decrease in cell-specific grazing rates corresponded to the
decreasing proportion of the most important bacterivore
within the ciliate communities, H. grandinella (cf. Figs.
3 and 4). The grazing rates on both types of pica-sized
food particles per ciliate ccl1 were significantly correlated
with the proportion of II. grandinella in the ciliate community (Fig. 6). On the other hand, when the grazing rate
per ciliate cell was correlated with water temperature or
mean ciliate cell volume, no significant relationships were
found (not shown).
Mean cell volumes of 14 ciliate taxa are presented in
Table 1. These species were inspected for uptake of FLB
r2=
m
22
September
Fig. 5. Bacterial production and protozoan grazing in the
epilimnion and metalimnion, August-September 1993. Total
protozoan bacterivory is divided into flagellate and ciliate bacterivory.
n
i
01
orn@
’
0
’
IO
q
’
’
20
’
’
30
I
1
40
Proportion of Halteria
1
0.61***
I
50
I
I
60
(%)
Fig. 6. Relationships between the proportion of Halteria
grandinella within the ciliate community vs. mean cell-specific
grazing rates of ciliates on bacteria (above, n = 32) and picocyanobacteria (below, n = 18). ***-I? < 0.001.
and FLC to calculate species-specific grazing rates on both
food items (Table 2). The highest grazing rates were observed in vorticellids (4,200 bact. cell-l h-l), followed
by oligotrichous ciliates (380-2,130 bact. cell-l h-l) and
Cyclidium sp. (470 bact. cell-l h-l). Among oligotrichs,
larger species of Strobilidium (partly represented by Strobilidium hyalinum, although not very abundant) and H.
grandinella had the highest ingestion rates. Considering
the abundance of the species (Fig. 4), II. grandinella was
the most important bacterivore in the ciliate community
(see Fig. 7). Two small oligotrichs, S. hexakinetum and
an unidentified oligotrichous ciliate species attached by
a stalk to Staurastrum cells, grazed -400 bact. cell-l h-l.
1084
&mek et al.
EPILIMNION
mother
m
oligotrichsmScuticociIiate:
C. mucicola
Coleps sp.
m
Others
‘HG
OS
a+CY
SH
I
‘SL
n UN
#CO
Cl
n BA
10
METALIMNION
100
1000
Bacteria ciliad
281
10000
K’
Fig. 8. Grazing rates of the 12 most abundant ciliate species
on bacteria vs. their grazing rates on picocyanobacteria. VOVorticella aquadulcis-complex; SL- Strobilidium sp. (large N 35
pm); HG - Halteria grandinella; OS - oligotrich on Staurastrum; CY- Cyclidium sp.; SH-Strobilidium
hexakinetum;
CM-Cyrtolophosis mucicola; UN-small Urotricha sp.; COColeps sp.; CI - Cinetochilum margaritaceum; BA --Balanionlike ciliate; UR- Urotricha spp. (> 90% Urotrichafurcata). ****P < 0.0001.
20
16
12
8
4
0 ,
I,
--‘-I
16
23
30
August
’ 171-l
6
IV
13
20
September
Fig. 7. Proportion of various ciliate groups or taxa in the
total ciliate bacterivory in the epilimnion and metalimnion,
August-September 1993. Other oligotrichs-,!Qrobilidium
hexakinetum, Strobilidium sp. (large -35 pm), and oligotrich on
Staurastrum; scuticociliates- Cyclidium sp. and Cinetochilum
margaritaceum; prostomatids- Urotricha spp. (> 90% Urotricha furcata), small Urotricha sp., and Balanion-like ciliate; others-vorticellids and unidentified ciliate species.
Surprisingly, larger ciliates such as Lacrymaria sp. and
Litonotus sp. (Table I), known as raptorial feeders (Fenchel 1968), ingested bacteria in amounts comparable to
those ingested by small filter-feeding oligotrichs (Table
2). A low mean grazing rate of 173 bact. cell-l h-l was
found for the dctritofagous ciliate C. mucicola. Prostomatids showed grazing rates of I 100 bact. cell-’ h-l.
The ingestion rates were lowest for the small (mean cell
volume 450 pm’), unidentified
Urotricha species, the
crawling scuticociliate C. margaritaceum, and Urotricha
species (> 90% dominated by U.furcata). However, a high
proportion
of prostomatids and C. margaritaceum ingested the larger phytoplankton
species, whereas ingestion of algae > 2 pm by vorticellids and oligotrichs was
rare (Table 2).
Figure 7 shows the proportion of bactcrivory of the
respective ciliate taxa or groups in the total ciliate bacterivory. This figure clearly documents the dominant role
of H. grandinella and of other oligotrichs in the ciliate
bacterivory in both layers for most of the study. Only
during the last 10 d was there a significant increase of
bacterivory of scuticociliates and detritofagous species,
such as C. mucicola and Coleps sp., which numerically
dominated at this time (cf. Fig. 4). Bacterivory of the
ciliate group designated as “others” in Fig. 7 was more
significant at the beginning of the study, mainly due to
grazing of Vorticella aquadulcis-complex, which was more
abundant during the first 10 d, but Fig. 7 also indicates
that the grazing ofprostomatids
on bacteria was negligible
throughout the study. The same pattern as for the ciliate
bacterivory was also found for ciliate grazing on picocyanobacteria (not shown), although these grazing rates
were measured only during the first half of the study (cf.
Fig. 3, lower panels). Again, oligotrichs, especially H.
grandinella, were responsible for > 70% of ciliate grazing
on picocyanobacteria
during this part of the study.
Regardless of the ciliate feeding mode, all species ingested both picoplankton food particles, although in some
cases at low rates (Table 2). Generally, grazing on picocyanobacteria was proportional to that of grazing on bacteria, but roughly an order of magnitude lower (Fig. 8).
This relationship was tested on 12 ciliate species; at least
10 individuals
of each species were inspected for uptake
of both FLB and FLC (Table 2). A tight linear relationship
between grazing rates on bacteria and picocyanobacteria
Ciliate picoplankton gruzing
(Fig. 8) could also be shown by nonparametric Spearman
rank order correlation (rs = 0.89; rr = 12; P < 0.001).
Without regard to prey density, Fig. 8 indicates a ratio
of 10 ingested bacteria to 1 ingested picocyanobacterium.
This type of analysis, however, does not enable us to
distinguish whether ciliates feed selectively on the respective kind of picoplankton particles, which one might
expect considering the differences in size and shape of
prey. Therefore, we determined clearance rates (Fig. 9,
upper panel) by dividing the species-specific grazing rates
(Table 2) by the picoplankton abundance (Fig. 1).
The clearance rates generally showed that all ciliate taxa
but one (Balanion-like ciliate) preferred larger picocyanobacteria. These differences were mostly on the order
of 2 : 1 to 3 : 1 for picocyanobacteria : bacteria. Algivorous
species of Urotricha, where U. furcata accounted for
>90%, had only a slightly higher clearance rate on picocyanobacteria. The other ciliate species strongly preferred the larger picocyanobacteria.
The strongest prcference (by a factor of 3 to 4) toward the larger particles
was found for typical fine filter feeders such as Cyclidium
sp., all oligotrichs except for the large Strobilidium sp..
and for the small unidentified Urotricha species. In terms
of absolute values, there were highly significant differences in clearance rates among the inspected ciliate taxa.
These rates ranged from 7 and 1 1 nl cell- * h- 1(Urotricha
spp.) to 1,240 and 3,150 nl celll’ hh’ (V. aquadu/ciscomplex) for bacteria and picocyanobacteria,
respectively. Most of the typical fine suspension feeders (i.e. small
oligotrichs and Cyclidium sp.) had clearance rates between 130 and 465 nl cell -I h-l for bacteria and between
426 and 1,176 nl cell-l h-l for picocyanobacteria
(Fig.
9, upper panel).
The clearance rates of the ciliate species were divided
by their mean cell volumes (Table 1) to obtain volumespecific clearance rates for both bacteria and picocyanobacteria (Fig. 9, lower panel). With one exception, a Balanion-like ciliate, all ciliate species showed higher volume-specific clearance rates on picocyanobacteria
(by a
factor of -2-4). This parameter showed the ability of I .
aquadulcis-complex, small oligotrichs, and Cvclidium sp.
to compete efficiently for the picoplankton food resources
in the reservoir. Alternatively,
volume-specific clearance
rates were very low for detritofagous Coleps sp. and (‘.
margaritaceum and especially for raptorial prostomatids.
except for the small unidentified species of Urotricha.
Potential top-down and bottom-up control of ciliate
abundance and biomass expressed as nonparametric
Spearman rank correlations (Statgraphics) were analyzed
with parameters of phyto- and zooplankton using an approach similar to that of McQueen et al. (1989). Data
from both layers were analyzed together and separately
for the epilimnion
and metalimnion
(Table 3). We list
only those correlations with the respective plankton groups
that were significant in at least one layer. The correlations
clearly indicate a strong influence of the total phytoplankton biomass characterized as Chl a and dominated by
Chlorophyceae. All correlations with cyanobacteria were
inverse and significant, except for their relationship with
ciliate biomass in the metalimnion. Correlations with the
1085
14
HG
m
12 )
SL
IO i
n
0
2
4
6
/
8
10
12
14
Clearance rate on bacteria
(1 O2nl cilrate-’ he’)
I
1
a
y-jj,Lk
CYm HG
n
401
I
SH v’
j
0s:
301;
/
20-l
l.$l
/
/”
lo-CM
SL
n
n,”
/
2- co
m
,’
Volume-specific clearance rate
on bacteria (1 O4h-l)
Fig. 9. Comparison of clearance rates and volume-specific
clearance rates ofthe 12 most abundant ciliate speciescalculated
from ingestion rates measured on bacterioplankton and picocyanobacteria. Insets show the position of the four species with
the lowest clearance rates and volume-specific
Abbreviations
as in Fig. 8.
clearance rates.
other taxonomic groups (sco Fig. 2) were insignificant.
When the data were correlated with different ciliate species, wc found a strong positive correlation only between
the abundance of II. ,~randirzc//u and the abundance of
picocyanobacteria
(r, = 0.58, n = 32, Y < 0.001). Although bacteria and HNF also were considered as possible
Simek et ui.
1086
Table 3. Coefficients of Spearman rank correlations (r,) of
ciliate numbers and biomass with selected parameters of phytoplankton biomass, its community composition, and selected
parameters of zooplankton for the time series from 16 August
to 22 September. Chl a in both layers, YE= 32; for data from
n = 16. For relations with taxoepilimnion and metalimnion,
nomical groups of phytoplankton,
n = 22 for all data and n =
11 for data from epilimnion
and metalimnion,
respectively.
All-data
from both layers; Epi-epilimnion;
Meta-metalimnion; Chl a-chlorophyll
a concentration; Chloro-Chlorophyceae; Cyano-Cyanophyceae.
Asterisks: *--P 5 0.05; **-P 5
0.01; ***-P 5 0.001.
Chl
a
Cyan0
Chloro
Cyclops
All
Epi
Meta
o.tso***
0.9 1***
0.73**
Ciliate number
0.61**
-0.72***
0.83**
-0.73*
0.59
-0.66*
-0.35
-0.45
-0.65*
All
Epi
Meta
0.63***
0.78**
0.37
Ciliate biomass
0.53*
-0.58**
0.70*
-0.64*
0.61*
-0.56
-0.23
-0.3 1
-0.61*
Rotifers
0.42
0.51*
0.22
0.22
0.43
-0.12
food sources for ciliates, we found no significant relationships between the total ciliate abundance and biomass
vs. abundances of bacteria and HNF.
Zooplankton > 100 pm were sorted into the following
six groups: Daphnia (mean abundance 12 individuals liter-l; range 3-29), Ceriodaphnia and Diaphanosoma
summed together (12; 2-4 l), Cyclops (39; 1O-74), nauplii
(9; 3-23), Diaptomus (9; l-32), and rotifers (38; 2-129).
Abundances were generally low and similar in both layers
without any clear trend, except for rotifers, which were
significantly more abundant in the epilimnion but only
by the end of the study period (data not shown). Our data
indicate a rather negligible grazing impact of zooplankton
in general, and the only inverse significant correlations
were those between ciliate abundance and biomass and
abundance of Cyclops in the metalimnion (Table 3); however, the ciliate biomass was positively correlated with
the abundance of rotifers in the epilimnion,
likely indicating a similar food niche for both groups.
Discussion
Our data support a rough balance between bacterial
production and protozoan grazing previously found in
the reservoir during summer (Simek and Straskrabova
1992) because > 90% of bacterial production was grazed
by protozoa (Fig. 5). Both methods can have relatively
large errors (B. F. Sherr et al. 1989) so this point needs
to be considered carefully to avoid potential biases of
these data. For the preservation of samples, we used the
protocol recommended to avoid an egestion of the food
vacuole content by protozoa (B. F. Sherr et al. 1989).
Several possible sources of over- and underestimates of
our production data exist (see Smits and Riemann 1988;
Simek and Straskrabova 1992), but the main source of
inaccuracy for bacterial production estimates is likely the
factor used for converting thymidine incorporation
to
bacterial cell production. In this study, we tried to minimize this error by use of empirically derived conversion
factors (ECF), i.e. 2.25 and 2.74 x lOi cells per mol of
thymidine incorporated for the epilimnion and metalimnion, respectively. Our ECFs are very close to the theoretical ones, and they are in the range of values reported
for other freshwater eutrophic systems (Riemann and
Sondergaard 1986a,b; Smits and Riemann 1988).
One ofthe purposes of this study was to simultaneously
measure bacterial production and mortality due to protozoan grazing, paying special attention to the role of
ciliates. In previous studies, ciliates were more important
bacterivores than HNF during the summer phytoplankton bloom in the reservoir (Simek et al. 1990a,b) because
they grazed >50% of bacterial production daily (Simek
and Straskrabova 1992). During the 1987 and 1988 seasons (Simek et al. 1990a,b; Simek and Straskrabova 1992)
scuticociliates were the dominant ciliates, especially Cyclidium sp., which had a high per-cell ingestion rate of
bacteria. However, in the present study HNFs were the
main grazes of bacteria in the reservoir, as is the case in
many freshwater lakes (e.g. Sanders et al. 1989; Weisse
et al. 1990; Chrzanowski and Simek 1993).
A slight successive decrease in abundance of ciliates
during 1987-l 990 was observed in the Rimov Reservoir
in parallel with an increase in HNF abundance (see StraSkrabova et al. 1993). In 1991-1993, HNF abundances
during summer phytoplankton
peaks did not show any
trend and were mostly between 1 and 4 x lo3 cells ml-’
(Simek unpubl. data). Ciliate abundance in the course of
our study was slightly higher than it was in summer 1990
(Macek 1994) but similar to abundances reported for
other meso-eutrophic lakes (e.g. Beaver and Crisman 1989
and references therein; Miiller et al. 199 1). In accordance
with recent reports from both freshwater and marine systems (Sanders et al. 1989; Sherr and Sherr 1987; E. B.
Sherr et al. 1989) ciliates were important bacterivores
because they grazed -20% of bacterial production in the
reservoir. However, due to the lower abundance and different species composition, ciliates were not the dominant
bacterial feeders in summer 1993 (cf. Simek et al. 1990b;
Simek and Straskrabova 1992).
Cladocerans might have a strong grazing impact on
ciliate populations (Porter et al. 1979; Jiirgens 1994), especially on small species (Jack and Gilbert 1993). Generally, control by zooplankton on ciliates seemed to be
rather weak because zooplankton were at very low abundances except for Cyclol~, which likely had more significant impact on ciliate abundance and biomass, at least
in the metalimnion (Table 3). The important role of ciliates in the diet of copepods has already been reported
(e.g. Korniyenko
1976; Stone et al. 1993). Along with
indications of the probable weak top-down control from
zooplankton (Table 3) the principal factor determining
ciliate growth and species composition seems to be the
dynamic changes in phytoplankton
composition (Finlay
et al. 1988; Beaver and Crisman 1989; Miiller et al. 199 1).
Considering potential food limitations of ciliates in the
reservoir, WC:divided the study period into two parts.
Ciliate picoplankton grazing
During the first part, until the end of August, fine suspension-feeding ciliates did not seem to be food limited
because along with bacterioplankton,
picocyanobacteria
also were available. During the second part of the study
period, fine suspension-feeding ciliates were likely food
limited because most of the bacteria present had very
small cell volumes. Moreover, picocyanobacteria dropped
to a negligible level (Fig. 1) and almost no algal cells
within the size range of 2-6 pm were available. Typical
raptorial feeders, such as prostomatids, had to be at least
partly food limited because the phytoplankton
composition became dominated by large algae throughout the
study. Although the total phytoplankton biomass was very
high for most of the study, only a minor portion of the
biomass (mainly cryptomonads) was directly available as
food for the ciliates.
An alternative food resource, especially for ciliate raptorial feeders, could bc HNFs (reported also by Wcisse
et al. 1990), which were sufficiently abundant throughout
the study (Fig. 1). We did not measure grazing of ciliates
on HNF, however, this potential food linkage was not
indicated by a significant correlation at either the community or ciliate species level (Simek unpubl. data). Ciliate abundance and biomass showed very tight correlations with Chl a concentration
and, more specifically,
with respective groups of phytoplankton
(Table 3). Phytoplankton development during summer 1993, however,
showed a pattern different from patterns observed in previous years (Komarkova 1994). Instead of a strong dominancc of cyanobacteria, this taxonomic group was soon
replaced by the bloom of chlorophytes dominated by S.
pingue. There was a corresponding shift in ciliate species
composition (Fig. 4).
H. grandinella, which dominated the ciliate community at the beginning of the study, was the most important
picoplanktivorc
within the system (Figs. 6, 7; Table 2).
H. grandinella was dependent not only on the bacterial
diet, but also on the availability
of picocyanobacteria
because the abundances of both were correlated and declincd in parallel during the course of the study. During
the second part of the study, there was an increased proportion of another oligotrich (i.e. S. hexakinetum, an unidentified oligotrich on Staurastrum) and a scuticociliate
Cyclidium sp. However, the relationships between their
appearances and changes in picocyanobacterial
abundance were unclear. The common features of these species
were small cell volumes, relatively high uptake rates of
picoplankton, and rare ingestion of algae >2 pm (cf. Tables 1 and 2). The unidentified oligotrichous ciliate attached on Staurastrum cells appeared in higher abundance in parallel with increasing density of this algae,
which likely created a very specific niche for this ciliate.
Our data demonstrated that small oligotrichs were the
most important ciliate picoplanktivores
in this eutrophic
reservoir (Fig. 7) and moreover that their volume-specific
clearance rates (Fig. 9) indicate the ability of these ciliate
species and Cyclidium sp. to compete for the picoplankton food resources with some typical bacterivorous HNF
(see Fenchel 1986). The numerical dominance of the oligotrichs within the ciliate assemblages during the first half
1087
of the study seemed to be partly related to the occurrence
of larger picoplankton because picocyanobacteria
at this
time accounted for 20-45% of the organic carbon available as picoplankton
prey (not shown). Bacterivory by
oligotrich ciliates has been reported for many species (e.g.
Fenchel and Jonsson 1988; Ohman and Snyder 1991;
Pierce and Turner 1992). Nevertheless, some studies have
suggested that ciliates cannot survive exclusively on bacteria in pelagic systems because they require high bacterial
concentrations (Fenchel 1980; Gast 1985; Stoecker 1988)
that are usually found only in specific environments, such
as the chemoclinc, oxycline, etc. (Fenchel et al. 1990).
However, more recent data from coastal waters (E. B.
Sherr et al. 1989) indicate that lo6 bacteria ml-’ might
be sufficient to allow choreotrich ciliates < 15 pm to grow
at a rate of one doubling per 48 h.
WC have preliminarily
reported that Cyclidium sp., the
species regularly occurring in the pelagic system of the
reservoir during summer-fall (Macek 1994), could meet
most of its carbon requirements by feeding exclusively
on bacterioplankton
(Simek and Stragkrabova 1992).
Bacteria alone have been reported as able to support
growth of oligotrich ciliates under laboratory conditions
(Rivier et al. 1985; Ohman and Snyder 199 1). Our recent
data address the question of whether the reservoir picoplankton was sufficient as a sole food source to support
growth of the small oligotrichs and Cyclidium sp. From
the six most abundant species (Fig. 4), 87-235 individuals
were inspected for feeding on bacteria and picocyanobacteria in the course of the study, which also makes our
data more robust statistically than data of previous studies. Bacterial abundances (2-4.5 x 1O6ml- I) and bacterial
mean cell volumes measured by image analysis were rather low (cf. Krambeck 1988; Bjornsen et al. 1989; Psenner
and Sommaruga 1992) considering the trophic state of
the reservoir. However, an almost equal amount of organic carbon was available from picocyanobacteria, which
was slightly higher than during the picocyanobacterial
maximum reported for Lake Constance (Weisse et al.
1990).
Species-specific cell volumes of the ciliates and cell
volumes of their ingested picoplankton prey were transformed to carbon using the following conversion factors
(in fg C pm-3): ciliates- 140 (Putt and Stoecker 1989);
bacteria-287
(calculated according to Norland 1993,
corresponding to the mean cell volume of the reservoir
bacteria, i.e. 0.047 ,um3); picocyanobacteria-200
(Wcisse
1993). If we assume gross growth effrciencics for bacterivorous ciliates between 20 and 40% (Turley et al. 1986;
Capriulo 1990) and use our estimates of ingestion, the
doubling times for the following fine suspension-feeding
ciliate species would be: H. grandinella (24-48 h), S. hexakinetum (35-70 h), the oligotrich on Staurastrum (3875 h), Cyclidium sp. (27-54 h), and V. aquadulcis-complex (29-58 h). These rates are consistent with those in
other studies in which different methods were used to
estimate the gcncration times of ciliates (e.g. Miiller 1989;
Taylor and Johannsson 199 1). Thus, picoplanktivory
possibly could be the prevailing feeding mode for some
typical pelagic ciliates in a reservoir dominated by small
1088
&mek et al.
species. Moreover, these ciliate taxa rarely ingested algae
>2 pm (Table 2).
For other species of ciliates listed in Table 2, procaryote
picoplanktivory
did not stem to be sufficient to support
the assumed ciliate growth without additional food from
larger autotrophic
and heterotrophic
eucaryotes. The
finding of relatively high ingestion rates of bacteria by
raptorial feeders (Fenchel 1968) such as Litonotus sp. and
Lacrymaria sp. (Table 2) however, calls for attention
because probably there are no comparable data from natural aquatic systems in the literature. One might argue
that the estimated ingestion rates are artifacts because
Litonotus sp. and Lacrymaria sp. could be eating ciliates
that had ingested labeled picoplankton;
however, the
probability
of this seems extremely low. We estimated
grazing rates in time series as short as 3-10 min. Even if
a ciliate “labeled” by picoplankton had been ingested by
the predator ciliate, the preyed ciliate would not have
been digested during the 10 min; hence, its nuclei (with
typical morphology)
would be clearly distinguishable
within a food vacuole of the predator ciliate.
The low grazing rates of detritofagous species, C. mucicola (173 bacteria and 27 cyanobacteria cell-l h-l) and
Coleps sp. (63 bacteria and 8 cyanobacteria cell-l h-l),
indicate that free-living suspended picoplankton
played
only a minor role in the diet of these ciliates. Similar
grazing rates also were found for C. margaritaceum (Table
2), which is considered to be partly detritofagous (reviewed by Foissner et al. 1994). However, we frequently
observed this species with ingested single cells of M. aeruginosa, which seemed to be an opportunistic food niche
of this ciliate that probably crawled around floes of cyanobacteria in the reservoir. Generally, the numerical
dominance of “browsers” and mostly detritofagous ciliate
species (i.e. C. mucicola, Coleps sp., and C. margaritaceum) during the last third of the study (Fig. 4) seemed
to be related to the increasing abundance of large or flocforming phytoplankters,
such as S. pingue and F. crotonensis (Fig. 2). Moreover, volume-specific clearance rates
of these species (Fig. 9) indicate their inability to compete
for picoplankton food resources. Even when these species
were not effective fine suspension-feeders, however, their
grazing accounted for a significant proportion
of total
ciliate bacterivory (mainly due to their high abundances)
at the end of the study (Fig. 7).
The low grazing rates of predominantly
algivorous
prostomatids show that picoplanktivory
covered only a
small portion of carbon requirements of these ciliates.
Their grazing rates on bacteria were generally - 2-4 times
lower than grazing rates of a ciliate community dominated by Balanion planctonicum and U. furcata in an
oligotrophic alpine lake, Piburger Set (Sommaruga and
Psenner 1993). The only prostomatid in our study for
which picoplanktivory
might be of some signihcance was
the small unidentified species of Urotricha (Table l), which
ingested 6 1 bacteria and 14 cyanobacteria cell- I h- I. This
finding also supports the conclusions of Miiller (1989)
and Foissner et al. (1994) that small species of Urotricha
can be partly bacterivorous.
Ciliates appear to be significant grazers of algal pico-
plankton, as was recently reported from both marine and
freshwaters (see Stockner 1988; Weisse 1993; and references therein). Our data (Table 2, Fig. 3) demonstrate the
importance of picocyanobacteria
as a carbon source for
ciliates. For the four most important picoplankton feeders
in the reservoir, we calculated that, compared with bacteria, picocyanobacteria
constituted the following percentage of organic carbon ingested: H. grandinella, 5 3.4%;
S. hexakinetum, 59.5%; oligotrichous species on Staurastrum, 60.1%; and Cyclidium sp., 55.6%. Picocyanobacteria were not only ingested, but also digested by ciliates because we frequently observed, especially in food
vacuoles of II. grandinella, autofluorescing picocyanobacterial cells at various degrees of digestion.
Volume-specific
clearance rates based on uptake rates
of FLB and FLC for the 12 ciliate species (Fig. 9) ranged
from 0.5 x lo4 to 1.6~ lo5 h-l for bacteria and from
1.2 x lo4 to 4.2 x 1O5 h-l for picocyanobacteria.
These
values are - l-2 orders of magnitude higher than the
previous volume-specific
clearance rates estimated in
studies that used latex beads for ciliates feeding on bacteria (Fcnchel 1980), but our values fit well with the values
reported for several fine filter-feeding marine ciliates when
the FLB were used as prey (Sherr and Sherr 1987). The
high specific clearance rates of H. grandinella, S. hcxakinetum, V. aquadulcis-complex, and Cyclidium sp.
confirmed their ability to bc highly eficient filter feeders.
Although our values are much higher than those reported
by Fenchel (1980, 1986), the positions of these taxa in
Fig. 9, compared with other inspected ciliate species, is
consistent with a theoretical concept of filter feeding (Fenchel 1986). The food collection of small oligotrichs enables them to select larger particles (e.g. for H. grandinella
particles of - 2 pm), and thus clearance rates on bacteriasized particles were estimated to be lower by - 50% (Fenchel 1986).
The feeding mechanism of Vorticella and Cyclidium
sp. enabled preferential collection of particles of - 1 pm
(Fenchel 1986; and references therein). The cell length of
reservoir bacterioplankton
was mostly between 0.4 and
0.7 pm (data not shown), so it is not surprising that we
found much higher clearance rates based on consumption
of picocyanobacteria
compared to those found for bacteria, indicating size-selective feeding toward the larger
prey. In extreme cases, some oligotrichs and Cyclidium
sp. had clearance rates for picocyanobacteria
that were
3-4 times higher than those for bacteria (Fig. 9, upper
panel). Our laboratory experiments with Cyclidium sp.
isolated from the reservoir (Simek et al. 1994) also showed
selection of larger cells within bacterioplankton
or a bacterial culture. Our results generally support previous reports of size-sclectivc grazing of natural ciliate communities on bacterioplankton
(Gonzalez et al. 1990; Epstein
and Shiaris 1992). However, our data show that small
scuticociliates and oligotrichs are cspccially likely to have
the strongest grazing impact on the size structure of picoplankton.
Typical species living in organic debris, such as C. margaritaceum and C. mucicola, may prefer feeding on small
particles (Foissner et al. 199 1, 1994), but they do not
Ciliate picoplankton grazing
behave as filter feeders. Also, the feeding apparatus of
prostomatids (Urotricha spp., namely U. furcata, partly
Coleps spp.) is not equipped for filter feeding, and they
are considered to bc raptorial feeders. Correspondingly,
these detritofagous and raptorial feeders showed only weak
or no size selection (see Balanion-like
ciliate in Fig. 9)
toward larger prey. A common feature of these species
was very low uptake rates on picoplankton; thus, we speculate that the feeding mode of these species does not
enable them to distinguish
relatively fine differences
among picoplankton-sized
prey, which likely were ingested as a side effect of their feeding activity on larger
particles.
In the reservoir studied, ciliates dominated by small
species were only weakly top-down controlled by larger
zooplankton, and their development seemed to be significantly related to changes in biomass and species composition of phytoplankton.
Ciliates consumed a significant proportion of bacterial production in the reservoir,
and we determined grazing rates of ciliates on bacteria
and picocyanobacteria at the community, genus, and species level. We identified several taxa of pelagic ciliates
that probably can survive exclusively on a picoplankton
diet under its natural concentrations when availability of
organic carbon in picocyanobactcria
is almost equal to
that in bacteria. Especially small oligotrichs and scuticociliates showed selection of larger picoplankton prey,
as has been reported for frcshwatcr heterotrophic flagellates (Simek and Chrzanowski 1992). This size-selective
grazing may have ecological impacts on natural bacterial
assemblages (Gonzalez et al. 1990; &mek et al. 1994).
Our study showed a different role for picoplankton and
phytoplankton
in the diets of various taxa of pelagic ciliatcs, and their in situ feeding behavior and ecology remain poorly documented in freshwater systems.
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Submitted: 6 October 1994
Accepted: 27 February 1995
Amended: I1 April 1995