Download Impacts of nutrients and zooplankton on the microbial food web of

Journal of Plankton Research Vol.20 no.8 pp.1501-1525, 1998
Impacts of nutrients and zooplankton on the microbial food web
of an ultra-oligotrophic lake
Carolyn W.Burns and Marc Schallenberg
Department of Zoology, University ofOtago, PO Box 56, Dunedin, New Zealand
Abstract In ultra-oligotrophic lakes and the sea, calanoid copepods are the dominant mesozooplankton and cladocerans are generally sparse or absent. To determine the effects of predation and
nutrient enrichment on the pelagic microbial food web of an ultra-oligotrophic lake, we added
copepods and cladocerans at low biomasses (<60 ug I"1) to in situ enclosures in Lake Wakatipu, New
Zealand, in the presence and absence of added nutrients (nitrogen and phosphorus). In response to
nutrient fertilization, the concentrations of phototrophs >3 um and heterotrophic bacteria increased
by SO and 15%, respectively, over 4 days, but those of cyanobacterial picoplankton decreased by 68%.
The presence of calanoid copepods (Boeckella dilatata) at ambient densities (1 and 41*1) rapidly and
severely suppressed ciliate population growth over 4 days and also lowered that offlagellates>3 um,
even when microbial growth was enhanced by added nutrients. The presence of a small cladoceran,
Ceriodaphnia dubia, at double the densities, but similar biomasses, to those of copepods, depressed
the net growth rates of ciliates andflagellatesto a lesser degree. The net growth rate of heterotrophic
bacteria after 4 days declined with flagellate abundance, consistent with the possibility of regulation
byflagellates.Although bacteria and algae increased in response to nutrient fertilization (bottom-up
control), predation (top-down control) appeared to play an important role in structuring the microbial
food web of this ultra-oligotrophic lake in summer.
Introduction
Pelagic microbial food webs manifest trophic complexities and hierarchies
comparable to those involving the classical food chain (nutrients-phytoplankton-zooplankton-fish) in lakes and the sea (e.g. Fenchel, 1988; Porter et al.,
1988; Stockner and Porter, 1988). The relative importance of planktonic microbial communities (protozoa, bacteria, picoalgae) to carbon flux in the pelagic
zone is highest in oligotrophic systems where nutrients are limited and regenerated rapidly (Azam et al., 1983; Fenchel, 1988; Weisse, 1991). Thus, oligotrophic
waters are particularly sensitive to the effects of nutrient enrichment and grazermediated recycling of nutrients.
In a review of interactions in classical aquatic food chains in lakes, McQueen
et al. (1986) concluded that top-down (consumer-driven) effects should prevail
over bottom-up (resource-driven) effects in oligotrophic lakes, and this imbalance should extend further down the classical food chain in oligotrophic systems
than in eutrophic ones. Thus, mesozooplankton are more likely to control phytoplankton biomass in oligotrophic systems than in eutrophic ones in which high
nutrient loads sustain higher algal biomasses. If the same reasoning is applied to
the relative strengths of top-down and bottom-up control within the pelagic
microbial food webs of ecosystems of different trophic status, consumer-driven
impacts might prevail down to the level of picoplankton in oligotrophic lakes, and
prevail over resource-driven impacts. In contrast, based on a simple food web
model involving bacteria and heterotrophic nanoplankton, and data from a range
of natural ecosystems, Sanders et al. (1992) argue that bottom-up control (food
supply) is more important in regulating bacterial abundances in oligotrophic
© Oxford University Press
1501
CW.Bums and IVLScfaaOenberg
systems, while top-down control (predation) is more important in eutrophic
systems.
There have been few descriptive or experimental studies of microbial food
webs in temperate oligotrophic and ultra-oligotrophic lakes, or in lakes in which
calanoid copepods rather than cladocerans are the dominant mesozooplankton.
Exceptions include studies on ultra-oligotrophic Sproat and Chilko Lakes,
Canada (Stockner and Shortreed, 1989, 1994), and Lake Njupfatet, Sweden
(Vrede, 1996), and in the oligotrophic Great Lakes, North America (e.g. Carrick
et al., 1991,1992; Fahnenstiel et al., 1991).
Daphnia dominate the zooplankton of many mesotrophic and eutrophic lakes
in summer, and their impacts on microbial food webs of these lakes are well documented (reviewed by Riemann and Christoffersen, 1993; Jiirgens, 1994). Ceriodaphnia, rather than Daphnia, are the dominant cladocerans in numerous other
lakes throughout the world, however (e.g. Green, 1971; Anderson, 1974; Fryer,
1985; Chapman and Green, 1987). The effects of this common, small, suspensionfeeding cladoceran on microbial communities have not been assessed.
Suspension-feeding calanoid copepods {Boeckella, Calamoecia) dominate the
zooplankton of many New Zealand lakes throughout the year, Ceriodaphnia and
Bosmina are the most widespread and abundant cladocerans (Chapman and
Green, 1987). In a study of the relative impacts of grazing by Boeckella and
Daphnia on the microbial food web of Lake Mahinerangi, a mesotrophic New
Zealand lake, in summer, Boeckella hamata at ambient densities severely
suppressed ciliate population growth, but effects on other components of the
microbial community were slight; in contrast, Daphnia carinata at the same densities lowered the concentrations of large bacterial rods, some photosynthetic
picoplankton and the dominant alga, Cyclotella. Thus, in Lake Mahinerangi, topdown effects of the copepod did not extend beyond ciliates, whereas those of the
cladoceran affected most components of the microbial food web (Burns and
Schallenberg, 1996).
The aim of this study was to evaluate the effects of inorganic nutrients and the
dominant mesozooplankton grazers, Boeckella and Ceriodaphnia, on the microbial food web of an ultra-oligotrophic New Zealand lake in summer. We tested
these effects by manipulating nitrogen (N) and phosphorus (P) levels, and
zooplankton densities, in a multifactorial design. The extent to which effects
propagated through the food web were determined from changes in the
abundances of micro-organisms to gain insight into the relative importance of
grazers and nutrients in structuring the microbial food web of an ultra-oligotrophic lake.
Study site and method
Lake Wakatipu is a large (surface area 289.17 km2), deep (mean depth 210 m),
warm monomictic, glacier-fed, oligotrophic lake. A calanoid copepod, Boeckella
dilatata Sars, is the dominant crustacean zooplankton in the lake; it is present
throughout the year at adult densities of 0.2-4.61"1 (mean 1.3 I"1). Cladocera are
scarce; the most abundant cladoceran, Ceriodaphnia dubia Richard, is present in
1502
Mkrobial food web interactions
summer. Daphnia carinata King and Bosmina meridionalis Sars also occur (Jolly
and Chapman, 1977; C.W.Burns, unpublished data).
This study was carried out from 30 January to 3 February 1995 in Frankton
Arm, a large, but shallow (33 m), bay at the outflow of the lake. This bay does
not stratify thermally and at the time of the study was isothermal at 14°C and well
oxygenated; Secchi depth was shallow (8.5 m) relative to commonly recorded
Secchi depths in the range 10-20 m; chlorophyll a concentration was 0.3 ug I"1.
Average zooplankton densities in the water column at this time were: adult
Boeckella 4.61"1 (juvenile copepodites 231"1), Cdubia 0.91"1, Daphnia 0.11"1 and
Synchaeta pectinata 12.9 H.
An integrated water sample collected from depths of 9 m to the surface was
screened through 150 um mesh to remove mesozooplankton, including
Synchaeta, and mixed in three large, covered barrels. Thirty, 4.25 1 polyethylene
enclosures (Cubitainers) were filled with well-mixed water, after which nutrients
and zooplankton were added in a combined factorial and gradient design (Table
I). A further six enclosures were filled with water that was screened through 25
um mesh to remove any remaining rotifers and large algae. Inorganic nutrients
(NH4CI and KH2PO4) were added to half of the enclosures to give added concentrations of 50 ug N I"1 and 2.5 ug P H (N:P molar ratio of added nutrients 44:1).
At the time of sampling, the dissolved inorganic nitrogen (DIN) concentration in
Lake Wakatipu was 8.3 ug I"1 and soluble reactive phosphorus (SRP) was 1.4 ug
I"1 with a molar ratio of total nitrogen:total phosphorus (TN:TP) of 35:1.
Zooplankton for addition to the enclosures were collected in Lake Wakatipu on
the previous evening by vertical hauls from 20 m, sorted to species and kept in
filtered (74 um) lake water in the dark at ~14°C. Egg-bearing Cdubia [length 0.90
± 0.009 (SE) mm; dry biomass ~7 ug] were added at densities of 2 and 8 I"1, and
ovigerous B.dilatata [prosomal length 0.97 ± 0.007 (SE) mm; dry biomass -11 ug]
were added at densities of 1 and 41"1 to equalize approximately the biomasses of
each species of crustacean in a treatment (Table I). These densities of added
zooplankton are within the reported range for B.dilatata in Lake Wakatipu and
probably also for Ceriodaphnia, although few records exist for this species owing
to its relatively brief appearance in summer (Jolly and Chapman, 1977).
Zooplankton were not added to the enclosures containing 25-um-filtered water.
Enclosures were assigned randomly to one of nine anchored buoys from which
they were resuspended in the lake at a depth of 7 m below the surface for 4 days.
Samples of the water used to fill the enclosures were collected at time zero for
analysis of chlorophyll a, nutrients and micro-organisms (see below).
The enclosures were sampled after 1 day when the water that was removed
(13% of total volume) was replaced with freshly collected 150-um-filtered, or 25um-filtered, lake water from 0.5-9 m and the nutrient-fertilized enclosures
received a second dose of nutrients at the same levels as the initial dose. Care was
taken to ensure that none of the added zooplankton were removed during
sampling. The enclosures were resuspended at the incubation depth for a further
3 days.
When the enclosures were retrieved on day 4, they were subsampled again for
micro-organisms, after which the entire contents of each enclosure were filtered
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CW.Bnrns and MJScfaaDeiiberg
Table L Treatments of enclosures and abbreviations for treatments. All treatments were replicated
three times
Low nutrients
(No added nutrients)
High nutrients
(Added nutrients)
<25-um-filtered water
OL25 No Ceriodaphnia or Boeckella
<150-um-filtered water
OL No Ceriodaphnia or Boeckella
2CL Ceriodaphnia 2 H
8CL Ceriodaphnia 8 H
1BL Boeckella 1H
4BL Boeckella 41-'
<2S-um-filtered water
OH25 No Ceriodaphnia or Boeckella
<150-um-filtered water
OH No Ceriodaphnia or Boeckella
2CH Ceriodaphnia 21"1
8CH Ceriodaphnia 8 H
1BH Boeckella II"1
4BH Boeckella 4 H
through 150 um mesh (25 um mesh for six enclosures) to retrieve the zooplankton for biomass determinations; samples of the filtrate were taken for analysis of
inorganic nutrients and chlorophyll a. Zooplankton were rinsed in distilled water,
sorted under a dissecting microscope to remove any debris and decaying animals,
dried at 50°C for 48 h and weighed on a Sartorius microbalance to the nearest
1 ug. All living zooplankton that were retrieved from an enclosure contributed to
the total biomass for that enclosure. Chlorophyll a was measured fluorometrically
(Turner Model 450) on acetone-extracted samples (0.81) that had been collected
on GF/F filters (Wetzel and Likens, 1991). P and N concentrations were determined spectrophotometrically using a Chemlab autoanalyser according to
standard procedures (Wetzel and Likens, 1991); SRP was analysed using the
antimony-ascorbate-molybdate method, NH4-N was measured by the phenolhypochlorite method, NO3-N was measured after reduction to NO2-N in a
cadmium-copper column, and total particulate N and P (TN and TP) were
measured after oxidation in the presence of boric acid and sodium hydroxide
(Valderra, 1981). Samples of 500 ml for ciliates, small rotifers (<150 um) and algae
were preserved with 1% Lugol's iodine, concentrated by sedimentation, and
counted at X300 magnification under an inverted light microscope. Samples for
picoautotrophs (5 ml) and flagellates (12 ml) were fixed with equal volumes of
ice-cold 4% glutaraldehyde and collected on black polycarbonate filters (0.8 um
for flagellates; 0.2 um for picoautotrophs) before staining with primulin (Bloem
et al, 1986). Samples for bacteria were fixed with 0.5% formalin and frozen until
analysis. Subsamples (2 ml) of the thawed samples were collected on black
0.2 pm filters, stained with DAPI (Porter and Feig, 1980) and counted immediately using fluorescence microscopy. Bacterial concentrations of samples stored
in this way were not significantly different from those of duplicate samples that
were filtered at the time of collection and stored dry (C.W.Burns and
M.Schallenberg, unpublished data). Counts were made at X1000 magnification
under a Zeiss Axiophot microscope equipped with filter sets for UV excitation
(BP365/11/FT395/LP397) for heterotrophic flagellates and bacteria, and green
excitation (BP546/12/FT580/LP590) for picoautotrophs and photosynthetic
flagellates. Few picoplankton cells fluoresced under blue excitation (BP450490/FT510/LP520), so we followed the procedure of Weisse and Kenter (1991)
1504
Microbial food web interactions
and used green excitation for enumerating all autotrophic picoplankton. This
procedure may underestimate the abundance of eukaryotic picoplankton (see
Results, and Weisse and Renter, 1991).
Net population growth rates (g, day 1 ) of micro-organisms in each enclosure
were calculated as:
ft (day 1 ) = [ln(C,/C0)]/l and g4 = Iln(CVCo)]/4
where Co, Q and C4 are, respectively, the concentrations of a taxon or category
initially, after 1 day, and after 4 days; 1 and 4 are the incubation times in days.
The effects of zooplankton biomass on micro-organism growth rates were
examined by regression with biomass as the independent variable, followed,
where appropriate (i.e. if the slopes were not significantly different at P > 0.10),
by analyses of covariance (ANCOVA). Clearance rates, or the rates at which
Ceriodaphnia and Boeckella removed micro-organisms from the water [ml ug
(dry wt)"1 day 1 ], were derived from the slopes of significant simple linear regressions (P < 0.05) that related the micro-organisms' growth rate to zooplankton
biomass (ug dry wt I"1) (Lehman, 1980).
Data were log transformed to equalize variances and tested by General Linear
Model (GLM) ANOVA, followed by orthogonal contrasts (F test) of the effects
of nutrients, zooplankton species and density, and their interactions, using the
Statistical Analysis Systems (SAS) 6.10 statistical package. Nine planned
contrasts were tested: (1) with versus without added nutrients (nutrient main
effect, N); (2) with versus without added zooplankton (zooplankton main effect,
Z); (3) added Ceriodaphnia versus added Boeckella (zooplankton species main
effect, S); (4) low densities versus high densities of added zooplankton (zooplankton density main effect, D); (5) with versus without added zooplankton in the
presence versus absence of added nutrients (nutrient by zooplankton interaction,
N x Z); (6) added Ceriodaphnia versus added Boeckella in the presence versus
absence of added nutrients (nutrient by species interaction, N x S); (7) low densities versus high densities of added zooplankton in the presence versus absence of
added nutrients (nutrient by density interaction, N x D); (8) low densities versus
high densities of Ceriodaphnia compared to low densities versus high densities of
Boeckella (species by density interaction, S x D); (9) nutrient by species by
density interaction (N X S X D). Additional contrasts between water screened
through 25 um mesh (OL25, OH25) and 150 um mesh (OL, OH) were tested
where appropriate. A significance level of P = 0.05 was adopted, unless stated
otherwise.
Results
The water that had been filtered through 25 um mesh contained significantly
lower initial densities of ciliates and algae (Cyclotella, desmids) than water that
had been screened through 150 um mesh (Table II). The survival of added
zooplankton and their offspring in the enclosures at the end of 4 days was 97%
for Ceriodaphnia and 100% for Boeckella. In addition to the added zooplankton,
1505
CW.Bnrns and M-SchaDenberg
Table IL Mean concentrations (1 SE) of chlorophyll a and micro-organisms in Lake Wakatipu at the
start of the experiment
Initial mean concentration (1 SE)
<25-jim water
<150-|im water
0.296 (0.031)
1494 (571)
36.3 (5.22)
0.67 (0.67)
0.324 (0.004)
2832 (1333)
33.1 (10.0)
5.4 (2.47)
58.89(0.25)
53.53 (0.57)
212.4(13.9)
62.5 (9.92)
274.9(21.85)
197.2 (20.9)
71.8(11.6)
269.1 (32.62)
Total flagellates
1.65 (0.38)
2.54 (0.31)
4.19 (0.07)
1.67 (1.88)
2.82(0.14)
4.49 (0.26)
Ciliates (cells 1-')
Oligotrichs
Other
Total ciliates
9.43 (3.78)
0.67 (0.67)
10.1 (4.24)
23.53 (2.32)
333 (0.67)
26.9 (1.69)
Algae
Chlorophyll a (ygH)
Cyclotella (cells 1"')
Other diatoms (cells I"1)
Desmids (cells H)
Picocyanobacteria (cells x 1061"1)
Bacteria (cells x 10* I"')
Cocci
Rods
Total small bacteria
Flagellates (cells x 10* H )
>3 ^m
<3jim
low densities of the following small zooplankton were retrieved from most of the
150-um-filtered enclosures after 4 days: Bosmina (0-0.2 I"1), B.dilatata nauplii
(0.2-4.21"1) and first instar copepodites (0.2-0.7 I"1).
Nutrients
Enrichment increased the concentrations of dissolved inorganic and total N and
P in the enclosures significantly (Figure 1, Table III). The mean concentrations
of TN and TP in the fertilized enclosures after 4 days were 199 and 5.4 ug H,
respectively, which are similar to the highest natural levels of 180 ug I"1 TN and
8 ug I"1 TP that have been recorded in the lake (N.M.Burns, NIWA, Hamilton,
personal communication). Levels of DIN were 9-17% higher in the presence of
Boeckella than in the presence of Ceriodaphnia in the fertilized and unfertilized
enclosures (species effect; Table III). Concentrations of SRP were higher in
enclosures to which zooplankton were added, but showed significant interactive
effects with species and density of zooplankton (Table III). In the absence of
added phosphate, SRP levels were higher at high zooplankton densities than at
low densities, and were 43% higher in the presence of Boeckella than Ceriodaphnia (Figure 1); in the fertilized enclosures, the pattern was reversed so that
SRP concentrations were higher at low zooplankton densities than at high densities and 9% higher in the presence of Ceriodaphnia than Boeckella (Table III).
1506
Microbial food web interactions
100 n
I
CO
OL
8CL 4BL
OH 8CH 4BH
OL
TREATMENT
CL
BL
OH
CH
8H
TREATMENT
300-1
200-
100-
OL
8CL 4BL
OH
8CH 4BH
TREATMENT
OL
CL
BL
OH
CH
BH
TREATMENT
Fig. 1. Nutrient concentrations (|jg I"1, ± 1 SE) in enclosures after 4 days. Abbreviations for treatments are as in Tables I and III; where zooplankton density is not specified, the results are means for
enclosures containing both densities of Ceriodaphnia or Boeckella (n = 6).
Algae
Nutrient enrichment had a significant effect on chlorophyll a (Figure 2; P =
0.0001). In the absence of added zooplankton, chlorophyll a increased to 1.74 ug
I"1 after 4 days, which was 2.3 times that in the unfertilized enclosures and 5.4
times the initial concentration. Concentrations of chlorophyll a were 8-50%
lower when Ceriodaphnia were present than when Boeckella were present
(Figure 2), but this species effect was only marginally significant (P = 0.08).
Diatoms, primarily Cyclotella sp., and a few desmids, dominated the phytoplankton in the enclosures. The abundance of these algae increased after 4 days
in response to enrichment in the absence of added zooplankton (P £ 0.001 for
diatoms; P < 0.01 for desmids). There was a significant interactive effect between
nutrients and zooplankton species on the concentrations of Cyclotella after 4 days
1507
CW.Bnrns and MSdudlenberg
OL
CL
BL
OH
CH BH
TREATMENT
Fig. 2. Algal biomass expressed as chlorophyll a (ug I'1, ± 1 SE) in enclosures after 4 days. Abbreviations for treatments are as in Table I.
Table III. Probabilities for contrasts of the main effects and interactions in nutrient concentrations in
30 enclosures after 4 days. P values > 0.1 are not shown
Contrast
1.
2.
3.
4.
5.
6.
7.
8.
9.
Nutrients (N)
Zooplankton (Z)
Species (S)
Density (D)
NXZ
NxS
NXD
SXD
NXSx D
DIN
TN
SRP
TP
0.0001
0.0001
0.0001
0.0484
0.0199
0.0005
0.0227
0.0703
_
_
0.0942
0.0003
0.0073
0.0972
DIN, dissolved inorganic N; TN, total nitrogen; SRP, soluble reactive phosphorus; TP, total phosphorus.
(P = 0.0465). In the fertilized enclosures, the growth rates of Cyclotella after 4
days declined significantly with Boeckella biomass (r2 = 0.574, P = 0.018), amounting to a 26% decline with 1 Boeckella I"1.
Picoautotrophs
Cells that fluoresced bright orange with the green filter, and were probably the
chroococcoid cyanobacterium, Synechococcus (Stockner, 1988, 1991; Stockner
and Shortreed, 1994), dominated the autotrophic picoplankton in Lake
Wakatipu. At the start of the study, these orange-fluorescing picoplankton
(referred to here as picocyanobacteria) were present at concentrations of -55 x
103 cells ml"1 (Table II). Their concentrations in the unfertilized enclosures
containing water that had been screened through 25 um mesh (42 X 103 ml"1) did
not differ significantly from those in unfertilized enclosures containing water that
had been screened through 150 um mesh (46 x 103 ml"1) on day 1 or on day 4 (72
X 103 ml"1 and 65 X 103 ml"1, respectively); similarly, in the fertilized treatments,
there were no significant differences in picocyanobacterial concentration
1508
Microbial food web interactions
100000-1
8CL
4BL
OH
8CH
4BH
TREATMENT
Fig. 3. Picocyanobacteria (cells ml"1, ± 1 SE) in enclosures after 1 day (open columns) and 4 days
(solid columns). Abbreviations for treatments are as in Table I.
between OH25 and OH enclosures on day 1 and day 4 (range 22-30 X 103 ml"1)
(all P > 0.3).
The abundances of picocyanobacteria were 34% lower in the fertilized enclosures than in the unenriched ones after 1 day and 68% lower after 4 days (Figure
3, Table III). After 1 day, the effects of fertilization on picocyanobacterial abundance showed significant interactive effects with zooplankton species and density
(Table III). These interactive effects did not persist, however. By day 4, the
concentrations of picocyanobacteria were lower at high zooplankton densities
than at low densities in all treatments; in the fertilized enclosures, their net growth
rate decreased significantly with increasing Ceriodaphnia biomass (r2 = 0.384, P
= 0.0316).
Heterotrophic bacteria
The heterotrophic bacteria in Lake Wakatipu were dominated by small cocci
(75%) with small rods comprising -25% of the total heterotrophic bacteria
(Table II). Bacteria increased in response to fertilization and the proportion of
rods was higher in the presence of zooplankton. After 1 day and 4 days, bacterial
densities in unfertilized enclosures with reduced micrograzers (OL25) did not
differ significantly from those in unfertilized enclosures containing OL water (day
1, P = 0.199; day 4, P = 0.137) (Figure 4). In the fertilized enclosures, there were
no significant differences in bacterial concentrations between OH25 and OH
enclosures on day 1 (P = 0.91), but by day 4, concentrations of both cocci and
rods were significantly higher in the enclosures with reduced micrograzers (total
bacteria, P = 0.0009). There was also a significant stimulatory effect of nutrients
on bacterial abundance after 4 days in enclosures with lowered micrograzers
1509
CW.Bums and M-Schallenberg
800-1
I
DAY 1
600-
D cocci
•
DAY 4
RODS
o
£ 400£
200-
g
TREATMENT
Fig. 4. Heterotrophic bacteria (x 103 ml'1) in enclosures without added zooplankton and containing
water that had been filtered through ISO um mesh (OL, OH) and 25 urn mesh (OL2S, OH2S), after
1 day and 4 days. The relative abundances of cocci (open columns) and rods (solid columns) are
shown.
(Figure 4; OH25 versus OL25, P < 0.03). Bacterial population doubling times in
the enclosures were in the range 3.1-6.6 days.
The concentrations of cocci in enclosures containing 150-um-filtered water
were unrelated to nutrient enrichment or to species and density of added
zooplankton after 1 and 4 days (all P > 0!05). In contrast, there were significant
effects on the concentration of rods (Table IV). After 1 day, rods were more
abundant at high densities of Boeckella than at low densities, in both fertilized
and unfertilized enclosures; they were also more abundant at high densities than
at low densities of Ceriodaphnia in the fertilized enclosures. By day 4, rods were
considerably more abundant in fertilized than unfertilized enclosures.
Flagellates
The main nanoflagellates in Lake Wakatipu were small, spherical heterotrophic
flagellates, -2 um in diameter, and loricate, photosynthetic flagellates (cell width
4-5 um; lorica width 6-7.5 um); these two types of flagellates were distinguished
as <3 um (small) and >3 um (large), respectively (Table II). The concentrations
of large and small flagellates in water screened through 25 um mesh did not differ
significantly from concentrations in water screened through 150 um mesh after 1
day and 4 days (contrasts, all P > 0.1). The population doubling time of large
flagellates in the absence of mesozooplankton was 3.2-4.6 days.
Small flagellates decreased in abundance by -50% during the 4 day incubation
period. After 1 day, there was a significant interactive effect of nutrients and
zooplankton species on small flagellates which were more abundant in fertilized
enclosures in the presence of Ceriodaphnia and in unfertilized enclosures in the
presence of Boeckella (Table IV). This interaction did not persist to day 4.
In contrast, large flagellates increased in abundance by 1.6- to 2.4-fold in the
1510
1. Nutrients (N)
2. Zooplankton (Z)
3. Species (S)
4. Density (D)
5.NXZ
6.NXS
7.NXD
8.SXD
9. N x S x D
Contrast
0.0001
0.0165
_
_
_
_
0.0836
•
0.0803
_
_
_
_
-
-
0.0339
-
0.0001
_
0.0292
0.0019
0.0508
-
Day 4
Dayl
_
0.0396
0.0355
_
0.0353
0.0479
-
Dayl
Day 4
Rods
Picocyanobacteria
0.0770
_
0.0390
_
_
-
Dayl
_
_
_
_
_
-
Day 4
Flagellates <3 um
0.0219
0.0030
0.0002
0.0437
_
0.0270
_
_
-
Dayl
_
_
_
-
_
0.0226
-
Day 4
Flagelliites >3 um
Table IV. Probabilities for contrasts of the main effects and interactions in concentrations of picocyanobacteria, heterotrophic bacteria (rods only) and
flagellates in 30 enclosures after 1 day and 4 days. P values £ 0.1 are not shown
CW.Burns and MSchallenberg
absence of mesozooplankton to reach 5.6 x 103 ml"1 after 4 days (Table IV). After
1 day, these flagellates were more abundant in fertilized than unfertilized enclosures (nutrient effect) and less abundant in the presence of Boeckella than Ceriodaphnia (species effect), but decreased in abundance with increasing density of
both species. These differences related to zooplankton species and density
persisted on day 4, but were no longer significant (Table IV).
The net growth rates of large flagellates decreased 4- to 10-fold with increasing zooplankton biomass after 1 day in the nutrient-enriched and unenriched
enclosures, and 3.7-fold after 4 days in the enriched enclosures (Figure 5). Net
growth rates were also significantly lower after 1 day and 4 days in fertilized treatments when Boeckella were present than when Ceriodaphnia were present
(Figure 5; interaction term, P = 0.02 on day 1, P = 0.047 on day 4), and marginally significant in unfertilized treatments after 1 day (P = 0.108).
DAY 1 LOW NUTRIENTS
02
y = -O.OOSx-0.O6S £ = 0.343
y= -O.OlOx- 0.072 I1 = 0.661
O
09
DAY 4 LOW NUTRIENTS
0.30.2, ,
0.1
s
M
•02*
°
•0.4-
•0.1-
Ul
-0.6
25
0.5n
SO
75
100
• O
-0.2 •
0
25
75
-O.010X - 0.093 £ = 0.434
0
y= -0.003X + 0.149 fi = 0.524
y= -0.006X + 0.127
-0.75
100
DAY 4 HIGH NUTRIENTS
DAY 1 HIGH NUTRIENTS
y-
50
I3.0.653
-0.3
0
25
SO
75
100
ZOOPL BIOMASS (ug/l)
0
25
50
75
100
ZOOPL BIOMASS (ug/I)
Fig. 5. Relationships between net growth rates (day 1 ) of large flagellates (>3 um) on days 1 and 4,
and biomass (ug I"1) of added Boeckella (squares, solid line) and Ceriodaphnia (circles, broken line)
in unfertilized (low nutrients) and fertilized (high nutrients) enclosures. Linear regression lines are
shown only when the relationships are statistically significant (P £ 0.05). The lower of the two equations refers to Boeckella.
1512
Microbial food web interactions
Ciliates
Ciliates were sparse in Lake Wakatipu (initial concentration -27 H) and were
dominated by oligotrichs which comprised 87.5% of the total ciliates in the water
initially (Table II). Most of these oligotrichs (71 %) were large (>20 urn) and were
not present in the 25-um-filtered water. For this reason, and because the density
of small ciliates (10-20 um) was so low, the ciliate data from enclosures filled with
25-um-filtered water were not analysed further. Ciliates <10 um may have been
overlooked at the magnification used (X300), although small ciliates were never
seen at X1000 magnification in 12 ml samples for flagellate enumeration. To
reduce variance associated with low numbers, the counts of small oligotrichs were
pooled with those of larger ones to produce total oligotrich numbers.
In the absence of mesozooplankton, ciliate densities increased in both the
enriched and unenriched enclosures after 1 day to -52 H, and after 4 days to -178
I"1 (Figure 6). Ciliate abundance on days 1 and 4 was unrelated to enrichment,
and there were no interactive effects between nutrients and zooplankton (Table
V). Ciliates were less abundant when Boeckella were present than when Ceriodaphnia were present, and less abundant at high than at low zooplankton densities (Figure 6, Table V).
The net growth rates of total ciliates in the presence of zooplankton were fitted
best by simple linear regressions in all treatments (Figure 7). The slopes of the
regressions for Boeckella and Ceriodaphnia were significantly different (day 1
interaction terms, P < 0.003; day 4 interactions, P < 0.04 in unfertilized enclosures, P < 0.08 in fertilized enclosures). After 1 day, ciliate growth rates were unrelated to the presence of Ceriodaphnia, but decreased after 4 days with increasing
Ceriodaphnia biomass in the presence and absence of added nutrients (Figure 7).
1000-j
100M
UJ
O
10-
8CL
4BL
OH
8CH
4BH
TREATMENT
Fig. 6. Total ciliates (number H, ± 1 SE) in enclosures after 1 day (open columns) and 4 days (solid
columns). Note the logarithmic scale. Abbreviations for treatments are as in Table I.
1513
CW.Bums and M^cfaaOenberg
Table V. Probabilities for contrasts of the main effects and interactions in concentrations of oligotrich
ciliates and total ciliates in 30 enclosures after 1 day and 4 days. P values £ 0.1 are not shown
Contrast
1.
2.
3.
4.
5.
6.
7.
8.
9.
Nutrients (N)
Zooplankton (Z)
Species (S)
Density (D)
NXZ
NXS
NxD
SXD
Nx Sx D
Oligotrichs
Total ciliates
Dayl
Day 4
Dayl
Day 4
_
0.0217
0.0061
0.0817
_
_
_
0.0448
-
0.0059
0.0039
0.0125
_
_
-
0.0006
0.0001
0.0087
0.0011
0.0011
0.0026
_
_
_
_
-
0.0650
0.0018
-
Boeckella had highly significant negative effects on total ciliate population growth
after 1 day and 4 days, in nutrient-enriched and unenriched enclosures (all
P < 0.007).
To ascertain whether there were differences between Boeckella and Ceriodaphnia in their effect on oligotrichs and other ciliates, the relationships between
zooplankton biomass and growth rates of non-oligotrich ciliates were examined
in both nutrient treatments after 1 day and 4 days. The net growth rates of nonoligotrich ciliates were unrelated to the biomass of Ceriodaphnia in the fertilized
and unfertilized enclosures on both days (all P > 0.15), but declined with
Boeckella biomass after 1 day in the fertilized enclosures (r* = 0.593, P = 0.015)
and after 4 days in the unfertilized enclosures (r2 = 0.508, P = 0.031) and fertilized enclosures (r2 = 0.486, P = 0.037).
The weight-specific rates at which Ceriodaphnia and Boeckella were estimated
to clear micro-organisms from the water after 4 days are shown in Table VI. The
potential maximum clearance rates for ciliates were more than twice those for
large flagellates and algae. Boeckella cleared the mixed ciliate assemblage from
the water at rates that were 2.4-2.6 times higher than those at which Ceriodaphnia removed ciliates.
Discussion
The very low concentrations of nutrients, chlorophyll a, heterotrophic bacteria,
nanoplankton and crustacean zooplankton in Lake Wakatipu are similar to those
recorded in ultra-oligotrophic Canadian lakes (Stockner and Shortreed, 1989,
1994), and the relatively high densities of picocyanobacteria in the lake
(40 000-50 000 ml"1) are also typical of ultra-oligotrophic lakes in the northern
hemisphere (Stockner, 1991). Flagellate concentrations at the time of our study
were 10-fold higher than are commonly recorded in Lake Wakatipu in winter
(C.W.Burns, unpublished data).
1514
Microbial food web interactions
Table VL Net loss rates, expressed as clearance rates [ml fig (dry wt)-' day-'], derived from the slopes
of significant linear regressions on day 4 that relate net growth rates negatively to biomass of
Ceriodaphnia and Boeckella in enclosures without (Low) and with (High) added nutrients. Dashes
indicate that linear regressions were not significant
Day 4
Low
Algae
Desmids
Diatoms
Picoautotrophs
Bacteria
Flagellates
>3|im
<3 um
Ciliates
Oligotrichs
Other ciliates
Total ciliates
Boeckella
Ceriodaphnia
High
Low
High
5.59
2.51
2.76
8.99
6.99
6.12
29.17
14.62
23.04
26.98
14.44
16.44
Responses after 1 day: Bottom-up effects .
The doubling in concentration of Cyclotella and photosynthetic flagellates in the
fertilized enclosures after 1 day implies strong bottom-up effects on autotrophs.
The 40% increase in bacterial rods after 1 day with increasing density of crustacean zooplankton (Table IV) suggests that rods may be a morphological
response to enrichment from added nutrients and recycling by zooplankton.
Responses after 1 day: Top-down effects
The biomasses of mesozooplankton that we added to the enclosures in Lake
Wakatipu (15-60 ug I"1) were low relative to those in comparable studies (Carrick
et al, 1991; Elser and Goldman, 1991; Pace and Funke, 1991; Wiackowski et al.,
1994; Burns and Schallenberg, 1996), but are realistic for this ultra-oligotrophic
lake.
Large flagellates (>3 um) and ciliates were 21 and 44%, respectively, less abundant in the presence of zooplankton after 1 day. Boeckella and Ceriodaphnia
differed in their effects on these protozoa. Flagellate concentrations were 27%
lower when Boeckella was present than when Ceriodaphnia was present. The net
growth rates of ciliates declined sharply with increasing biomass of Boeckella, but
not Ceriodaphnia (Figure 7). These results are consistent with stronger grazing
pressure on these protozoa by the copepod than the cladoceran. Decreases in
autotrophic picoplankton and larger algae that may be related to grazing by
either species were not detected after 1 day.
1515
CW.Bnms and IVLSchallenberg
DAY 1
2-
DAY 4 LOW NUTRIENTS
LOW NUTRIENTS
y -O.023X +0.396 i* = 0.728
y - -0.009X + 0.503 1**0.482
CILIATES (d"i
2
y - •O.O3SX + 0.716 l = 0.669
1-
1-
I
I
O
O°
0-
.
-1-
5
-225
2-1
•
xX
50
75
100
DAY 4 HIGH NUTRIENTS
DAY 1 HIGH NUTRIENTS
y--0.036x
y » -O.016X*0J296 1**0.685
y = -O.OOTx + 0.366 1**0.459
+ 0ST8 i*-0.8S7
UJ
o
i -1
o
0
25
50
75
100
ZOOPL BIOMASS (ag/1)
0
25
50
75
100
ZOOPL BIOMASS (ug/l)
Fig. 7. Relationships between net growth rates (day 1 ) of ciliates on days 1 and 4, and biomass (ug
l~') of added Boeckella (squares, solid line) and Ceriodaphnia (circles, broken line) in unfertilized
(low nutrients) and fertilized (high nutrients) enclosures. Linear regression lines are shown only when
the relationships are statistically significant (P £ 0.03). The upper of the two equations refers to
Boeckella.
Responses after 4 days: Bottom-up effects
In the unfertilized enclosures, SRP concentrations were elevated in the presence
of added zooplankton, and increased with Boeckella biomass to give a mean ratio
of DIN:SRP of four, compared to a mean of five in the presence of Ceriodaphnia. These observations are consistent with the stoichiometry of N and P cycling
by crustacean plankton (Sterner et al. ,-1992), and the tendency of copepods to
regenerate more P per unit biomass than do cladocerans (Sterner et al., 1992;
Brett et al., 1994; Lyche et aL, 1996), largely because copepods are able to regenerate P that is bound in their microzooplankton prey (Lyche et aL, 1996). In
mesotrophic Castle Lake, the SRP concentration in enclosures containing Diaptomus was approximately twice that in enclosures containing Daphnia after
adjustments for the differences in zooplankton biomass (Brett et aL, 1994).
1516
Microbial food web interactions
The level of DIN in the unfertilized enclosures in Lake Wakatipu increased in
response to added zooplankton and was higher in the presence of Boeckella than
Ceriodaphnia. This result is contrary to our prediction based on studies of
Daphnia species (Andersen and Hessen, 1991; Sterner et al, 1992; Brett et al,
1994) and it suggests that Ceriodaphnia may be less efficient than Daphnia at
recycling N.
Fertilization by inorganic nutrients stimulated the growth of diatoms and
desmids in Lake Wakatipu, and the biomass of chlorophyll a increased 2.3-fold
over 4 days. Heterotrophic bacteria also responded to added nutrients. The mean
doubling time of these bacteria in the zooplankton-free enclosures, 4.8 days, is
within the range of several days to weeks reported for temperate lakes (Jiirgens
and Gvide, 1991,1994). Stimulatory effects of enrichment on bacterial growth in
the absence of mesozooplankton (Figure 4) are consistent with a high affinity of
bacteria for inorganic N and P (e.g. Caron et al, 1988; Le et al., 1994). Positive
correlations between bacterial density and TP comparable to that in Lake
Wakatipu have been recorded in other studies in response to inorganic fertilization (Le et al, 1994; Vrede, 1996).
As Synechococcus responded positively to nutrients added to ultra-oligotrophic Canadian lakes (Stockner and Shortreed, 1988,1994), we expected that
the picocyanobacteria in Lake Wakatipu would also increase in the fertilized
enclosures. Instead, their abundance decreased by 68% in response to enrichment (Figure 3). This decline, which was detectable after 1 day, cannot be attributed to artifacts of enclosure or the incubation techniques because the
concentration of picocyanobacteria increased in the unfertilized enclosures after
4 days at a rate of 0.149 day 1 in the absence of added zooplankton. A possible
explanation for their decline is light limitation brought about by the rapid
increases in biomass of larger phytoplankton (Cyclotella, desmids) in response to
added nutrients. Alternatively, the species or strains of picocyanobacteria in Lake
Wakatipu at the time of our study may have been adversely affected by high N,
P or the N:P ratio. Postius et al (1996) speculate that differences in growth of
various strains of Synechococcus isolated from the pelagic zone of Lake
Constance may have been related to the ambient nutrient levels in the lake at the
time they were isolated. A smaller, negative response of picocyanobacteria to
enrichment also occurred in mesotrophic Lake Mahinerangi (Burns and Schallenberg, 1996). If these negative responses persist in the longer term, they would
be consistent with observations that the abundance of picoautotrophs tends to
decrease with eutrophication of a lake (Stockner and Antia, 1986; Burns and
Stockner, 1991; Le et al, 1994; Wehr et al, 1994; Perin et al, 1996). Although
heterotrophic flagellates can remove Synechococcus from a lake at high rates
(Shriek et al, 1997), the concentrations of autotrophic picoplankton in the enclosures in Lake Wakatipu were unrelated to those of total flagellates or large flagellates (>3 um). If grazing by flagellates were largely responsible for the dramatic
decline in abundance of picoautotrophs in the fertilized enclosures, a negative
correlation between picoautotrophs and large flagellates might have been
expected.
1517
CW.Bums and M^cfaaflenberg
Responses after 4 days: Top-down effects
Strong top-down effects are shown by negative responses in net growth rates of
micro-organisms, which can be expressed as clearance rates (e.g. Carrick et al,
1991). Clearance rates of mesozooplankton calculated in this way represent the
net grazing impact of all consumers of that resource. Boeckella depressed the
population growth of the diatom Cyclotella in fertilized enclosures after 4 days.
Copepods feed selectively, preferring large algae to small algae and bacteria. In
Lake Biwa, Eodiaptomus japonicus appeared not to consume picoautotrophs or
bacteria directly (Nagata et al., 1996). The clearance of Cyclotella, by Boeckella,
rather than the more abundant picocyanobacteria in Lake Wakatipu, may reflect
this preference. In a seasonal study of the effects of grazing by calanoid copepods
on the phytoplankton of a eutrophic New Zealand lake, Cyclotella stelligera was
consistently suppressed by calanoids (Edgar and Green, 1994). The rate at which
Boeckella removed Cyclotella, 5.59 ml ug (dry wt)-1 day 1 (Table VI), or -2.6 ml
copepod"1 h"1, is high, but less than the maximal rate recorded for Boeckella in a
dilute food environment (Burns and Hegarty, 1994). In contrast, Ceriodaphnia,
which does not actively select food, had no detectable impact on the net growth
rate of Cyclotella after 4 days.
Ceriodaphnia can feed on picophytoplankton (e.g. Geller and Miiller, 1981;
Porter et al., 1983) and individuals with guts packed with picocyanobacteria have
been reported in another oligotrophic New Zealand lake (Burns and Stockner,
1991). The net loss rate of picocyanobacteria in Lake Wakatipu in the presence
of Ceriodaphnia, 0.73 ml animal'1 h"1 [or 2.51 ml ug (dry wt)-1 day 1 ; Table VI],
may overestimate the impact of Ceriodaphnia if protozoan consumers are
removing picocyanobacteria at high rates, but this rate lies within a calculated
range of 0.5-1.5 ml animal"1 h"1 based on relationships between the length of
Ceriodaphnia and the clearance rate for bacteria and algae, derived by Porter et
al. (1983).
The net growth rates of large flagellates (>3 um) in Lake Wakatipu in the
absence of mesozooplankton, 0.1-0.15 day 1 , are similar to those recorded in
Lake Michigan at a comparable water temperature (Carrick et al., 1992). In the
presence of Boeckella, large flagellates were lost from Lake Wakatipu at estimated maximal clearance rates of 9-11 ml ug (dry wt)-1 day 1 (-4-5 ml animal"1
h"1) during the first day and 6 ml ug (dry wt)-1 day 1 (-2.7 ml animal"1 h"1) after
4 days in fertilized enclosures (Figure 5, Table VI). These estimated clearance
rates (which include those of other micrograzers on flagellates) exceed those of
1.6-6.2 ml ug (dry wt)-1 day 1 at which mixed zooplankton (Diaptomus, Daphnia)
removed nanoflagellates in oligotrophic Lake Michigan (Carrick et al, 1991), but
they are similar to rates at which Boeckella clear some filamentous cyanobacteria
(Burns and Hegarty, 1994). If the losses are attributed to Boeckella alone, they
correspond to daily consumption rates of 26% of the flagellate standing stock by
adult Boeckella, when at a density of 41"1; this implies that Boeckella exerts relatively strong top-down control on larger flagellates in this lake. The lack of a
significant relationship between the net growth rate of flagellates and Boeckella
biomass in the unfertilized enclosures after 4 days is possibly an outcome of high
1518
Microbial food web interactions
variance in the data of enclosures containing 0 and 1 Boeckella I"1 (Figure 5). The
presence of Ceriodaphnia had weak or no detectable impact on large flagellates
in day 1, but by day 4 they were cleared from enriched enclosures at a net rate of
2.76 ml ug (dry wt)"1 day 1 (Table VI). This rate, if attributed to Ceriodaphnia
alone, is similar to maximal clearance rates of 2.16 and 3.99 ml ug (dry wt)"1 day 1
for Ceriodaphnia reticulata feeding on monocultures of the flagellates Spumella
sp. and Bodo saltans, respectively (Jiirgens et aL, 1996). There is strong correlational evidence for a negative effect of Ceriodaphnia quadrangula and
Diaphanosoma brachyurum on the abundance and biomass of heterotrophic
nanoflagellates in Rimov reservoir, Czech Republic, in late summer (Simek etaL,
1997).
Whereas large flagellates (>3 um) increased in abundance during the 4 day
incubation period, the density of small heterotrophic flagellates (<3 um)
decreased in the presence and absence of nutrient enrichment. As they also
declined in enclosures lacking microzooplankton grazers (OL2S and OH25),
predation by microzooplankton cannot account for the decrease. It is conceivable
that consumption by micro-organisms of <25 um may have caused the decline,
but we lack information to evaluate this possibility. Alternatively, the physical or
chemical conditions in the enclosures may have inhibited the growth of these
flagellates. The presence of Boeckella and Ceriodaphnia did not affect the net
growth rate or density of these small flagellates, possibly because they are too
small to be captured effectively by Boeckella, but there is no obvious reason why
they were not removed by Ceriodaphnia.
Heterotrophic and mixotrophic nanoflagellates may consume bacteria and
autotrophic picoplankton (e.g. Weisse, 1991; Christoffersen, 1994; Nagata et al,
1996; Raven, 1997; Simek et al, 1997), and tight coupling between bacteria and
flagellates has been noted in a wide range of marine and freshwater ecosystems
(e.g. Sanders etai, 1992; Gasol, 1994 and references therein; Jiirgens era/., 1994b;
Pace and Cole, 1994,1996; Solic and KrstuloviC, 1994; Pernthaler et al., 1996).
Top-down control (predation) of bacteria by heterotrophic and mixotrophic
nanoflagellates is considered to be more important in eutrophic lakes than in
oligotrophic ones, where bacterial abundance is under strong bottom-up control
(Sanders et al., 1992; Pace and Cole, 1994). We examined the coupling between
flagellates and picoplankton in Lake Wakatipu from correlations on day 4
between concentrations and net growth rates of flagellates, heterotrophic
bacteria and picocyanobacteria. When data from all enclosures, except those
containing <25 um water, are pooled (n = 30), the abundance and net growth rate
of heterotrophic bacteria are negatively related to flagellate abundance (r =
-0.616, P < 0.001). This negative correlation is consistent with flagellate control
of bacterial growth in Lake Wakatipu, although zooplankton-mediated elevations
in nutrients which accompanied the decline in flagellates may also have
contributed to high bacterial abundance at low flagellate densities. At the time of
our study, flagellates were unusually abundant, thereby enhancing the potential
for top-down control of bacterial population growth. Based on an assumed clearance rate of 2.5 nlflagellate"1h"1 (Pace et al., 1990),flagellatescould have reduced
the bacterial biomass by 10-25% day 1 . Higher bacterial densities after 4 days in
1519
CW.Bums and IVLScfaaOenberg
fertilized water that had been screened through 25 urn mesh than in enriched
water that had been screened through 150 um mesh (Figure 4) imply that
bacterivorous micro-zooplankton in the size range 25-150 um may also regulate
bacterial populations in Lake Wakatipu. Concurrent increases in bacteria with
decreases in flagellates have also been recorded in sub-Antarctic lakes in which
calanoid copepods are the dominant zooplankton (Tranvik and Hansson, 1997).
Although the concentrations of picocyanobacteria in the enclosures were unrelated to those of total flagellates or large flagellates (see above), the net growth
rates of picoautotrophs in the unfertilized enclosures after 4 days decreased with
increasing density of small heterotrophic flagellates (n = 18, r2 = 0.3402, P =
0.011), and the net growth rates of the small flagellates declined with increasing
abundance of picocyanobacteria (r2 = 0.2893, P = 0.021). These negative relationships are consistent with small flagellates exerting top-down control over picoautotrophs by grazing (Simek et al., 1997), although we have no direct evidence
of this trophic coupling in Lake Wakatipu.
The concentration of ciliates in the surface waters (9 m) of Lake Wakatipu at
the time of our study, 27 I"1, is among the lowest ciliate densities that have been
recorded (Sherr and Sherr, 1984; JUrgens et al., 1994b). The average growth rate
of ciliates over 4 days in the absence of crustacean zooplankton was 0.41 day 1 ,
or doubling every 1.7 days. This rate falls within the range of 0-1.4 day 1 for ciliates in Lakes Michigan and Ontario (Taylor and Johannsson, 1991; Carrick et al,
1992) and in the range 0.12-0.85 day 1 for ciliates in oligo-mesotrophic Piburger
See (Macek et al., 1996), and is approximately five times higher than that of ciliates in mesotrophic Lake Mahinerangi in summer (Burns and Schallenberg,
1996). Therefore, the ciliate populations in Lake Wakatipu had the potential to
increase. There were no significant correlations between the net rate of ciliate
increase in the enclosures after 4 days and the densities of their potential food
resources of bacteria, picocyanobacteria, chlorophyll a and flagellates that would
suggest strong resource constraints to ciliate population growth (four regressions:
all n = 30, all r2 < 0.027, all P > 0.388).
Although the abilities of different-sized Daphnia and Bosmina to depress
ciliate growth rates are well documented (Jack and Gilbert, 1993; Jurgens, 1994,
and references therein; Burns and Schallenberg, 1996), the potential ability of
Ceriodaphnia to do so has not been recorded. The net loss rates of ciliates in Lake
Wakatipu, expressed as potential maximal clearance rates by Ceriodaphnia, were
7-9 ml ug (dry wt)"1 day 1 (2.0-2.6 ml animal"1 h"1). These rates are twice the
maximal rates recorded for Bosmina and small Daphnia pulex feeding on 1.3 ciliates ml"1 (Jack and Gilbert, 1993). They are also approximately twice those at
which Ceriodaphnia cleared picocyanobacteria and flagellates in our study (Table
VI); they would allow Ceriodaphnia at a density of 4 adults I"1 to clear the water
of ciliates over 4 days.
Boeckella depressed the net population growth rate of oligotrichs more effectively than did Ceriodaphnia (Figure 7), and also lowered that of non-oligotrich
ciliates. The per capita rates of ciliate removal in the presence of B.dilatata
(prosomal length 970 um), 159-320 ml copepod"1 day 1 (Table VI), are considerably higher than a rate of 61 ml copepod"1 day 1 that is predicted by an equation
1520
Microbial food web interactions
that relates clearance rate of ciliates to prosomal length of diaptomids (Burns and
Gilbert, 1993). Although high, these rates are less than rates of ciliate removal
that have been recorded in the marine calanoids Acartia clausi (386-630 ml
copepod"1 day 1 ) and Centropages typicus (1394 ml copepod"1 day 1 ) by Wiadnyana and Rassoulzadegan (1989).
Boeckella dilatata was -2.5 times as effective as Ceriodaphnia in depressing
ciliate growth rate in Lake Wakatipu when results are expressed as weightspecific clearance rates (Table VI). These findings are consistent with earlier ones
in mesotrophic Lake Mahinerangi where Boeckella hamata was 10 times more
efficient than the dominant cladoceran, Daphnia carinata, in clearing ciliates from
the water (Burns and Schallenberg, 1996). Our results differ from those of
Wiackowski et al. (1994) who found that Daphnia and a calanoid, Diaptomus
novamexicanus, were equally effective in clearing ciliates from Castle Lake, California, although a cyclopoid, Diacyclops bicuspidatus thomasi, was considerably
more effective.
Zooplankton-mediated depression of ciliate growth rates in Lake Wakatipu
amounted to a loss of 39-50% of ciliate standing stock per day in the presence of
8 H adult Ceriodaphnia, and 72-100% day 1 when 4 I"1 adult Boeckella were
present. Effective removal of ciliates by Boeckella undoubtedly contributes to
remarkably low ciliate densities in this lake, and is consistent with our hypothesis
that low ciliate densities in New Zealand lakes are related to the dominance of
calanoid copepods in them (Burns and Schallenberg, 1996). This strong,
copepod-mediated control of both ciliates and flagellates >3 um in Lake Wakatipu contrasts with the situation in mesotrophic Lakes Schohsee (Jiirgens et al,
1994a) and Mahinerangi (Burns and Schallenberg, 1996) where copepods
controlled the net growth of ciliates, but flagellates were regulated weakly or not
at all.
In pelagic microbial food webs, the provision of resources (bottom-up effects)
and their consumption (top-down effects) operate continuously, often at different time scales; when these opposing effects balance, there is no net change in
biomass or net growth rate of the relevant micro-organisms. Bottom-up effects
prevail when the biomass or net growth rate of micro-organisms responds positively to increased resources; conversely, top-down impacts appear stronger when
biomass or net growth rate responds negatively to increased biomass of the
organisms that consume them.
In Lake Wakatipu, inorganic nutrients (N and P) stimulated the growth of
bacteria and phytoplankton, including phototrophic flagellates >3 um and
Cyclotella, either directly or indirectly (Table VII). These results are consistent
with the view that substrate supply controls bacterial abundance in oligotrophic
environments (Sanders et al, 1992; Pace and Cole, 1994). In contrast, strong topdown effects of zooplankton on protozoa were evident. Boeckella and Ceriodaphnia at low densities suppressed the population growth of flagellates >3 um
and ciliates; Boeckella were more effective at doing so than Ceriodaphnia.
In a comparable study of the effects of nutrients and mesozooplankton on the
microbial community of mesotrophic Lake Mahinerangi in early summer, the
effects of top-down perturbations virtually ceased at the level of ciliates in the
1521
CW.Bams and IVLSchalleiiberg
Table VEL Responses of micro-organisms to additions of nutrients and crustacean grazers, in Lake
Wakatipu after 4 days. Symbols refer to statistically significant (ANOVA or linear regression*, P £
0.0S) increases (+), decreases (-) or no change (ns) in concentration, or net population growth rate, in
treated enclosures relative to untreated enclosures
(a) Nutrients. Responses of micro-organisms in <25-um-filtered water and <150-um-filtered water to
nutrient additions in the absence of added crustaceans. The responses of organisms that were removed
differentially by filtration through 25 um netting (..) were not analysed
(b) Grazers. Responses of micro-organisms in <150-um-filtered water (with and without added
nutrients) to additions of Ceriodaphnia and Boeckella
(a) Nutrients
Algae
Chlorophyll a
Desmids
Diatoms
Picoautotrophs
Bacteria
Cocci
Rods
Total bacteria
Flagellates
>3um
<3 um
Ciliates
Oligotrichs
Other ciliates
Total ciliates
(b) Grazers
<25um
<150 urn
Ceriodaphnia
Boeckella
••
+
+
+
ns
ns
ns
-
-
„ •
ns
ns
—«
ns
+
+
+
ns
+
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
_•
ns
ns
ns
ns
ns
ns
-
presence of Boeckella, whereas bottom-up effects stimulated by nutrient additions were evident at all levels in the microbial food web and involved phytoplankton, bacteria, flagellates and ciliates (Burns and Schallenberg, 1996). Our
finding in Lake Wakatipu that top-down effects extended to the level of flagellates, and possibly also to bacteria, implies that predation may play an important
role in determining the structure of the microbial food web of this ultra-oligotrophic lake in summer.
Acknowledgements
This study was supported by grants from the Foundation for Research, Science
and Technology (grant number UOO313) and the University of Otago. We are
grateful to Dr C.R.Thompson, Department of Mathematics and Statistics,
University of Otago, for statistical advice, R.Wass and S.Twombly for help in the
field and laboratory, C.Mitchell for nutrient analyses and R.Keedwell for
bacterial counts.
1522
Microbial food web interactions
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Received on January 15,1998; accepted on April 3, 1998
1525