J. Great Lakes Res. 25(1):61–77Internat. Assoc. Great Lakes Res., 1999

The Impact of Zooplankton Grazing on Phytoplankton SpeciesComposition and Biomass in Lake Champlain (USA-Canada)

Suzanne N. Levine1,*, Mark A. Borchardt2, Moshe Braner1, and Angela d. Shambaugh1

1School of Natural ResourcesAiken Center

University of VermontBurlington, Vermont 05405

2Marshfield Medical Research FoundationMarshfield, Wisconsin 54449

ABSTRACT. Rates of grazing on phytoplankton by macrozooplankton (cladocerans and copepods > 220 µm in length) and microzooplankton (animals < 220 µm, mostly rotifers and nauplii) were deter-mined for Lake Champlain on three occasions using a modified version of the Lehman-Sandgren method.Gradients in grazer density were created in fertilized cubitainers incubated in situ, and clearance rateson specific phytoplankton taxa determined from regressions of algal growth rates on herbivore biomass.Grazers consumed 3 to 26% of the total phytobiomass present and 22 to 139% of net primary productivitydaily. Macrozooplankton fed most heavily on algae 5 to 25 µm in size and generally selected dinoflagel-lates and green algae (6 to 26% of biomass removed per day) over cryptophytes (1 to 8%/day), diatoms(0 to 10%/day) and blue green algae (0 to 6%/day). However, variability in grazing vulnerability amongthe species within divisions was high. Microzooplankton had greater weight-specific clearance rates thanmacrozooplankton when consuming diatoms, blue-green algae, and cryptophytes, but were less efficientat harvesting green algae. An experiment in which nutrients and zooplankton were manipulated in a 2 × 3factorial design indicated that both variables have a net positive impact on phytoplankton growth rates inLake Champlain, the zooplankton because they excrete required nutrients. Indirect effects of the nutrientsvs. grazers experiment included rotifer growth in response to increased algal productivity and harvestingof rotifers and Cladocera by cyclopoid copepods. It was concluded that both nutrients and grazing influ-enced the structure of Lake Champlain’s phytoplankton community, but that nutrients were generallymore important.

INDEX WORDS: Lake Champlain, zooplankton, phytoplankton, grazing, nutrients, primary productiv-ity, mortality, edibility.

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INTRODUCTION

Successful manipulation of forage fish communi-ties through addition or removal of piscivorous fishand of zooplankton communities through change inplanktivorous fish densities (Reynolds 1994) hasfueled the development of trophic cascade theory(Carpenter et al. 1985), and raised questions aboutthe relative roles of nutrients and grazers in control-ling algal biomass and species composition inlakes. While eutrophication management continuesto focus on phosphorus control, there is growing in-

terest in biomanipulation as a tool for reducingalgal biomass when P reductions are difficult(Reynolds 1994).

The potential for grazing to influence algal bio-mass has long been recognized. The “clear-waterphase” that is a feature of many lakes in early sum-mer has been attributed to grazing rates that out-pace reproduction, the latter being constrained bynutrient depletion (Sommer et al. 1986). That graz-ing may account for significant mortality overlonger periods of time is apparent in recent re-eval-uations of the chlorophyll a- phosphorus relation-ship in lakes (Hansson 1992, Persson et al. 1992,Mazumder 1994). These show chlorophyll a con-centrations increasing about four times more*Corresponding author. E-mail: Slevine@nature.snr.uvm.edu

62 Levine et al.

tems. Because zooplankton are major recyclers of Nand P (Lehman 1980), and these nutrients frequentlylimit phytoplankton growth rate, manipulation ofzooplankton density almost certainly alters repro-duction rates under ambient nutrient conditions. Toeliminate the effects of zooplankton nutrient regen-eration, nutrients may be added to experimental sys-tems. These saturate nutrient uptake and raisephytoplankton growth rate to a maximum (and uni-form) velocity (Elser 1992). Lehman and Sandgren’soriginal method did not include nutrient additions,so that its use was restricted to consideration of thegeneral direction (positive, negative, neutral) of nettaxon response to grazer density (Lehman and Sand-gren 1985, Berquist and Carpenter 1986). Elser andGoldman (1991) and Elser (1992) used experimentalsystems with and without zooplankton present andwith nutrient saturation to obtain the first estimatesof species-specific grazing mortality in lakes. Cyrand Pace (1992) and Cyr (1998) then combined thenutrient additions of Elser with the Lehman-Sand-gren method to estimate zooplankton grazing ontotal phytobiomass (chlorophyll a) in 16 northeast-ern U.S. lakes, and grazing on Cryptomonas in twolakes.

In this work the Lehman-Sandgren technique isused with nutrient addition to assess taxon specificphytoplankton loss rates in Lake Champlain. Bothmacrozooplankton (animals > 220 µm in length;Cladocera and copepodite and adult copepods) andmicrozooplankton (< 220 µm; mostly rotifers andnauplii) grazing were assessed on three occasions(spring, summer, and fall), and compared with phy-toplankton division rates (or primary productivity).A 2 × 3 factorial experiment was also conducted tocompare the importance of nutrients (N + P + C)and grazers as controls on phytoplankton biomassin the lake. Lake Champlain is just the fourth lakeworldwide, and the first lake outside of California,for which rates of macrozooplankton grazing onspecific taxa have been obtained. It is the secondlake for which microzooplankton grazing has beenassessed (the first was Castle Lake, CA; Elser andFrees 1995). As Lake Champlain shares manyphysicochemical and biological characteristics withthe Laurentian Great Lakes, the data presented heremay contribute to an understanding of the phyto-plankton dynamics in these important lakes as wellas in Lake Champlain. This work was conducted inconcert with studies of bacterivory and zooplank-ton-protozoan interactions in Lake Champlain, andwas part of a larger effort to describe and model thelake’s food web (Levine et al. 1998, 1999).

rapidly with rising total P concentration in lakeswith impoverished zooplankton communities thanin lakes with dense populations of large daphnids.Biomanipulation experiments have been highly suc-cessful in reducing algal biomass when one of theiroutcomes has been increased densities of Daphnia(Liebold 1989, Reynolds 1994). By contrast, ma-nipulations yielding zooplankton communitiesdominated by copepods or small-bodied cladocer-ans rarely have affected phytoplankton.

When zooplankton manipulations affect phyto-biomass, phytoplankton species composition isoften altered as well (Porter 1973, Berquist andCarpenter 1986, Proulx et al. 1996, Svensson andStenson 1991). Discrepancies in the relative propor-tions of phytoplankton species found in the guts ofzooplankton with the proportions present in lakecommunities suggest that zooplankton frequentlyfeed selectively (Porter 1973, Infante 1978,Berquist and Carpenter 1986), with prey discrimi-nation on the basis of size, shape, toxicity, or nutri-ent content (Reynolds 1984).

Given the complexity of the pelagic foodweb andthe many factors that affect phytoplankton growthand mortality, progress in understanding phyto-plankton community structure and succession, andan enhanced ability to predict the outcome of bio-manipulation or nutrient management, may requirethe use of numeric models. Modeling of phyto-plankton dynamics requires quantitative data on re-productive and mortality rates, however, and theseare rarely obtained at the species level. While zoo-plankton grazing on the total phytoplankton stand-ing stock has been quantified in a large number oflakes, there is only one substantial data set on themortality of individual phytoplankton species dur-ing zooplankton grazing, that of Elser and col-leagues for three California lakes (Tahoe, Castle,and Clear; Elser and Goldman 1991, Elser 1992,Elser and Frees 1995). The intense labor investmentneeded to track individual phytoplankton specieshas discouraged species-level analysis, as has slowmethods development.

The most reliable method for grazing assessmentis that of Lehman and Sandgren (1985), in whichclearance rates on individual phytoplankton taxa areestimated by manipulating grazer densities in exper-imental systems to multiple levels, measuring the re-sulting growth rates, and regressing algal growthrates on grazer density. The slope of the regressionline is the clearance rate. However, the approachprovides meaningful rates only if phytoplankton re-production is “constant” across experimental sys-

Zooplankton Grazing of Phytoplankton in Lake Champlain 63

SITE DESCRIPTION

Lake Champlain is a large (170 km long, surfacearea 1,130 km2), deep (maximum depth 122 m;mean depth 23 m), but narrow (maximum width, 20km) lake straddling the border between Vermontand New York and extending a short distance intoQuebec. Numerous islands, sills, peninsulas, andcauseways divide the lake into partially isolatedsub-basins. This study took place in the largest ofthese, known as the “Main Lake” (at 44°27′ N,73°17′ W, about 1 km northwest of Juniper Island,at the broadest expanse of the lake).

The Main Lake is marginally dimictic; it strati-fies in summer, establishing an epilimnion with adepth of 10 to 12 m, and, in winter, sometimesfreezes for as long as 2 months (February to April).For several months between October and June,however, it mixes to the bottom. The Main Lake isoligotrophic-mesotrophic, with a mean total phos-phorus concentration of 0.4 µM (VT Dept. Environ.Consev., unpub. data) and a mean chlorophyll aconcentration of 5 µg/L (NY Dept. Environ. Con-sev., unpub. data). Its phytoplankton community isdominated by diatoms and cryptophytes duringmuch of the year, although blue-green algal domi-nance is common in late summer (McIntosh et al.1993). Seasonal phytoplankton biomass distributionis bimodal, with a major peak in spring and asmaller peak in fall. The zooplankton community isa mixture of cycloploid copepods, cladocerans(principally Daphnia galeata mendota and Daphniaretrocurva), and rotifers (McIntosh et al. 1993).Calanoid copepods are comparatively scarce. In atypical year, copepods and rotifers are most abun-dant in spring, while cladocerans peak in summer.Over 80 species of fish are present in Lake Cham-plain; Osmerus mordax dentex (rainbow smelt) isthe dominant planktivore (Myer and Gruendling1979).

METHODS

Grazing Experiments

The Lehman-Sandgren method with nutrient ad-dition was used to estimate zooplankton grazingrates. Three levels of grazer density were created induplicate in transparent 10-L cubitainers for bothmacrozooplankton (animals retained by a 220 µmmesh; adult and copepodite copepods, and clado-cerans) and microzooplankton (< 220 µm; rotifersand nauplii) and the cubitainers incubated in situfor 2 to 3 days. Phytoplankton growth rates in cu-

bitainers were obtained by measuring phytoplank-ton biomass four or more times during an incuba-tion, and calculating the slope of the regression ofln (phytobiomass) against time. These rates thenwere regressed against grazer biomass to yield esti-mates of grazer-biomass specific clearance rates(from the slope). The fraction of phytoplankton bio-mass removed per day was calculated as the prod-uct of clearance rate and ambient zooplanktonbiomass and carbon flow from the product of thisvalue and the C content of the phytoplankton (about0.5 µg C/ µg dwt; Reynolds 1984).

Grazing losses were estimated for several abun-dant species, for the phytoplankton divisions, andfor the phytoplankton community as a whole. Sepa-rate estimates for micro- and macrozooplanktongrazing were possible because the slope of the rela-tionship between algal growth rate and grazer bio-mass was not affected by an across-the-boardsubtraction of grazer biomass from all data points(provided that the biomass subtracted is of the sametype, which it was). The sieve used to manipulatemacrozooplankton densities allowed microzoo-plankton to pass freely and thus attain similar den-sities in all macrozooplankton treatments, while themicrozooplankton treatments used water pre-sievedto remove macrozooplankton (some contaminationoccurred; see below). The experiment was repeatedthree times, in mid-summer (25 to 27 July) and fall(27 to 29 September) 1994, and in spring (8 to 11May) 1995.

Water and plankton for the experiment were ob-tained with a clear vertically-oriented 8-L van Dornbottle which was deployed at 1-m intervals over thedepth of the epilimnion (10 m in summer, 18 m infall) and 2 m into the metalimnion. In spring, whenthe lake mixed to the bottom, water collectionstopped at 20 m. The collected water was pooled ina 240-L tank, and mixed with a paddle, prior to dis-persal to cubitainers or sieving. Zooplankton for the“high macrograzer” treatment were obtained with a202 µm-mesh Wisconsin net (collar off; mouth di-ameter 0.5 m; a mason jar attached) towed verti-cally over the same depth interval used for watercollection. The captured animals were added to 30L of lake water (under the water surface to mini-mize air trapping under cladoceran carapaces) tocreate a macrozooplankton “concentrate.”

64 Levine et al.

The procedure for preparing the experimentaltreatments was as follows. The “ambient macro-grazer” (AMA) cubitainers received unaltered lakewater from the reservoir, while the “high macro-grazer” (HMA) treatment was created by filling cu-bitainers with lake water and adding sufficientzooplankton concentrate to increase animal densi-ties about 4 fold (300 to 500 mL). The water re-maining in the reservoir then was passed througheither a 202 µm plankton net (July) or a 220 µmsieve (September and May) to remove macrozoo-plankton. The “low macrograzer” (LMA) treatment(which also served as the “ambient micrograzer”(AMI) treatment) consisted of cubitainers filledwith this coarsely-sieved water. To create the re-maining treatments, some of the 220 µm (or 202µm) “sievate” was passed through a second 20 µmsieve. The finer “sievate,” which had been largelycleared of micrograzers, was poured into cubitain-ers to become the “low micrograzer” (LMI) treat-ment, while animals collected on the sieve were washed back into 20 L of the 220/202 µm“sievate” to produce the “high micrograzer” (HMI)treatment.

Once all the cubitainers were filled, KH2PO4,NH4Cl, and dextrose (C6H12O6) were added to eachto increase P, N, and C concentrations by 3, 5, and34 µM, respectively. Air was squeezed out of thecubitainers to prevent animal trapping in an airspace, and the cubitainers were suspended from an-chored floating frames in Burlington Harbor at adepth of 1.5 m. They were incubated 44, 48, and 64h in summer, fall, and spring, respectively. Light in-tensities in the cubitainers were generally below thethreshold for photoinhibition for most phytoplank-ton groups (< 1,000 µmol/m2/s), but still suffi-ciently high during mid-day (> 300 µmol/m2/s) tosaturate photosynthesis.

Sixty-milliliter phytoplankton samples were col-lected in triplicate from the cubitainers immediatelybefore incubation, on 3 or 4 occasions during thefirst 24 h of incubation, and daily thereafter. Onepercent acid Lugol’s solution was used as a phyto-plankton preservative. Zooplankton were collectedat the beginning of each experiment by passing 10L of the water from the mixing tank through eithera 64 µm (July and September 1994) or 20 µm-meshsieve (May 1995), and washing the retained animalsinto sample bottles. These samples were used to es-timate lake standing stocks. To estimate zoobio-mass in the cubitainers, the entire contents of eachcubitainer were sieved (same mesh sizes as the ini-tials) at the end of each experiment. The animals

collected on sieves were anesthesized in carbonatedwater prior to preservation in 5% buffered sucroseformalin (Haney and Hall 1973) with rose bengalstain.

Algal species composition and cell densities weredetermined by direct microscopic counts of settledsamples using an Olympus inverted light micro-scope (150 random fields, or > 300 cells of thedominant species). The biovolume of each phyto-plankton species was estimated from its cell dimen-sions and geometry. For common species, celldimensions were measured during every incubationand in all treatments; scarce species were measuredduring at least one incubation. Algal dry weightwas estimated from biovolume using the conversionfactor 0.47 pg dwt/µm3 (Reynolds 1984).

Zooplankton were enumerated and measuredwith the same compound microscope. The entiresample was counted for macrozooplankton, whilesubsamples were used to estimate microzooplank-ton numbers. Dead and moribund animals and non-feeding stages (eggs and embryos) were countedbut not included in biomass estimates. The biomassof rotifers was estimated from shape dimensionsand geometry (Ruttner-Kolisko 1977), and that ofcopepods and cladocerans from biomass-length re-lationships (McCauley 1984, Culver et al. 1985).To the extent possible, the relationships used werefor animals preserved as in our study. Preservationcan cause shrinkage of animals, although it is gen-erally minimal (5 to 10%) in 4 to 5% sucrose for-malin (Dumont et al. 1975, Culver et al. 1985). Thefirst two copepodite instars of cyclopoid copepodswere included in the estimates of herbivore biomassas these animals consume substantial amounts ofphytoplankton. Later stages (especially adults,which contributed > 90% of copepod biomass in theexperiment) are predominantly carnivorous, andthus were not included.

Primary Productivity

The y-intercepts of the Lehman-Sandgren regres-sion lines provided estimates of algal division ratesin cubitainers (growth in the absence of grazers).Multiplying these rates by phytoplankton biomassand C content yielded estimates of net primary pro-ductivity (NPP) under the conditions of the experi-ment (1.5 m incubation depth, and nutrientenrichment). To assess primary productivity withinthe lake, the 14C technique in a laboratory incubatorwas used (Fee et al. 1992). Water was collected asin the grazing experiments on the second day of the

Zooplankton Grazing of Phytoplankton in Lake Champlain 65

last two grazing studies (in September and May),and incubated with 14C-labelled bicarbonate at fivelight intensities and at ambient temperature. ALiCor light meter and submersible probe were usedto estimate light extinction rates at the samplingsite, while a second light meter with an aerial probemonitored solar irradiance over the course of thegrazing experiments. The numerical model of Fee(1990) was employed to estimate daily primary pro-duction from the data on solar irradiance, light ex-tinction, and productivity-light relationships.Dissolved inorganic carbon was estimated fromsample alkalinity and pH.

The Nutrients vs. Grazers Experiment

To evaluate the relative importance of nutrientsand grazers in determining Lake Champlain’s phy-toplankton standing stocks, a 2 × 3 factorial experi-ment was performed (31 July to 4 August 1995).Water and zooplankton were collected as in thegrazing experiments (depth interval, 0 to 15 m) andthe same “high,” “ambient,” and “low” macrograzertreatments were created, except that each treatmentwas repeated in six, rather than two, cubitainers.Half of the cubitainers at each grazer level thenwere enriched with C, N, and P (as in the grazingexperiments), while the other half were left unfertil-ized. Duplicate chlorophyll a samples (750 mL)were collected at the beginning of the experimentfrom the tanks used to fill cubitainers, and from thecubitainers after 23 and 92 hr of incubation at 1.5 mdepth. Pigments collected on GFF filters (nominalpore size 0.7 µm) were extracted in hot ethanol(Sartory and Grobbelaar 1984), and their ab-sorbance read on a Shimadzu UV-VIS 160U spec-trophotometer, using the monochromatic procedure(665 nm) with phaeophytin correction (Lorenzen1967). Phytoplankton growth rates were calculatedfrom the rate of change in chlorophyll a concentra-tion over the incubation. Triplicate phytoplanktonsamples (for microscope counts) also were takenfrom cubitainers at the beginning and end of the 4-day experiment and those in the fertilized systemswere used to obtain a fourth set of grazing and NPPrate estimates. Zooplankton were collected at theexperiment’s end using the procedures describedearlier. Two-way analysis of variance (ANOVA) al-lowed assessment of the relative contributions ofnutrients and zooplankton in controlling phyto-plankton growth rates.

RESULTS

Grazing Studies

Experimental Conditions

The three grazing studies were conducted at thebeginning, middle, and end of the growth season tomaximize the number of phytoplankton species ex-amined and to provide a sense of grazing rate vari-ance. Physicochemical and biological conditionswere markedly different during the three experi-ments (Table 1). In May, the lake was isothermal,with a water temperature of 5°C, and nutrient con-centrations relatively high. The lake was fully strat-ified in July, with a mixed layer depth of 10 m andan epilmnion temperature of 22°C, whereas in Sep-tember, thermocline erosion was underway (epil-imnion temperature, 17°C; mixed-layer depth, 18m). Phytoplankton biomass was far greater in Maythan during the other experiments (Fig. 1) andstrongly dominated by diatoms (96%), in particularAulacoseira sp. (67%). The July phytoplanktoncommunity was dominated by green algae(Mougeotia sp., Oocystis spp., and Eudorina ele-gans) and cryptophytes (Chroomonas sp. and Cryp-tomonas sp.), and the September community, bydiatoms and cryptophytes, although blue-green andgreen algae were also common.

During two of the three grazing experiments,Cladocera (Daphnia galeata mendotae, Daphniaretrocurva, and Eubosmina coregoni) were the prin-cipal herbivores present (Fig. 2). The calanoid cope-pods Diaptomus sicilis and Diaptomus minutusshared dominance with Cladocera during the Mayexperiment, but otherwise were scarce. The rotifercommunity was consistently dominated by Keratellaspp, but Polyarthra spp., Noltholca sp. (May 1995),and Kellicottia longispina were present as well. Thepredaceous rotifer Asplanchna sp. was common inJuly and large enough to be included in the macro-zooplankton. Cyclopoid copepods were the most sig-nificant components of the carnivorous zooplanktoncommunity. These animals were almost as abundantas Cladocera in May (dominants Diacyclopsthomasi, Acanthocyclops robustus, and Tropocyclopsprasinus prasinus), scarce in July, and the major con-tributors to zoobiomass in September (A. robustus,Mesocyclops edax, and D. thomasi).

Extent of Zooplankton and PhytoplanktonManipulation Through Sieving and Addbacks

Through sievings and additions of netted ani-mals, 6 to 60 fold gradients in herbivore biomass

66 Levine et al.

were created within the macrograzer portion of ourexperiments (0.06 to 3.4, 0.003 to 0.25, and 0.09 to0.58 mg.dwt zoop./L, in May, July, and September,respectively). The biomass of micrograzers presentin the macrograzer treatments was both fairly uni-form, and a minor portion of total zoobiomass (Fig.2). Thus the estimates of macrograzing can be con-sidered to be relatively free of a “micrograzer influ-ence.” Duplication of macrozooplankton biomasswithin treatments was sometimes poor, but this wasnot an issue, as data from each cubitainer were en-tered separately into the regression analysis.

In the microzooplankton portion of the study, thegreatest herbivore biomass present in cubitainersexceeded the lowest by 200 to 1500 fold. Some ofthe LMI cubitainers were almost devoid of animals(0.0001 to 0.0005 mg dwt/ L), while zoobiomass inthe HMI cubitainers reached 0.61, 0.019, and 0.24

mg.dwt/ L in May, July, and September, respec-tively. Sieving of lake water through a 220 µm sieveor a 202 µm net prior to micrograzer manipulationsucceeded in removing > 95% of macrozooplanktonbiomass. However, it was not uncommon for one ortwo macrograzers to slip through or around thesieve. Because these animals had biomasses threeorders of magnitude greater than those of the indi-vidual microzooplankton, and microzooplanktonwere relatively scarce, the estimates of total herbi-vore biomass in the micrograzer treatments were af-fected by macrozooplankton contamination;macrograzers made up from 13 to 57% of the bio-mass in the AMI treatments, and 36 to 44% of thatin the HMI treatments. In estimating microzoo-plankton clearance rates, the influence of macro-zooplankton grazing was accounted for as follows:macrozooplankton clearance rates for the taxa of in-

TABLE 1. Conditions at the sampling site during the 4 experiments. The chemistry data are from theVermont Department of Environmental Conservation, and pertain to samples taken within a week of theexperiments. All values pertain to the mixed layer, and are for daytime.

Grazing Studies Nutrients vs. GrazersJuly 1994 Sept. 1994 May 1995 July 1995

Physicochemical ParametersMixed layer depth (m) 10 18 70 12Secchi depth (m) 5 6.5 — 6Temperature (°C) 22 17 5 22TP (µM) 0.32 0.29 0.35 0.29TDP (µM) 0.13 0.13 0.19 0.23TN (µM) 29 38 49 23DIN (µM) 12 10 — —TN:TP 89 130 139 79DIN:TP 36 33 — —DSi (µM) 2.9 25 — 8.2

Biological ParametersChlorophyll a (µg/L) 1.5 2.1 6.3 1.8Phytobiomass (µg dwt/L) 146 274 1,264 20Zooplankton Biomass (µg dwt/L) 79 1,282 695 1,144Herbivore Biomass (µg dwt/L)1 79 371 485 648Primary Productivity

in fertilized cubitainers (mg C/m3/d) 15 13 119 5in the lake, areal (mg/m2/d)2 — 349 1,660 —

Limiting Nutrient3 N + P N + P None1Excludes biomass associated with late-stage cyclopoid copepods.2Cloudless conditions are assumed for the sake of comparability. The estimates include phytoplankton exudates andthus are approximately gross primary productivity. The values for the cubitainers are net productivity, without respira-tion and exudates).3Results of enrichment experiments conducted within a week of the grazing studies at the same sampling site (Levine etal. 1997).

Zooplankton Grazing of Phytoplankton in Lake Champlain 67

terest (from the macrozooplankton experiment)were multiplied by the biomass of macrograzers incubitainers to yield estimates of the fraction of phy-tobiomass removed per day. These were added tothe measured growth rates, yielding estimates ofgrowth rates in the absence of macrograzer contam-ination. Finally, the corrected growth rates were re-gressed on micrograzer biomass to obtain theclearance rates reported.

Phytoplankton communities were not altered bymacrozooplankton sievings and addbacks duringthe July and September experiments (Fig. 1). In

May, however, colonial diatoms (Aulacoseira sp.,Tabellaria spp., and Fragilaria crotonensis) domi-nated the phytoplankton. These colonies were suffi-ciently large to be partially retained by thezooplankton net and added to the HMA treatmentalong with zooplankton. Thus the HMA treatment

FIG. 1. The biomass of different phytoplanktongroups in cubitainers at the beginning (I) andending (F) of incubations with macrozooplanktonat three different densities (low (LMA), ambient(AMA), and high (HMA). The two sets of columnsindicate duplicate treatments. Incubation timeswere 44, 48, and 64 h in July, October, and May,respectively.

FIG. 2. The biomass of major zooplanktongroups in experimental cubitainers at the end ofthe three grazing studies. HMA, AMA, and LMArefer to high, ambient, and low macrograzer den-sities; HMI, AMI, and LMI, to high, ambient, andlow micrograzer densities. The two sets of columnsindicate duplicate treatments. The “lake” sampleswere taken at the initiation of the experimentsfrom the 240 L tank used to fill cubitainers. Lackof rotifers and nauplii in the “lake” in July is dueto these two samples accidentally being concen-trated with a 90 µm rather than a 64 or 20 µmsieve prior to counting; the sieve probably allowedrotifers to pass.

68 Levine et al.

had a phytoplankton biomass 2 to 3 times greaterthan the AMA and LMA treatments. Manipulationof micrograzers with a 20 µm sieve did not influ-ence cryptophyte abundance in cubitainers, but didaffect the densities of larger algae, especially thoseforming colonies (Fig. 3). The HMI cubitainerstherefore contained about three times as much phy-tobiomass, and the LMI cubitainers only one half asmuch, as was present in the lake (AMI). The ex-pected effects of these manipulations on grazingrate estimates are considered below.

Primary Productivity

The y-intercepts of the Lehman-Sandgren curvesprovided estimates of phytoplankton division ratesin the cubitainers, albeit under the nutrient- andlight-enhanced conditions of the incubations. Theserates were 0.20, 0.09, and 0.18 /d in July, Septem-ber, and May, respectively. Multiplying these valuesby phytobiomass and C content yielded net primaryproduction (NPP) estimates of 15, 13, and 119 mgC/m3/d. These rates can be compared with 14C esti-mates of gross primary productivity (GPP) at theincubation depth during September and May, 56and 184 mg C/m3/d respectively. The latter weregreater, despite the fact that they were for unfertil-ized conditions, because they include C exudedfrom algae or respired during the 2 day incubationsused to estimate NPP.

For the lake as a whole, areal primary production(GPP) estimates of 236 and 1,093 mg C/m2/d wereobtained in September and May, respectively. Theserates (the first ever reported for Lake Champlain)reflect partially cloudy weather. Under cloudlessconditions, the areal rates would be 349 and 1,660mg C/m2/d. The mixed layer was much deeper inMay than in September (70 vs. 18 m), and the phy-toplankton, although more abundant, were morelight limited (Levine et al. 1999). The mean volu-metric GPP rates for the mixed layer during Sep-tember and May were 13 and 16 mg C/m3/d,respectively. Under cloudless conditions, theywould be 19 and 24 mg C/m3/d.

Grazing Rates

Rates of phytoplankton loss from Lake Cham-plain as a result of macrograzing were very low inJuly and September 1994, < 1% of total phytobio-mass per day, and only modestly higher in May(10% per day). Losses to microzooplankton wereslightly greater and followed the same seasonaltrends, 4, 3, and 15% per day, in July, September,and May, respectively. Microzooplankton made upa relatively small proportion of total zoobiomass,but had weight-specific clearance rates four or moretimes those observed among macrozooplankton(Tables 2 vs. 3), hence the similarity in grazingpressure.

Expressed as a percentage of the NPP in cu-bitainers, macrozooplankton and microzooplanktonremoval rates for total phytobiomass were 2, 0, and56% and 20, 33, and 83% in July, September, andMay, respectively. Thus in May, grazers removedmore phytobiomass per day than was produced,

FIG. 3. The biomass of different phytoplanktongroups in cubitainers at the beginning (I) andending (F) of incubations with microzooplanktonat three different densities (low (LMI), ambient(AMI), and high (HMI). The two sets of columnsindicate duplicate treatments. Three levels ofmicrozooplankton density were used. Incubationtimes were 44, 48, and 64 h in July, October, andMay, respectively.

Zooplankton Grazing of Phytoplankton in Lake Champlain 69

contributing to the collapse of the spring bloom.For the September and May experiments, grazingcan also be expressed as a percentage of the meanGPP rate in the 18 and 70 m-deep mixed layers; thevalues are 22 and 658%.

Discrepancies in taxon vulnerability to macro-grazers were apparent both at the Division level andamong species within a Division (Table 2). DuringJuly, dinoflagellates lost 15% of their standingstock to grazing daily, while green algae lost 6%,

TABLE 2. Clearance rates (CR; mL/day / µg dwt zoopl.) for macrozooplankton feeding on phytoplank-ton and estimates of percentage algal biomass lost per day during the four studies in Lake Champlain.Rates are given for the total phytoplankton community, for algal divisions containing > 10% of phyto-plankton biomass, and for a selection of individual species. Clearance rates significantly different fromzero at P > 0.05 are indicated with an asterisk. GALD = greatest axial linear dimension.

July 1994 Sept. 1994 May 1995 July 1995

Taxon GALD C R % Lost C R % Lost C R % Lost C R % Lost

Total — 0.06 0.4 –0.02 0 0.26 10 0.17 11

By Division:Greens — 0.92 6 — — — — 0.41* 26Cryptophytes — 0.12 1 0.18* 7 — — 0.13* 8Diatoms — –0.68 0 –0.01 0 0.19 10 0.08 5Blue greens — –0.39 0 0.15 6 — — 0.07 4Dinoflagellates — 2.11 15 — — 1.69* 89 — —

By Species:Mougeotia sp. 18 1.8 13 — — — — — —Coelastrum scabrum 11 — — 1.8* 58 — — — —Cryptomonas sp. 7 1.79 12 0.28 9 — — 0.46 28Chroomonas sp. 4 –0.59 0 0.06 2 — — 0.09 6Aphanizomenon flos-aquae 32 — — –0.05 0 — — — —Chroococcales 2 — — — — 0.14 9Fragilaria crotonensis 27 — — 0.00 0 — — — —Tabellaria sp. 17 — — –0.01 0 — — — —Aulacoseira sp. 13 — — — — 0.16 8 — —Centrales1 2 –5.63 0 — — — — 0.05 3

1An unidentified, small species, not colonial. This estimate does not include grazing on other Centrales, like Aulacoseira.

TABLE 3. Clearance rates (CR; mL/day / µg dwt zoopl.) for microzooplankton feeding on phytoplanktonand estimates of percentage phytobiomass lost per day, during the three grazing studies in Lake Cham-plain. Rates are given for the total phytoplankton community and for algal divisions containing > 10% ofphytoplankton biomass. Clearance rates significantly different from zero at P > 0.05 are indicated with anasterisk.

July 1994 Sept. 1994 May 1995

Taxon C R % Lost C R % Lost C R % Lost

Total 2.28 4 1.07* 3 0.83* 15

By division:Greens 0.81 1 — — — —Cryptophytes 4.40 7 2.15* 6 — —Diatoms — — 0.33 1 1.12* 20Blue-greens — — 2.64* 7 — —

70 Levine et al.

and cryptophytes, 1%. Blue-green algae had nega-tive clearance rates (grew more rapidly in the pres-ence of grazers), and thus probably were largelyinvulnerable to grazing. In September, when di-noflagellates and green algae were scarce, crypto-phytes and blue-green algae were subject to greatergrazing mortality, 7 and 6% per day, respectively.Diatoms also were present at this time, but appar-ently not utilized by grazers (loss rate, 0% per day).In May, diatoms, the biomass dominant, lost an av-erage of 10% of their biomass per day to macro-grazers, which in this case included copepods aswell as Cladocera. Micrograzers consumed crypto-phytes, blue-green algae, and diatoms at higherrates than did macrograzers (Tables 2 vs. 3). Duringthe spring bloom, they removed 20% of diatom bio-mass daily. Micrograzers were somewhat less effi-cient than macrograzers at green algal consumption,perhaps because the green algae present were rela-tively large.

Table 2 includes rates of macrozooplankton graz-ing on some of the more abundant phytoplanktonspecies. Grazing by microzooplankton at thespecies-level was not measured because of the lowdensities of the species involved. Among the cryp-tophytes, Cryptomonas sp. was more vulnerable tomacrozooplankton predation (11 to 12% loss perday) than Chroomonas sp. (loss rate, 0 to 2% perday). Among the blue-green algae, the Chroococ-cales were more heavily grazed than Aphani-zomenon flos-aquae , and, among diatoms,Aulacoseira sp. suffered greater grazing losses thanFragilaria crotonensis and Tabellaria sp.

Many of the clearance rates given in Tables 2 and3 are not significantly different from zero at a Plevel of 0.05. This is partly because clearance rateswere, in fact, very low, but also because of scatterrelated to the patchy distribution of large (espe-cially colonial) and rare phytoplankton species andthe strong influence of individual animal length onzoobiomass estimates (all animals were assumed tohave a mean length, when length actually varied).Although the regressions would have been greatlyimproved by the omission of outliers, this wasavoided, given that there was no knowledge of whatthe correct grazing rates were. The reporting oflarge grazing rate estimates not significantly differ-ent from zero should be viewed as provisional.

Nutrients Versus Grazers

The nutrients vs. grazers experiment took placeat the same time of year as the July grazing experi-

ment, but a year later. Epilimnion depth, tempera-ture, and nutrient concentrations were nearly identi-cal during the two studies. Phytoplankton speciescomposition was similar in the 2 years (Figs. 1 vs.4), but phytobiomass was 7 times lower and herbi-vore biomass, 8 times greater in 1995. In addition,cyclopoid copepods (mostly Mesocyclops edax)were abundant in 1995, but scarce in 1994 (Fig. 2vs. 5). This was essentially a “clear water” phase.

Samples taken 23 h into the experiment indicatedlittle change in chlorophyll a concentrations rela-tive to initial values (Fig. 6), suggesting that phyto-plankton may need a day or more to “gear up” fornutrient assimilation and division (Reynolds 1984),and also that grazing was not particularly intense.

FIG. 5. The biomass of major zooplanktongroups in experimental cubitainers at the end ofthe nutrients vs. grazers experiment, July 1995.The three sets of columns indicate triplicate treatments.

FIG. 4. The biomass of major phytoplanktongroups in experimental cubitainers at the begin-ning (I) and end (F) of the nutrients vs. grazersexperiment, July 1995. The three sets of columnsindicate triplicate treatments.

Zooplankton Grazing of Phytoplankton in Lake Champlain 71

After 92 h, however, most nutrient-enriched cu-bitainers had chlorophyll a concentrations substan-tially above initial values, as did cubitainers withelevated zooplankton densities. By contrast, thosecubitainers that were left unfertilized and with zoo-plankton densities at or below ambient levelsshowed no increase in chlorophyll a. Two-wayANOVA indicated that both nutrient and zooplank-ton levels had a statistically significant (p < 0.05),and positive, impact on phytoplankton growth ratesduring the 3 days of the experiment when chloro-phyll a levels increased (23 to 92 h). The nutrient-zooplankton interaction term was trivial.

A plot of phytoplankton growth rate in cubitain-ers versus total zooplankton biomass (herbivoresplus carnivores) revealed only a weak relationshipbetween the two variables under nutrient enrich-ment, but a strong positive relationship when no nu-trients were added (Fig. 7). Growth rates declinedsharply with herbivore biomass in the fertilized sys-

tems, but, in the unfertilized systems, they first in-creased and then decreased as herbivore biomassrose above 0.5 mg/L. The slope of the relationshipbetween growth rates and herbivore biomass forfertilized and unfertilized systems was similarabove the 0.5 mg/L threshold, suggesting that inboth treatments, phytoplankton divided at maximalrates when zooplankton were dense enough to keepthe nutrient supply high. Thus the slopes obtainedreflected grazing impacts alone.

Analysis of zooplankton communities at the endof the experiment revealed some unexpected treat-ment side effects. As intended, total zooplanktonand also cyclopoid biomass in cubitainers increasedalong the gradient LMA to AMA to HMA, whilenauplii densities were uniform (micrograzers werenot manipulated). Cladoceran biomass, however,proved to be greater in the AMA than in the HMAcubitainers, and rotifer biomass peaked at the low-est macrozooplankton level (Fig. 5). Furthermore,

FIG. 6. Chlorophyll a concentrations in experi-mental cubitainers 0, 23 and 92 h into the nutri-ents vs. grazers experiment, July 1995. Initial con-centrations were measured for the water used tofill cubitainers of similar treatment. The three setsof columns indicate triplicate treatments.

FIG. 7. Phytoplankton growth rates vs. (a) totalzooplankton biomass and (b) herbivore biomass(late stage cyclopoids excluded) of July 1995.Trend lines are linear regressions. The one shownin Panel b is for fertilized cubitainers.

72 Levine et al.

rotifer biomass (mostly Polyarthra and Keratella)was significantly greater in fertilized than in unfer-tilized cubitainers. These results suggest that the 4-day incubation was too long for maintenance of thezooplankton community structure imposed by thetreatments. Rotifers, which can reproduce rapidlydue to small size and parthenogenesis, were able toexploit the greater productivity of the nutrient-en-riched systems and increase in numbers, while cy-clopoid copepods managed to exert a grazingimpact on rotifer densities. Cyclopoid predationmay have also been responsible for the reducedcladoceran densities in the HMA treatments.

Macrograzer densities created in fertilized cu-bitainers permitted an additional set of grazing rateestimates for mid-summer (Table 2). The findingswere similar to those obtained during the year be-fore: green algae were more heavily grazed than di-atoms, cryptophytes, and blue-green algae (26% vs.4 to 8% of biomass/d), and among the cryptophytes,Chroomonas sp. was grazed less than Cryptomonassp. (6% vs. 28% loss/d). The clearance rate for thephytoplankton community as a whole was 0.17 mLper g of zooplankton/d, or 11% of phytobiomassdaily. Using change in chlorophyll a concentrationin the fertilized cubitainers to calculate communitygrazing mortality yielded a nearly identical result(0.18 mL/g zooplankton/d or 11% of biomass/d).All grazing estimates derived from the nutrientsversus grazers experiment may be slightly belowreal values because of the growth of rotifers in thefertilized LMA treatment. These animals likelycontributed to a lowering of the algal growth rate inthe cubitainers involved, with the result that thegrowth rate vs. zoobiomass curve was less steepthan it otherwise would be. Net primary productiv-ity was especially low during this experiment (5 mgC/m3/d).

DISCUSSION

Macrozooplankton Grazing on Phytoplankton

This study provided much needed data on taxon-specific rates of phytoplankton mortality due tozooplankton grazing. Modeling of phytoplanktoncommunity dynamics requires such information,but only one study prior to this has yielded rate esti-mates for taxonomic levels finer than the entirecommunity. With this study, the list of analyzedspecies is expanded from warm temperate speciesto those found in boreal regions and the GreatLakes.

Like Elser and Goldman (1991) and Elser (1992),

substantial variability in the vulnerability of phyto-plankton taxa to macrozooplankton consumptionwas found. Some species lost as much as 58% oftheir biomass per day to grazers, while others wereunaffected. A few had negative grazing rates, indi-cating that they grew more quickly when zooplank-ton were abundant than when they were rare. Thesespecies are likely immune to grazing and benefitfrom the suppression of more vulnerable phyto-plankton species, either because this suppression re-leases resources for their use or because thesuppressed species are active in allelochemical production.

Much of this assessment was done at the Divisionlevel. Most conclusions were in agreement withprevious findings and with widely-accepted con-cepts about the role that grazers play in influencingalgal community structure; a few were not. BecauseCladocera dominated during all but the May experi-ment (when calanoid copepods were also impor-tant), these observations pertain primarily to thisgroup of herbivores.

One important paradigm of phytoplankton ecol-ogy is the existence of low grazing pressure onblue-green algae, which presumably are toxic(Arnold 1971), of low nutrient content (Lampert1981), or too large and filamentous for easy con-sumption (Lampert 1981, Infante and Abella 1985).Blue-green grazing mortality was consistently mini-mal (< 1% per day) during the experiments.

Another group frequently described as relativelyinedible is the dinoflagellates (Porter 1977, Som-mer et al. 1986). However, higher grazing rateswere measured on this group than on any other, andthese results are not unique. Elser (1992) reportedgrazing rates on Gymnodinium similar to ours, andProulx et al. (1996) described the blooming of di-noflagellates in a Quebec lake following the re-moval of zooplankton through planktivorous fishintroduction. Dinoflagellate vulnerability may de-pend on size, and the species seen during the exper-iments were relatively small (except for Ceratiumhirundinella).

Among the phytoplankton generally consideredmost edible are cryptophytes, chrysophytes, andsmall non-colonial diatoms (Porter 1977, Sommeret al. 1986). Chrysophytes were too rare in LakeChamplain during this study to permit estimation oftheir losses to grazers, while the response of crypto-phytes to grazing was mixed. Cryptomonas sp. wasas strongly grazed as green algae and dinoflagel-lates, while Chroomonas sp. suffered minimallosses. Many other researchers have reported heavy

Zooplankton Grazing of Phytoplankton in Lake Champlain 73

grazing on Cryptomonas (e.g., Porter 1977, Lehmanand Sandgren 1985, Knisely and Geller 1986). TheCryptomonas clearance rates measured by Elser andGoldman (1991) and Cyr and Pace (1992) wereeven greater than those reported here. ThatChroomonas sp. is poorly grazed may contribute toits status as the most abundant phytoplankter inLake Champlain (McIntosh et al. 1993).

Minimal grazing was observed on diatoms exceptfor Aulacoseira sp., which was grazed at a rate of8% per day in May 1995. Because the diatoms pre-sent during this study were primarily colonies largein at least one dimension, this was expected. Elserand Goldman (1991) measured similarly low ratesof diatom consumption in California lakes. That di-atom losses were greater in May than July and Sep-tember may be related to the greater densities ofcalanoid copepods present. Calanoids are more effi-cient than Cladocera at consuming large particles(Peters and Downing 1984).

Green algae are a highly diverse group which ap-parently contains both edible and inedible species(Porter 1977, Sommer et al. 1986). This group wasfound to be one of the most heavily grazed in LakeChamplain, although some of the algae used as preyitems (e.g., Mougeotia sp.) have been previouslyclassified as inedible (Knisely and Geller 1986). Intheir study of California lakes, Elser and Goldman(1991; Elser 1992) measured low grazing rates onmost green algae, but relatively high rates for cer-tain species of Cosmarium, Oocystis, Quadrigula,and Selenastrum.

There were no obvious clues as to why somespecies within phytoplankton divisions were morevulnerable to grazing than others. In some cases,size may have been a factor. Three of the poorlygrazed species, Tabellaria sp., Fragilaria crotonen-sis, and Aphanizomenon flos-aquae, were coloniestoo large to fit easily into the gap of most clado-ceran carapaces or into the mouths of most cope-pods (Gliwicz 1977). Two colonial species,Aulacoseira sp. and Mougeotia sp., were moder-ately grazed, but these algae form singular fila-ments that grazers can attack by holding down oneend and pulling (or biting) cells off the other (Van-derploeg and Paffenhöfer 1985).

Very small algae may slip though the filteringsetae of grazers (Gliwitz 1977) and thus be capturedat a lower rate than larger algae. The two smallest-sized species that were assessed, Chroomonas sp. (4 µm) and an unidentified centric diatom (2 µm),were very poorly grazed. A third small-celledspecies, Chroococcus sp., was moderately grazed,

but it forms small colonies that increase its effec-tive size. The moderately to heavily grazed speciesin the experiments were 5 to 25 µm in their greatestaxial l inear dimensions (GALD), or formedcolonies of this size. The literature indicates thatthe optimal food size for most zooplankton is 10 to20 µm GALD (Svensson and Stenson 1991).

Differential species vulnerability to grazerswithin phytoplankton divisions presents a challengeto modelers of phytoplankton dynamics. Modelingthe dynamics of dozens of phytoplankton species iscumbersome. Therefore, phytoplankton are oftencompartmentalized by division rather than byspecies (e.g., Lehman et al. 1975, Scavia et al.1988). These results, and Elser’s, indicate that meandivision loss rates mask important dynamics at thespecies level. A more fruitful approach may be themodeling of a selection of particularly abundantspecies.

Microzooplankton Grazing on Phytoplankton

In a recent study of Castle Lake, CA, Elser andFrees (1995) showed that the traditional emphasisof grazing studies on macrozooplankton maygreatly underestimate overall grazing mortality.Phytoplankton removal rates by micrograzers inthis lake (5 to 22% of phytobiomass per day) over-lapped or exceeded removal rates by macrograzers(5 to 12% per day). This study of micrograzing inLake Champlain also indicated phytoplanktonlosses on par with the losses to macrozooplankton(3 to 14% vs 1 to 10% of phytobiomass per day).Micrograzers had significantly higher biomass-spe-cific clearance rates than macrograzers when feed-ing on most phytoplankton divisions (cryptophytes,blue-green algae, and diatoms). Only green algaeappeared to be more easily cleared from suspensionby macro- than micrograzers. The green algalspecies involved (e.g., Mougeotia sp., Oocystisspp., and Coelastrum scabrum) were relativelylarge or colonial in nature, and thus presumably dif-ficult for a small animal to handle.

Phytoplankton species > 20 µm in size were ma-nipulated substantially by the sievings and ad-dbacks preceding the micrograzer experiment. Allphytoplankton divisions except the Cryptophytawere affected. Because clearance rates decline withfood density (as the inverse of the cube root; Petersand Downing 1984) and growth rates rise with di-minished mortality, the impact of unequal phyto-plankton densities on the Lehman-Sandgren curvesis expected to be a reduction in slope (as the point

74 Levine et al.

at high grazer density rises), and thus an underesti-mation of grazing rates. Thus microzooplanktonmay be even more important grazers than the valuesin Table 4 suggest. A recent study by Cyr (1998)suggested that zooplankton communities dominatedby rotifers or copepod nauplii generally have higherbiomass-specific clearance rates than communitiesdominated by Cladocera or adult copepods.

Total Grazing Loss

Micro- and macrozooplankton together removedfrom 2 to 21% of the phytoplankton biomass in themixed layer of Lake Champlain daily. Cyr and Pace(1992) reported exactly the same range of removalrates for 30 phytoplankton communities in 16northeastern U.S. lakes (they analyzed grazers > 80µm, and thus included the bulk of micrograzers).The similarity of the two ranges indicates that tem-poral variability in grazing rates may be of a similarorder of magnitude to lake-to-lake variability, andshould be considered when phytoplankton grazingis modeled.

Comparisons of grazing loss rates with phyto-plankton replacement rates have more ecologicalsignificance than comparisons with standing stocks.However, estimates of species-specific reproductiverates are almost as rare as species-specific grazingmortality estimates. The estimates of reproductiverates reported here were for artificial conditions(more light and nutrients than normally enounteredin the lake), but they are useful in forming firstorder approximations of grazing as a fraction ofproduction. The combined mortality induced bymacro- and microzooplankton grazing balancedfrom 22% to 139% of the NPP measured in fertil-ized cubitainers. Thus, grazing is a more importantphenomenon in Lake Champlain than the slow bio-mass loss rates suggest. Grazing should be moresignificant yet when related to the lower NPP ratesexpected in the absence of nutrients. When con-trasted with the mean primary productivity of thelake’s mixed layer (the 14C estimates), grazing mor-tality was relatively minor in September (22%), butexceeded primary production by 6 fold in May. Cyrand Pace (1993) reported that, on average, zoo-plankton remove 51% of lacustrine primary produc-tion annually.

Reproductive rates in the cubitainers were on thelow end of values reported for natural phytoplank-ton communities in the literature, 0.1 to 0.2 versus0.1 to 0.9 per day (Reynolds 1984). Thus the imageof phytoplankton dynamics that emerges for Lake

Champlain is that of a slowly-growing phytoplank-ton community cropped by zooplankton feeding ata similarly slow pace. Analysis of zooplankton-pro-tozoan-bacterial interactions in the cubitainers indi-cated that, while phytoplankton were the principalfood source for zooplankton in May, bacteria andheterotrophic protozoa were more important in Julyand September (Levine et al. 1999).

Nutrients Versus Grazers as Phytoplankton Controls

The nutrients vs. grazers experiment supportedthe view that nutrients play a major role in regulat-ing algal biomass in Lake Champlain. Combinedadditions of N, P, and C to cubitainers increasedphytoplankton growth rates, even at the highestgrazer level (ANOVA indicated a nutrient level ef-fect at p < 0.03). Other researchers conducting fieldexperiments of a similar design have noted thesame effect (Lehman and Sandgren 1985, Berquistand Carpenter 1986, Vanni 1987, Kivi et al. 1993,Spencer and Ellis 1998). At the division level, re-sponse to nutrients was more variable. Diatoms andgreen algae generally increased with fertilization,while cryptophytes and blue-green algae sometimeswere negatively affected, or showed no consistentresponse (Fig. 4; also Levine et al. 1997). Lehmanand Sandgren (1985) and Berquist and Carpenter(1986) also found negative responses to fertilizationamong some species in their experiments.

While it is generally expected that grazers shouldreduce phytobiomass and thus growth rates,ANOVA indicated a significant (p = 0.01) positiverelationship between the phytoplankton growthrates in cubitainers and macrozooplankton level.Lehman and Sandgren (1985) and Berquist andCarpenter (1986) also observed phytoplanktonspecies whose growth rates increased on elevationof zooplankton levels. Both Kivi et al. (1993) andSpencer and Ellis (1998) reported a lack of relation-ship between chlorophyll a concentration andgrazer level in nutrient versus grazer experiments,although some positive trends were found inSpencer and Ellis’s study. Kivi et al. (1993) ob-served a slightly positive relationship between pri-mary productivity and grazer level, while Elser andMacKay (1989) reported a stronger relationship.What is implied by the outcome of this experimentand those of others is that nutrient recycling by zoo-plankton has a greater impact on phytoplankton dy-namics than does grazing mortality. By examiningalgal growth in fertilized and unfertilized cubitain-

Zooplankton Grazing of Phytoplankton in Lake Champlain 75

ers separately, it was confirmed that the positive re-lationship between growth rates and zooplanktonlevel was due to algal response in the nutrient-poorsystems. For the fertilized cubitainers, there was anegative relationship between growth rate and her-bivore biomass.

The nutrients vs. grazers experiment was perhapsmost valuable in revealing the tremendous inter-twining of feeding and recycling relationships inthe lake, and thus the potential for indirect conse-quences to resource or trophic level manipulations.Nutrient additions stimulated algal growth, whichin turn allowed rotifer growth to increase, providingmore food for cyclopoid copepods. This was justone of the “cascades” observed. The rotifer increasealso drove down protozoan populations, allowingbacterial densities to rise (Levine et al. 1999).These results argue strongly for a modeling ap-proach to phytoplankton ecology, as models areneeded to track multiple permeations of effects.They also suggest that time limits are necessary onthe field incubations used to calculate grazing rates.Four days is obviously too long. One to 2 days maybe more reasonable, given that rotifer increaseswere not observed during grazing studies of thislength. Brief incubations create problems whengrazing rates are low, however. Substantial samplereplication and intensive phytoplankton countingare necessary to obtain grazing curves with reason-ably high r2 values. Chlorophyll a analysis yieldedestimates of total phytoplankton grazing mortalitysimilar to those obtained for phytobiomass, andthus this low-labor method may be used whenspecies-level analyses are not required.

Future Research

Future research on zooplankton grazing in LakeChamplain should include separate night time andday time analyses. This study did not take into ac-count the fact that macrozooplankton undertake ex-tensive vertical migrations that cause them toconcentrate at the surface at night and disperse atdepth during the day. The analysis also should beextended to other lake sub-basins, and to winter-time sampling. Assessment of cyclopoid copepodfeeding dynamics is desirable, as these largely car-nivorous species are common in the lake and mayindirectly affect rates of herbivory. They may alsoengage in omnivory when animal foods are scarce(Adrian and Frost 1993), although they are believedto be inefficient feeders on algae of the small sizecommon to Lake Champlain (Knoechel and Holtby

1986). Had cyclopoids been included in the esti-mates of herbivore biomass, the values obtained forbiomass-specific clearance rates would have beenreduced by 5 to 98% (estimates of phytoplanktonloss rates would not be greatly affected as their de-termination involves multiplication of the lowerclearance rates by greater zoobiomass). Improvedassessment of micrograzing is another priority. Thedilution method of Landry and Hassett (1982) mayallow grazers to be manipulated without affectingphytoplankton. However, when animal and algalpopulations are already small, dilution further in-creases variability in density determinations andleads to low r2 values for grazer curves (J. Lehman,Univ. Mich.)

A larger goal is extension of species-specificgrazing analyses to a diversity of lakes around theworld, and comparison of these losses with repro-ductive rates. Cyr’s (1998) recent comparison ofgrazing in lakes with different zooplankton commu-nity types indicates that calanoid copepods and ro-tifers can exert as much grazing mortality onphytoplankton as Cladocera under oligotrophic con-ditions. Thus it is particularly important that studiesof species-specific grazing mortality expand be-yond the cladoceran-dominated lakes investigatedto date.

ACKNOWLEDGMENTS

We thank H. McKinney for assistance in the fieldand laboratory, S. Pomeroy for help with data entry,and the Vermont and New York Departments of En-vironmental Conservation for the sharing of unpub-lished data on nutrients and chlorophyll in LakeChamplain. J. Lehman, R. Stemberger, A. McIn-tosh, and an anonymous reviewer provided usefulcomments on the manuscript. This project wasfunded by the U.S. Environmental ProtectionAgency through the New England Interstate WaterPollution Control Commission (LC-RC92-6-NYRFP).

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Submitted: 8 December 1997Accepted: 18 October 1998Editorial handling: Marlene S. Evans