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

Rates of grazing on phytoplankton by macrozooplankton (cladocerans and copepods > 220 μm in length) and microzooplankton (animals < 220 μm, mostly rotifers and nauplii) were determined 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 rates on 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 productivity daily. Macrozooplankton fed most heavily on algae 5 to 25 μm in size and generally selected dinoflagellates 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 among the species within divisions was high. Microzooplankton had greater weight-specific clearance rates than macrozooplankton when consuming diatoms, blue-green algae, and cryptophytes, but were less efficient at harvesting green algae. An experiment in which nutrients and zooplankton were manipulated in a 2 × 3 factorial design indicated that both variables have a net positive impact on phytoplankton growth rates in Lake Champlain, the zooplankton because they excrete required nutrients. Indirect effects of the nutrients vs. grazers experiment included rotifer growth in response to increased algal productivity and harvesting of rotifers and Cladocera by cyclopoid copepods. It was concluded that both nutrients and grazing influenced the structure of Lake Champlain's phytoplankton community, but that nutrients were generally more important.