The impact of zooplankton grazing on phytoplankton species composition andbiomass in Lake Champlain (USA-Canada)

Rates of grazing on phytoplankton by macrozooplankton (cladocerans and cope pods > 220 mu m in length) and microzooplankton (animals < 220 mu m, mostly rotifers and nauplii) were determined for Lake Champlain on three occasion s using a modified version of the Lehman-Sandgren method. Gradients in graz er density were created in fertilized cubitainers incubated in situ, and cl earance 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 mu m in size and gener ally 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 g reen algae (0 to 6%/day). However, variability in grazing vulnerability amo ng the species within divisions was high. Microzooplankton had greater weig ht-specific clearance rates than macrozooplankton when consuming diatoms, b lue-green algae, and cryptophytes, but were less efficient at harvesting gr een algae. An experiment in which nutrients and zooplankton were manipulate d in a 2 x 3 factorial design indicated that both variables have a net posi tive impact on phytoplankton growth rates in Lake Champlain, the zooplankto n because they excrete required nutrients. Indirect effects of the nutrient s vs, grazers experiment included rotifer growth in response to increased a lgal productivity and harvesting of rotifers and Cladocera by cyclopoid cop epods. It was concluded that both nutrients and grazing influenced the stru cture of Lake Champlain's phytoplankton community, but that nutrients were generally more important.