Analysis of zooplankton grazing rates and the effect of grazing on phytoplankton populations (Idaho)

Zooplankton community grazing rates were measured in situ during the summer season in two montane oligotrophic lakes in the Sawtooth Valley of central Idaho. Assays of the amount of 14C-labeled phytoplankton consumed by zooplankton indicated that zooplankton biomass and temperature had the largest effect on community grazing rate. Taxonomic composition of the zooplankton community had a small but significant effect on grazing rates. Time of day had no influence on grazing rates. A statistical model was developed to compare seasonal and spatial variation in grazing with measurements of phytoplankton growth. Zooplankton grazing ranged from 10 to 13% of algal growth in the two lakes during the two summers measured. Results of this research indicated that grazing by crustacean zooplankton had little impact on phytoplankton population growth rates. Approximately 11% of phytoplankton mortality was attributed to zooplankton grazing in these lakes.