Discrimination by freshwater zooplankton between single algal cells differing in nutritional status.

Grazing rates of a freshwater copepod (Diaptomus kenai) and a freshwater cladoceran (Daphnia rosea) on the green alga Selenastrum minutum were determined to be dependent on the nutritional status of individual cells. Cells that were less nitrogen limited were ingested at a greater rate than cells reared under more nitrogen limited conditions. Diaptomus displayed active discrimination, possibly via chemodetection, while Daphnia expressed passive selection, probably via differential retention on filters. These results suggest that the impact of grazing zooplankton varies with the physiological state of components of the phytoplankton community.