Limnetic herbivory: effects on phytoplankton populations and primary production

Population-level and ecosystem-level responses of phytoplankton to grazing by zoo­plankton were determined m nutrient-enriched and unenriched enclosures in Peter Lake, Michigan. Species-specific net growth rates were determined, and chlorophyll a, primary production (PPR), and alkaline phosphatase activity (AP A) were determined for each of three size categories of phytoplankton: < 22 µm, 22-7 5 µm, > 7 5 µm. Thirteen of 16 dominant algal taxa were nutrient limited. Zooplankton reduced APA, an indicator of nutrient deficiency. Growth rates of grazed, nutrient-limited algae ( < 30 µm) increased at low levels of zooplankton biomass, but decreased at higher biomass levels. Growth rates of larger (> 30 µm) nutrient-limited algae increased or did not change as zooplankton biomass increased. These species-specific responses resulted in the four- to five-fold changes by in chlorophyll concentration and PPR. Overall the stimulatory effect of nutrient regeneration by zooplankton overrode losses due to grazing, and caused net increases in phytoplankton biomass and productivity.