Nutrient regeneration by zooplankton: effects on nutrient limitation of phytoplankton in a eutrophic lake

We tested the hypothesis that excretion of nutrients by zooplankton can reduce the severity of nutrient limitation of phytoplankton, and determine whether the phytoplankton community is limited by nitrogen or phosphorus. In situ experiments were conducted in eutrophic Lake Mendota (Wisconsin, USA) during the summer of 1988, where phytoplankton were limited by N and P, but periods of nutrient limitation were transitory Increased zooplankton biomass and the consequent increased excretion of nutrients by zooplankton reduced P limitation (as measured by specific alkaline phosphatase activity) in all experiments Excretion of nutrients also reduced N limitation (as measured by ammonium enhancement response) in one of three experiments. In additional experiments in the more highly eutrophic Lake Wingra, excretion of nutrients by zooplankton reduced both N and P limitation These results support the hypothesis that zooplankton have potentially important indirect effects on phytoplankton communities through recycling of nutrients