Nutrient enrichment and selective predation by zooplankton promote Microcystis (Cyanobacteria) bloom formation

An experiment was conducted with a natural freshwater phytoplankton community from a eutrophic pond to investigate the combined effects of phytoplankton community competitive interactions, nutrient enrichment and zooplankton on Microcystis bloom formation. The pond water initially had a very low concentration of Microcystis, but the total phytoplankton biomass as chlorophyll a reached 110 mg L in summer. The pond water was incubated outdoors at natural temperature and light with a 2 2 factorial manipulation of nutrient (nutrient additions versus no additions) with and without zooplankton. The interaction of a two-phase nutrient addition (a net increase in concentrations of 250.0 mM N and 16.1 mM P each time) and the presence of zooplankton significantly altered phytoplankton community composition. When the water initially contained zooplankton, nitrogen and phosphorus enrichment promoted a surface Microcystis bloom. However, when the zooplankton was removed from the water at the start of the experiment, no surface Microcystis bloom formed, regardless of nutrient additions. Chlorophyta dominated in the absence of zooplankton, when the same nutrient was provided. Our results demonstrate that Microcystis bloom formation in this eutrophic water body at a mean temperature of about 368C at 14:00 h was closely related to the initial presence of zooplankton and a sufficient supply of nitrogen and phosphorus. We believe this is one of the first demonstrations of zooplankton controlling Microcystis bloom formation in a water body previously free of surface cyanobacterial blooms.