TROPHIC CASCADES AND PHYTOPLANKTON COMMUNITY STRUCTURE

Two predators of zooplankton, fish (yellow perch Perca flavescens) and larval insects (Chaoborus), were manipulated to assess their indirect effects on phytoplankton. The two predators caused similar reductions in zooplankton size and biomass, and as a consequence, zooplankton grazing rates on phytoplankton, but phytoplankton increased relative to predator-free controls only in enclosures with fish and not in enclosures with Chaoborus, suggesting that fish have effects on phytoplankton independent of their effects on zooplankton grazing rates. Fish render P more available to phytoplankton: 1) by directly excreting and egesting P, and 2) by increasing rates of P excretion by the zooplankton community. Phytoplankton taxa exhibiting enhanced biomass in the presence of fish were those with high P requirements, including green and blue-green algae and dinoflagellates. -from Authors