Multiple effects of planktivorous fish on algae through a pelagic trophic cascade

We used a 2 x 2 factorial design enclosure study of biomanipulation (controls, with fish, with added nutrients, and with both fish and nutrients) during 1987 in Lake St. George, Ontario, to determine the impacts of planktivorous fish on phytoplankton populations and water clarity. Addition of nutrients had little or no effect on either communities of zoo- or phyto-plankton, or water clarity, but addition of fish caused dramatic changes. Fish biomass was negatively related to daphnid biomass and body size, but was positively related to bosminid and rotifer biomass. Fish also caused a marked shift in algal communities towards smaller cells and reduced water clarity. However, neither fish nor nutrient treatments had significant effects on concentrations of chlorophyll a. Our estimates of fish consumption and excretion rates and zooplankton community grazing and excretion rates indicated that fish-induced reductions in zooplankton biomasses were indirectly responsible for changes in the phytoplankton. By reducing herbivory, fish caused decreases in both zooplankton grazing and nutrient recycling. Our results show that changes in fish communities can influence water clarity, but that the simple trophic cascade is confounded by several other direct and indirect mechanisms involving shifts in grazing pressure, nutrient excretion, and phytoplankton cell size.