Effects of planktivorous fish mass mortality on the plankton community of Lake Mendota, Wisconsin: implications for biomanipulation

Abstract

Massive mortality of planktivorous fish had a dramatic impact on plankton community dynamics of Lake Mendota, Wisconsin, USA. After fish mortality, the larger Daphnia pulicaria replaced the smaller Daphnia galeata mendotae, resulting in greater grazing pressure on phytoplankton. This was accompanied by a much longer spring clear-water period and lower summer phytoplankton biomass compared to years before the fish mortality. Analysis of historical data (from the mid-1970’s) showed that previous fluctuations in planktivorous abundance had similar effects on Daphnia abundance and species composition, and on spring phytoplankton biomass. However, the mid-1970’s fish fluctuations had no detectable effect on summer phytoplankton. Concentrations of phosphorus were much higher in the 1970’s (spring P 80–135 μg 1−1 ’) than in the 1980’s (spring P 19–36 μg 1−1) and it is possible that high P concentrations may reduce trophic cascade effects on summer phytoplankton communities. This suggests that the success of biomanipulation programs may be dependent on lake nutrient status.