Opposing diel migration of fish and zooplankton in the littoral zone of a large lake

Abstract

Backscatter data from an acoustic Doppler current profiler are analyzed to provide simultaneous estimates of relative concentration of Daphnia and relative abundance of young-of-the-year perch in the vicinity of an artificial reef in the littoral zone of Lake Constance. These long-term (3 months) and highresolution (1 h) observations demonstrate the existence of opposing diel migration patterns in fish and zooplankton. According to the data, the migration pattern constrains feeding of young-of-the-year fish on vertically migrating zooplankton to a few hours around sunrise and sunset. Temperature recorded simultaneously with the backscatter signal indicates that the diurnal activity pattern and the vertical distribution of the fish are not affected by strong temperature fluctuations generated by high-frequency internal wave motions. The abrupt decline of internal wave activity following a storm-induced deepening of the surface mixed layer, however, seems to be a trigger for changes in the diurnal activity and hence the intensity of predator prey interactions.