The role of benthivorous and planktivorous fish in a mesotrophic lake ecosystem

The effectiveness of fish removal as a tool for biomanipulation and restora tion was studied from 1993 to 1997 in shallow Lake Pohjalampi (North Kareli a, eastern Finland). The external nutrient loading into this lake was low a nd nearly stable throughout the study period. During a period of 5 years, a total of > 200 kg ha(-1) of fish, mainly roach and bream, were caught and thus the total fish biomass was reduced by nearly 80%. However, in 1996 a v ery dense year-class of perch hatched and the number of planktivorous fish increased again. These changes in the fish community resulted in a decrease d fish predation on benthic invertebrates, whereas the predation of fish on zooplankton increased. The responses in the prey communities were consiste nt with these changes: the biomass of benthic invertebrates increased and t hat of zooplankton decreased. At the same time, nutrient and chlorophyll a concentrations declined. The decreasing chlorophyll a:P ratio indicated als o that the nutrient cycling in the lake was reduced and despite the increas ed predation on herbivorous zooplankton the lake was shifted to a more olig otrophic state.