Interactions between zooplankton and fish in a fertilized lake

Abstract

The effects of fish predation on the zooplankton community in an oligotrophic lake, Langvatn, near Trondheim in Central Norway, were investigated during a six‐year period (1973–1978), together with the added effects of changes produced by adding artificial fertilizer in 1975 and 1976. The improved nutrient conditions in 1975 resulted in a rapid increase in biomass and production of the largest herbivore zooplankton species and of the fish population. A change in the behaviour and food habits of the arctic char was recorded; they became more pelagic and fed mainly on zooplankton. An increased survival rate of 0‐group and biomass of planktivorous fish in 1975 enhanced the degree of fish predation on the zooplankton during subsequent years (1976–1978). As a consequence of fish predation, the composition of the zooplankton changed, from a mainly large‐sized to a mainly small‐sized community, dominated by Bosmina longirostris and rotifers. Since fish predation is size‐selective and visibility‐dependent, it induced a decrease in mean size and in body length at onset of maturity of the cladoceran populations and probably also weakened their ability to produce resting eggs.