Synthesis of theoretical and empirical experiences from nutrient and cyprinid reductions in Lake Ringsjon

The reduction in external phosphorus load to Lake Ringsjon during the 1980s , did not result in improved water transparency during the following ten-ye ar period. Furthermore, a fish-kill in the Eastern Basin of the lake, in ad dition to a cyprinid reduction programme (biomanipulation; 1988-1992), in c ontrast to theory, did not lead to any increase in zooplankton biomass or s ize. This absence of response in the pelagic food chain may have been attri buted to the increase in abundance of YOY (0+) fish, following the fish red uction programme. Despite the lack of effect on zooplankton, there was a de crease in phytoplankton biomass, a change in species composition and an inc rease in water transparency following biomanipulation. In 1989, one year af ter the fish-kill in Eastern Basin, the Secchi depth (summer mean) increase d from 60 cm to 110 cm. In the following years, water transparency increase d further, despite an increase in phosphorus loading. An unexpected effect of the biomanipulation was an increase in benthic invertebrate and staging waterfowl abundances, which occurred 2-4 years after fish reduction. Hence, the response in the benthic community following biomanipulation was consid erably stronger than in the pelagic community. A likely explanation is that reduction in abundance of the benthic feeding fish species bream (Abramis brama), strongly affected the benthic invertebrate fauna. In this paper, we present what we believe happened in Lake Ringsjon, and which processes are likely to have been important at various stages of the restoration process .