Management measures and long-term water quality changes in Lake Balaton (Hungary)

Abstract

A reduction in the external and internal P loadings along with the removal of benthivorous cyprinids resulted in a significant improvement of the water quality in the hypertrophic western part of Lake Balaton by 1996, some ten years following first management measure. Enhanced internal P loading after the reduction in external nutrient load is clearly an important factor explaining the duration of recovery in shallow lakes. Phytoplankton responded quite rapidly to reductions in nutrient loading and fish stock, whereas the effect on both the structure and abundance of the crustacean plankton was less pronounced. The role of top-down control of phytoplankton by crustaceans following nutrient reduction is not clearly understood in shallow lakes. Grazing did not play a significant role in the overall reduction of algal biomass in Lake Balaton. Since high concentration of suspended mineral particles from both sediment resuspension and lime precipitation causes permanent food limitation of daphnids, the top-down effects were negligible in Lake Balaton following fish stock reduction. Fish community structure did not respond to the lower external and internal TP loads cyprinids being dominant in both the pre- and post-management periods.