Restoration of a Stratified Lake (Feldberger Haussee, Germany) by a Combination of Nutrient Load Reduction and Long-Term Biomanipulation

Abstract

A long-term biomanipulation has been performed in the stratified Feldberger Haussee since 1985. Prior to manipulation, nutrient load to the lake had declined due to waste water removal. Planktivorous fish were reduced by seining and by enhancement of piscivorous fish. Changes in transparency, nutrients, phytoplankton, zooplankton and fish were documented for both the premanipulation period (1978-1985) and the manipulation period (1986-1998). Transparency increased in response to the manipulation (+54%), but strong year-to-year fluctuations were observed. These fluctuations were correlated to chlorophyll a, primary production and the proportion of piscivores in the fish community. We conclude that the success of the restoration was predominantly attributed to bottom-up forces as a result of the declining nutrient load and an intensified co-precipitation of phosphorus with calcite. However, the increased predation impact by the piscivorous fish may have caused a reduced nutrient recycling by the planktivorous fish thus contributing also to the improvement in water quality.