Restoration of a stratified lake (Feldberger Haussee, Germany) by a combination of nutrient load reduction and long-term biomanipulation

A long-term biomanipulation has been performed in the stratified Feldberger Haussee since 1985. Prior to manipulation, nutrient load to the lake had d eclined due to waste water removal. Planktivorous fish were reduced by sein ing and by enhancement of piscivorous fish. Changes in transparency, nutrie nts, phytoplankton: zooplankton and fish were documented for both the prema nipulation period ((1978-1985) and the manipulation period (1986-1998). Tra nsparency 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 f ish community. We conclude that the success of the restoration was predomin antly attributed to bottom-up forces as a result of the declining nutrient load and an intensified co-precipitation of phosphorus with calcite. Howeve r, the increased predation impact by the piscivorous fish may have caused a reduced nutrient recycling by the planktivorous fish thus contributing als o to the improvement in water quality..