UPSCALING BIOMANIPULATION IN 600-HA LAKE ZUIDLAARDERMEER

In the 600 ha Lake Zuidlaardermeer in The Netherlands enclosure experi ments were carried out in order to test the feasibility of biomanipula tion. In two enclosures of 625 m(2) each the effects of reduction of f ish and wind and waves on water chemistry, phyto- and zooplankton and submerged macrophytes were studied and compared to the lake. In the '' fishless'' enclosure transparency increased from 40 to 120 cm, chlorop hyll-a dropped from 70 to 20 mu g/l, total-P dropped from 0.20 to 0.10 mg/l, Daphnia sp. > 1 mm increased from 30 to 130 individuals/l and s ubmerged macrophytes returned. The ''fish'' enclosure largely behaved in a way intermediate between the ''fishless'' enclosure and the lake: transparency increased to 60 cm, chlorophyll-a dropped to 40 mu g/l, total-P dropped to 0.12 mg/l and Daphnia sp. > 1 mm increased to 80/1. Submerged macrophytes however did not develop.The experiments show th e potential of the lake for ecological rehabilitation. Nevertheless th e effect of wind and waves on a larger scale remains uncertain. Theref ore plans have been made to build a larger enclosure (40-100 ha). In t his way, the effects of fish and wind and waves can be studied on a la rger scale while at the same time recovery of the submerged macrophyte s within this enclosure will be a significant step towards ecological rehabilitation of the whole lake. Beside the return of the submerged m acrophytes the management is also aimed at the development of substant ial amounts of emergent vegetation, thus supporting enough Northern Pi ke (Esox lucius L.) for regulation of the planktivorous fish community .