SUBMERGED MACROPHYTES IN THE RECENTLY FRESHENED LAKE SYSTEM VOLKERAK-ZOOM (THE NETHERLANDS), 1987-1991

Extensive biological research has been carried out during the first fo ur years after isolation and freshening of Lake Volkerak-Zoom, a forme r part of the marine-tidal Oosterschelde. Freshening took place in 8 m onths. Research on submerged macrophytes has been an important part of the investigation because the aim of the water management was to esta blish a clear and stable freshwater ecosystem, despite high nutrient l oading. In the first year after closure the marine Zostera noltii vani shed. and the brackish water Ruppia maritima colonized the lake. In su bsequent years, several freshwater species colonized the lake (e.g. Po tamogeton pusillus, P. pectinatus, Zannichellia palustris, Callitriche sp. and Chara sp.). The total area in the shallow zone (0.5-5.0 m) co vered by submerged macrophytes, increased from less than 1 % in 1987 t o 22.5 % in 1991. Characteristics (height, flowering and fruiting) of the main species (P. pusillus, P. pectinatus, Z. palustris, R. maritim a) were closely related to water temperature and secchi depth. Waterfo wl consumed about half of the production of the submerged macrophytes.