THE STRUCTURE AND FUNCTION OF FEN LAKES IN RELATION TO WATER-TABLE MANAGEMENT IN THE NETHERLANDS

Most relatively large (> 250 ha) and shallow (< 2 m), Dutch lakes orig inate from large-scale peat mining. Originally, their water tables wer e regulated by natural processes such as evapotransporation, precipita tion, drainage and seepage (up and downwards). These lakes used to be oligo to mesotrophic, and the phytoplankton consisted mainly of diatom s and green algae. However, 30 years ago filamentous cyanobacteria wer e also present. Owing to the intensification of agriculture in the sur roundings of the lakes, their management developed more and more towar ds a man-made reservoir function. This management resulted in inflow o f eutrophic water from the agricultural areas in winter. In summer the lakes were increasingly used as a water supply. This supply is compen sated by water imported mainly from the River Rhine. The impacts of th is reservoir function on the limnology of the Dutch fen lakes are disc ussed with the Tjeukemeer, the Loosdrechtse Plassen and the Reeuwijkse Plassen as examples. All these lakes have become typical eutrophic wa ters with high turbidity, dominance of filamentous cyanobacteria and b ream and complete loss of submerged macrophytes. Finally, methods and results of manipulation procedures to restore these waters are discuss ed.