EUTROPHICATION AND ECOLOGICAL REHABILITATION OF DUTCH LAKES - PRESENTATION OF A NEW CONCEPTUAL-FRAMEWORK

Lake restoration is a major issue in water management in The Netherlan ds. In this context biomanipulation is considered an important tool fo llowing the reduction of the nutrient load. Here we present a new conc eptual framework and try to quantify the productivity level below whic h predator control of the fish population, which is considered a prere quisite for successful biomanipulation, occurs. The conceptual framewo rk focuses on a balance between preyfish production and piscivorous fi sh consumption. When the piscivorous fish consumption (top-down contro l) is too low to control the preyfish community (bottom-up forces), th e abundance of small planktivorous fishes will increase, leading to th e cascading effects of a heavy predation on zooplankton and an excessi ve development of algae resulting in an increase of turbidity. With th e framework the total-P concentrations characterising lake productivit y at such turning points can be calculated. The framework is used to e xplain the eutrophication process of Dutch lakes, which is described a s a series of stages between which shifts in community structure inclu ding the dominant piscivorous fish species occurred. Furthermore, base d on the framework some predictions and methods for ecological rehabil itation will be presented.