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

Lake restoration is a major issue in water management in The Netherlands. In this context biomanipulation is considered an important tool following the reduction of the nutrient load. Here we present a new conceptual framework and try to quantify the productivity level below which predator control of the fish population, which is considered a prerequisite for successful biomanipulation, occurs. The conceptual framework focuses on a balance between preyfish production and piscivorous fish consumption. When the piscivorous fish consumption (top-down control) is too low to control the preyfish community (bottom-up forces), the abundance of small planktivorous fishes will increase, leading to the cascading effects of a heavy predation on zooplankton and an excessive development of algae resulting in an increase of turbidity. With the framework the total-P concentrations characterising lake productivity at such turning points can be calculated. The framework is used to explain the eutrophication process of Dutch lakes, which is described as a series of stages between which shifts in community structure including the dominant piscivorous fish species occurred. Furthermore, based on the framework some predictions and methods for ecological rehabilitation will be presented.