BIOMANIPULATION ADDITIONAL TO NUTRIENT CONTROL FOR RESTORATION OF SHALLOW LAKES IN THE NETHERLANDS

Eutrophication control is one of the major issues in the environmental policy in The Netherlands. As a result of international action programmes the average phosphorus loading of freshwater systems should decrease by 50% between 1985 and 1995. However, in many cases the restoration of water quality requires additional measures. Recovery is hampered by the structure and functioning of the present food-chain. The feeding behaviour of the dominant fish species in Dutch lakes, bream and roach, tend to impose a homeostasis on the system, resisting restoration of water quality. In shallow lakes, biomanipulation, including drastic reduction of fish-stocks, may induce a shift from a stable 'turbid-water state' to a stable 'clear-water state'. To assess the possibilities of biomanipulation for the restoration of a particular lake, three questions are relevant: (1) is a drastic reduction of fish-stocks feasible?, (2) will a shift occur from 'turbid to clear' after the fish reduction? and (3) will the new situation of clear water be stable? This paper focuses attention on the last two questions. The increase in water clarity, following fish reduction, largely depends on the increase in the density of the Daphnia-population and the contribution of benthivorous fish to the resuspension of sediments. A 'turbid to clear' shift may be expected if the total biomass of planktivorous and benthivorous fish is reduced to levels<50 kg ha-1. The stability of the achieved clear-water state largely depends on the development of submerged macrophytes in the lake and on the level of nutrient loading. It is tentatively concluded that a stable clear-water state may be expected at initial total-P concentrations<0.10 mg l-1. Because the water managers in The Netherlands have no fishing rights, they have to.co-operate with anglers and commercial fishermen to apply biomanipulation as a tool for water management. © 1990 Kluwer Academic Publishers.