The eutrophic Loosdrecht Lakes: Current ecological research and restoration perspectives

Features of the Loodsrecht Lakes, with emphasis on the main lake, are discussed with reference to restoration.Characteristics of the present situation are: (1) very low water transparency-Secchi-disc readings around 0.3 m occur in all seasons; (2) relatively small seasonal changes in sestonic matter; (3) important input of resuspended particles into the seston; (4) predominance of filamentous blue-green algae for most of the year; (5) relative scarcity of crustacean zooplankton, while rotifers are abundant; (6) poor development of littoral communities, and absence of benthic producers. The blue-green algae maintain high population density at very low growth rates: rates of loss are low. The zooplankton grazing rate is low due to inefficient filtering, but predation of larger crustaceans by fish may also be important. Studies on epipelon indicated that loss by deposition may be largely compensated by resuspension.Starting in 1984, the external phosphorus loading was markedly reduced. Results for 1984 and 1985 indicate that complementary measures are needed in order to improve water quality. Action should be directed towards increasing the phytoplankton turnover rates. Accelerated specific growth rate can be expected to accompany lower biomass, more successful competition by other algal groups, and enhancement of grazing pressure.Considering the shallowness of the system, promotion of littoral development and return of submerged vegetation may be important in establishing a new equilibrium of the system.