An outdoor model ecosystem was designed for the ecotoxicological evaluation of xenobiotics. Two years were necessary before the artificial pond reached a steady state. During this time the composition of the community and its functions were investigated. We recorded the amount of nutrients and the O2/CO2-metabolism in the water, the density and diversity of the phytoplankton, the aquatic macrophytes and the fauna, the microbial activity in the sediment, and the environmental impacts on the ecosystem. A short time before the application of sodium pentachlorophenate (Na-PCP) the ecosystem was divided into three identical subunits. One of these was used as an internal control, the others were contaminated with two different concentrations of Na-PCP (0·1 and 0·3 mg litre-1). These concentrations were maintained over a period of eight weeks. Ecological changes in the contaminated compartments were investigated during a period of one year. The results were compared with those of single-species tests. Significant variations were observed only in the unit receiving 0·3 mg Na-PCP litre-1. A short time after starting the experiment, the number of rotifers and cyclopids decreased. Primary producers were not affected. An increase of the chloride levels in the water indicated degradation processes. One year after application of the chemical, the remineralisation of nutrients was disturbed. This resulted in a diminution of the phytoplankton and the aquatic fauna. © 1988.
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