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Between 1992 and 1997 the effects of contaminated sediment on an aquatic community were studied in outdoor mesocosms (2m3), one filled with 'naturally' polluted sediment from the sedimentation area 'Ketelmeer', the other with relatively clean sediment from the shallow lake 'Oostvaardersplassen'. Negative effects of the contaminatedsediment were observed on the population development of the amoeba species Arcella megastoma and Difflugia accuminata, the mysid shrimp Neomysis integer, the microlepidopteran Cataclysta lemnata and the cladoceran community, whereas the biting midge Culex sp. showed a preference for the contaminated system. The reduction of the grazing capacity of the cladoceran community resulted in higher algal densities in the 'Ketelmeer' mesocosm despite the fact that nutrient concentrations in the water column of both mesocosms were comparable. (c) 1998 IAWQ. Published by Elsevier Science Ltd. [ABSTRACT FROM AUTHOR]

Marine and freshwater mesocosm-scale experiments with contaminated sediments have shown that there is a direct relationship between the accumulated contaminant levels and the feeding habits of the organismsused. The highest levels of PAHs and PCBs were found in the sedimentfeeding lugworm Arenicola marina and in Tubifex worms. The levels ofcontaminants in the suspension feeding mussels Mytilus edulis and the zebra mussels, Dreissena polymorpha, were not influenced by the contaminant content of the sediments, but were related instead to level of contaminants in the sea water above. Intermediate levels were found in the baltic tellin, Macoma balthica, which is a filter feeder as well as a deposit feeder, depending on the availability of food. The results show that there is no simple relationship between contaminantconcentration in the sediments and bioavailability. Higher levels ofcontaminants do not necessarily lead to higher levels of these contaminants in Arenicola, due to differences in the sediment structure and the ageing of the contamination. On the other hand. toxic effects are related to the internal concentrations of certain chemicals. The internal concentrations observed in Arenicola may provide a good estimation of the true bioavailability of sedimentary contaminants and canalso be used as an indicator for potential environmental effects. (c) 1998 IAWQ. Published by Elsevier Science Ltd. [ABSTRACT FROM AUTHOR]