Study of the parameters which determine the distribution of benthic invertebrates in the brackish part of the Scheldt estuary, and in particular the pollutant load and the sediment dynamics therein. The central question here is to what extent pollution has an effect on the macrozoobenthos, and if it does have a significant effect, at which stage of the life cycle of these invertebrates these effects are situated and how this influences the structure of the entire benthic biocenosis. The study will consist of a combination of (1) population-dynamic oriented examination of the macrozoobenthos, (2) chemical analysis of the sediment, pore water and organisms and (3) colonisation experiments and toxicity tests. Pollution is one of the major problems in estuarine and coastal ecosystems. Toxic sediments in particular can affect the 'quality' of the marine environment, by altering various processes at population and ecosystem level. The effect of pollution on benthic invertebrates has been studied at different levels of biological organisation. It is stressed by Underwood and Peterson (1988) that further research on the biological aspects of pollution effects should be focused on a combination of measures of pollution at different levels of biological organisation, and also on a combination of chemical and biological aspects of pollution (Bayne, 1979). The Sediment Quality Triad approach is a concept which combines these aspects (Chapman, 1986; Chapman et al., 1991). It incorporates sediment chemistry which measures contamination, sediment bioassays which measure toxicity, and in situ parameters which measure benthic community structure. The combination of the three components provide strong evidence for determining pollution-induced degradation. The Westerschelde is known as a very polluted area and in the brackish part the distribution of benthic invertebrates might be influenced by pollution. The major goal of this proposal is therefore to investigate 1) to what extend the distribution of benthic invertebrates in the brackish part of the Schelde estuary is influenced by pollution 2) at what stage of the live cycle of the species are the effects situated 3) and how does this influence the overal benthic community structure. This will be done by studying at a limited number of places the chemical composition of the sediment, the benthic populations and the toxicity of the sediment by means of bioassays. The combination of these measurements will increase our understandings of the effects of pollution on benthic populations. The former Institute for Nature Conservation was a partner in this project