Monitoring the Bioavailability of Heavy Metals in Relation to the Sediment Pollution in the Weser Estuary (FRG)

Abstract

The geochemistry of heavy metals in aquatic ecosystems is dominated by solution, complexation, oxidizing and reducing chemical reactions, precipitation and remobilization processes. Their ecotoxicological properties, especially their bioavailability, change markedly in accordance with the respective chemical status. When heavy metals reach the tidal and/or brackish water region they are subjected to pronounced changes in hydrographic and chemical conditions. The accumulation of suspended solids, an increase of fine-grained and organic material in the sediments and growing salinity, are the predominating factors affecting their geochemical status within the estuarine environment. The usual heavy metal monitoring practices, however, do not regard these dynamics. The methods in use, i.e. total digestion of unfiltered water samples and occasionally of the silt fraction of the sediment, yield only gross values without any differentiation of the geochemical speciation. Thus, processes of geo- and bioaccumulation can hardly be described nor predicted, or controlled with regard to the available information. We made an attempt to improve the data base by following a highly integrated two-year-survey program in the Weser estuary. The River Weser has a well documented anthropogenic heavy metal pollution with particularly high cadmium levels (Schirmer et al., 1989). The Weser estuary (Fig. 1) comprises an inner part (km 0–50) which is slightly brackish due to effluents from the potash industry, a middle reach (km 50–70), naturally mixomesohaline with high salinity variations, and the outer estuary leading through the Wadden Sea into the German Bight. With average headwater discharge of 323 m3/sec and a tidal range of about 4 m at km 0 a suspended particle needs about eight days to reach the turbidity zone at km 60.