Fate and effects of micropollutants in the Gironde estuary, France: a multidisciplinary approach

Important Cd contamination has been observed in the Gironde estuary, France, but other metallic and organic pollutants are also present. Since sediment is well-known as a major compartment for the storage of numerous pollutants in aquatic environments, its contribution as a source of pollutants for the biota has been examined according to different methodologies. Geochemical studies have provided estimates of Cd mobility particularly with decreasing pHs and increasing salinity, a result in agreement with the relative abundance of exchangeable Cd and Cd bound to carbonates shown by sequential extraction. When in vitro assays tended to be more realistic with regard to the digestion process in bivalves, Cd extraction at low pH was lowered but was still important compared to Cu or Zn. Moreover, toxicity of Gironde sediments to copepod and sea urchin larval stages (not to oyster larvae) brought indirect evidence of the bioavailability of sediment-bound pollutants. Except the aromatic compound perylene, bioaccumulation in bivalves concerns mainly metals. In oysters they were found preferentially in the soluble phase and, in the cytosol, a strong relationship between cadmium and metallothionein-like proteins has been shown, suggesting a detoxication process in this species. This hypothesis is in agreement with the fact that neither cytopathological effects in gills and digestive glands nor marked changes of condition index were observed. On the other hand, no changes in MTLP levels in mussels were induced by metal accumulation in individuals transplanted from a comparatively unpolluted area (Bay of Bourgneuf). The stability of GST activity may be related to a poor accumulation of aromatic compounds. Changes in MDA concentration, AChE and catalase activities are discussed. Mobility, bioavailability of pollutants, significant responses of biomarkers suggest a potential environmental hazard. However, since interspecific differences occur, is the risk at the level of the whole estuarine ecosystem equilibrium or is it limited to a small number of species?