Oceans and lakes are characterized by large residence times and can thus be considered as being in an equilibrium or quasi-equilibrium state. In contrast, estuaries, which constitute the interface between the fresh water and marine ecosystems, are biogeochemically and physically very dynamic. Strong gradients in physical and chemical variables such as temperature, pH, salinity, dissolved oxygen, nutrients, amount and composition of particulates result from the mixing of fresh water and saline end members. These gradients provoke increased biogeochemical processes, generally resulting in a geochemical filter (the area of high turbidity, showing enhanced adsorption-desorption processes) and a biological filter (the area of high plankton activity showing, besides a high uptake of nutrients, a high accumulation of trace metals). Both filters disturb the conservative mixing pattern of the trace metals in an estuary. In addition, four aspects make the Scheldt estuary very unusual and different from other estuaries:
- the Scheldt is a tide-governed estuary which means larger residence times;
- the upper estuary receives large inputs of biodegradable organic matter which leads to oxygen depletion in winter and to anoxic conditions in summer;
- inputs of toxic pollutants occur in the upper estuary by the industrial park of Antwerp;
- the anoxic zone, the zone of pollutant input and the zone of the turbidity maximum coincide geographically.
Table of Contents1. General Description of the Scheldt Estuary; W. Baeyens, et al. 2. Biogeochemical Behaviour of Cd, Cu, Pb and Zn in the Scheldt Estuary during the Period 1981-1983; W. Baeyens, et al. 3. Biogeochemical Behaviour of Cd, Cu, Pb and Zn in the Scheldt Estuary: Results of the 1995 Surveys; W. Baeyens, et al. 4. Behaviour and Speciation of Mercury in the Scheldt Estuary (Water, Sediments and Benthic Organisms; W. Baeyens, et al. 5. Effect of Organic Complexation on the Behaviour of Dissolved Cd, Cu and Zn in the Scheldt Estuary; W. Baeyens, et al. 6. The Impact of the Scheldt Output on the Coastal Area (1981-1983 and 1995); W. Baeyens, et al. 7. A Box-model of Metal Flows Through the Scheldt Estuary (1981-1983 and 1992-1995); W. Baeyens, et al. 8. Water Quality Modelling of the Western Scheldt Estuary; J.P. Sweerts, et al. 9. A Time-Dependent Flow Model for Heavy Metals in the Scheldt Estuary; F. De Smedt, et al. 10. Evolution of Trace Metal Concentrations in the Scheldt Estuary (1978-1995). A Comparison with Other Estuarine and Ocean Levels; W. Baeyens.