| Abstrakt: |
The distribution of dissolved Cd, Cu, and Zn in the Scheldt estuary has been studied during eight axial surveys, carried out between February 1987 and February 1988. The observed metal-salinity profiles depend on the season. During spring and summer, when the river water is anoxic (containing traces of dissolved sulfide), the dissolved metal concentrations in the riverine endmember are extremely low. This observation is ascribed to formation of sparingly soluble metal sulfides in the water column. During winter, when the river water is not totally devoid of oxygen (10-40% saturation), the dissolved Cu and Zn concentrations in the riverine endmember are an order of magnitude higher, but rapid removal is apparent in the very low salinity zone. Flocculation (of organometal complexes) or coagulation (of colloid-associated metals), sediment resuspension and formation of particulate Fe and Mn oxyhydroxides are likely to be involved in the removal process. At higher salinities, maxima of dissolved Zn (at 6-9 x 10^-^3), Cu (at 9-18 x 10^-^3), and Cd (at 12-21 x 10^-^3) are consistently found over the year. These maxima are ascribed to dissolution and desorption of particulate metal forms with increasing salinity. Reoxidation of trace metal sulfides during transport from the anoxic (or suboxic) upper estuary to the fully oxygenated lower estuary is suggested as the first step in the mobilization process. During phytoplankton blooms, desorption of Cd and Zn (but not Cu) is suppressed, which is attributed to the pH increase related to primary production, and to biological uptake. The impact of mobilization processes in the Scheldt estuary is reflected by effective dissolved Cd and Cu concentrations which are much higher than the observed metal concentrations in the river water. Based on these findings, it is expected that restoration of the dissolved oxygen concentration, which is a major goal of the present-day management of the Scheldt estuary, will lead to an increase in the dissolved metal transport to the North Sea. |
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