Distribution model for particulate trace metals in the rhine estuary, Southern Bight and Dutch Wadden Sea

Abstract

Suspended matter in the Dutch Wadden Sea (sampled in the period September 1972 to March 1973) and the Southern Bight including the Rhine estuary (sampled in the period September to November 1973) has been characterized by correlation of the trace metals Mn, Fe, Zn and Cu. Fe:Mn ratios can be used to distinguish coastal and offshore samples. Away from the coasts the southern part, including the Straits of Dover, is characterized by minimum amounts of suspended matter (1 to 2 mg l−1). Leacheable particulate trace metal concentrations in this part are 0.4 to 1 μg l−1 Mn, 6 to 17 μg l−1 Fe, 1 to 2 μg l−1 Zn and 0 to 1 μg l−1 Cu.On a weight over weight basis (μg g−), concentrations of iron and manganese along the coast are high, those of copper and zinc are low compared to offshore samples. The distribution of phytoplankton and its metal concentrations cannot explain the observed levels.The combined data on trace metal concentrations in bottom sediments, particulate matter and in solution within and outside the estuary suggest that the Rhine estuary acts as a sink for particulate trace metals. Estimates of the loss of individual particulate trace elements vary between 20 and 60%. Mobilization of trace metals from suspended matter is of minor importance, if occurring at all.Additionally, river-borne dissolved Mn and Fe are precipitated, already in the early stages of mixing. As a result, considerable changes in the ratio of Mn to Fe, Zn and Cu occur in bottom sediment within the estuary and in particulate matter within and outside the estuary.Once in the coastal region, the relative particulate concentrations of the trace elements investigated remain essentially unchanged except for manganese that may be taken up from solution into the particulate phase with a time delay relative to iron.Although the relative amounts of particulate leacheable Mn, Fe, Zn and Cu in individual samples are more or less constant in the coastal and Wadden Sea area, and the amounts in μg l−1 are comparable in the two areas, the absolute values for the concentrations (ppm) in the shallow Wadden Sea are usually decreased due to mixing with heavier bottom particulates with lower metal content.