Behaviour of manganese in the Rhine and Scheldt estuaries: II. Geochemical cycling

Measurements of dissolved and particulate suspended Mn concentrations throughout the Rhine and Scheldt estuaries are interpreted in terms of cycling processes of Mn. Dissolved Mn is removed in the lower estuary into particulate form. This gives rise to elevated Mn concentrations in coastal suspended matter; particulate Mn is also partially returned to the upper estuary by estuarine circulation processes. Dissolved Mn is produced in the upper estuary by dissolution of particulate Mn either in the water column owing to the low pH and Eh prevailing at low salinities or in the anoxic sediments, from where it subsequently diffuses into the overlying water. Budget calculations and other arguments show that a significant part of the Mn carried down by the river is recycled between the lower and upper estuaries. An important part accumulates within the sediments; the fraction that escapes to the marine environment is mainly in particulate form. Very similar conclusions can be drawn for the two estuaries with quite different residence times of both water and particulates.

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