The dynamics of estuarine manganese cycling

Abstract

Seasonal observations of dissolved managnese in the Tamar Estuary,south-west England have demonstrated pronounced temporal and spatial variability in the distribution of this component. Simultaneous observations of physical and physico-chemical properties of the estuarine system, together with kinetic measurements of dissolved managenese behaviour in isolated samples, have demonstrated that the observed distributions are the instantaneous product of dynamic interactions involving external sources and internal input and removal processes. Dissolved manganese is mobilized internally from reduced sedimentsduring advective resuspension of bed material so that this input is regulated by temporal and spatial variations in tidal stress. Manganese is returned to the sediments by continuous uptake on to suspended particles. The rate of this process is responsive to changes in suspended particulate load, ionic strength pH and temperature. The reaction changes from zero-order kinetics in freshwater to first-order kinetics characteristic of the saline medium, with marked deceleration in effective, removal rate, following small increases in salt concentration. Relative changes in the first-order removal rate throughout the estuary are consistent, but there are considerable temporal variations in the absolute rates. This is partially attributable to variations in the quality, as well as the quantity, of the suspended particles. These results indicate that changes in the relative effectiveness of both the input and removal mechanisms, which lead to internal cycling of manganese and to variable distributions of dissolved managanese within the estuary, are closely coupled to the internal behaviour of particles and hence to the tidally-induced oscillations in bed stress. This observation has been confirmed by sequential recordings of dissolved manganese distributions through a spring tide to neap tide period. Furthermore, climatic events producing enhanced disturbance of bed sediment have been shown to induce a similar sequence of events to that arising from tidally induced net mobilization of bed material.