A comparative study of manganese dynamics in the water column and sediments of intertidal systems of the North Sea

The dynamics of the redox-sensitive trace metal manganese (Mn) and its response to biological activity were investigated in the water column and shallow pore waters of the German Wadden Sea (southern North Sea) in 2008 and 2009. Two systems, one from the southern (backbarrier area of Spiekeroog Island) and one from the eastern part (Sylt-Rømø Wadden Sea) of the German Bight were compared. The major aim was to examine to which extent biogeochemical findings are specific for a particular tidal basin or transferable to other basins of the German Wadden Sea. Although both study areas reveal hydrodynamical, sedimentological, and ecological differences, qualitatively pronounced similarities in Mn dynamics are observed. Thus, complex cycling of dissolved Mn with increasing values in spring and late summer due to elevated biological activity as well as a depletion period in early summer were observed in both study areas. This finding suggests a seasonal behaviour of Mn being generally representative for the tidal basins of the southern North Sea. Quantitative differences are significant as the backbarrier area of Spiekeroog Island shows much higher concentrations of dissolved Mn in the water column especially during spring. We suggest that this difference is due to a larger sediment area/water volume ratio in the Spiekeroog backbarrier area combined with a higher release of dissolved Mn from the tidal flat sediments as seen in pore water profiles from surface sediments. Site-specific differences are also seen in further tidal systems of the North Frisian Wadden Sea, which are characterised by individual hydrodynamical and sedimentological conditions. Several transects leading from offshore locations towards the coast also revealed a highly variable Mn level. Overall, the North Frisian Wadden Sea most likely represents a less important source for dissolved Mn than the East Frisian Wadden Sea. Furthermore, our data suggest that site-specific properties of the different tidal basins have to be considered in budget calculations for the entire Wadden Sea.