Manganese dynamics in tidal basins of the Wadden Sea: Spatial/seasonal patterns and budget estimates

Abstract

The concentrations of dissolved and particulate manganese (Mndiss and Mnpart) were studied in the Wadden Sea and adjacent estuaries to gain insights into spatial and regional differences, but also to assess the source function of the tidal basins for the open North Sea compared to river discharge. Ten transects across tidal basins of the southern and northern Wadden Sea as well as the estuaries of the rivers Elbe, Weser, and Ems were sampled. A case study in the tidal basin of Spiekeroog Island (southern Wadden Sea) was further used to evaluate seasonal and inter-annual Mn dynamics and the potential importance of deeper, groundwater-associated (up to 20 m sediment depth) Mn sources.In the estuaries and the Jade Bay, Mnpart is the predominant Mn fraction, whereas Mndiss is the major faction in the tidal basins. Higher Mndiss levels in the tidal basins of the southern compared to the northern Wadden Sea result from differences in tidal basin characteristics such as intertidal area and tidal asymmetry, influencing the extent of benthic-pelagic interaction and material import. Offshore-directed Mndiss fluxes exhibit less regional variability than Mndiss levels. Overall, the net export fluxes of total Mn from the tidal basins are variable but clearly exceed the river input for the considered summer situation. Annual Mndiss export fluxes, exemplified for the Spiekeroog tidal basin, exhibit high seasonal variations with highest fluxes during spring and late summer. Along with onshore material transport, budget calculations for the Spiekeroog tidal basin indicate that offshore-directed Mndiss fluxes are mainly controlled by surface and deep pore water discharge. Groundwater-associated Mn-rich pore waters in the deep sediments of the intertidal area are likely counteracting sedimentary Mn exhaustion on longer time scales.