Organisms influence fine sediment dynamics on basin scale

The amount of fine sediment in the water phase and in the bed is an important factor influencing the functioning of the ecosystem. Therefore, man-made and natural changes in the amount and distribution of fine sediment will affect the functioning of the ecosystem. Many bottom dwelling organisms engineer the temporal and spatial distribution of fine sediment in the bed by actively changing settling and erosion processes. In this way organisms influence local habitat quality to their own advantage. Impact on settling and erosion processes will through changes in the bed force changes in the fine sediment availability in the water column. This study elaborates the effect of such local ecosystem-engineers on sediment budget and suspended sediment concentrations at the scale of whole tidal basins. In the Wadden Sea yearly average settling and erosion fluxes are roughly known on the basis of long term monitoring of suspended sediment concentrations. It is shown that the effect of ecosystem engineers, occurring predominantly on shallow areas (less then 3 m water depth), causes erosion fluxes and suspended sediment concentrations to change significantly due to the seasonality of biomass variations. The effect could explain the seasonal variation in suspended sediment concentrations measured in the Wadden Sea. Absolute biomediated fluxes are comparable to exchange of fine sediment between North Sea and Wadden Sea. Biomediated fluxes are an order of magnitude larger than the multi-year averaged net import flux of fine sediment into the Wadden Sea. The study results are in agreement with field data and it is shown that the effect of ecosystem engineers on erosion fluxes and on suspended sediment concentrations is quite large and will result in lower turbidity in the summer and higher turbidity in the winter on basin scale.