Spatio-temporal analysis of sediment plumes formed by mussel fisheries and aquaculture in the western Wadden Sea

Abstract

The Wadden Sea is a highly dynamic tidal system and an important area for a variety of species, including wild and cultivated mussels. Increased suspended sediment concentrations may negatively affect ecosystem functioning across trophic levels; hence, insight into the effects of anthropogenic activities is of importance. The aim of this study was to investigate the scale of sediment resuspension associated with commercial mussel cultivation activities. Turbidity prior to and during fisheries and harvest was investigated by in situ measurements with towed and fixed sensors. Four case studies were selected to represent maximum disturbance. For one case study, sediment resuspension was further investigated by analysis of satellite images. The in situ measurements showed that turbidity is enhanced by mussel fisheries and harvest but that effects are limited in time and space. Turbidity enhancement during fisheries was several-fold higher than that during harvest, whereas the spatial scale of both activities was limited to their direct vicinity. In contrast to the in situ study, the satellite image analysis identified a spatial magnitude that extended over a somewhat larger spatial scale, in the form of a dark water plume, and further revealed that this plume consisted of high concentrations of coloured dissolved organic matter (CDOM), rather than suspended particles. Along with turbidity, CDOM thus seems an important consideration for determining light attenuation in a water body affected by anthropogenic seafloor disturbance. As we presented case studies that were expected to represent maximum impact, we conclude that there is no specific need for further in-depth studies that allow upscaling to ecosystem-wide effects.