Shallowness may be a major factor generating nutrient gradients in the Wadden Sea

Steep horizontal nutrient gradients are observed in the Wadden Sea. Along a transect from the German Bight to the mainland shoreline the concentrations of nitrate, ammonium, phosphate and silicate increase three to fivefold. This is surprising in view of the short residence times of the water in the basins of a few days only. In this paper, models are presented explaining the occurrence of such steep gradients. With decreasing depth and consequently lower mean tidal current velocities, phytoplankton removal by benthic suspension feeders and sedimentation of suspended particulate matter (SPM) increase. It is demonstrated that these factors alone lead to nutrient gradients. An assumed net inward transport of SPM amplifies these gradients. The considered models are 1D-equilibrium models with the distance from the shoreline as independent variable. The modelled processes are primary production, phytoplankton mortality, nutrient regeneration, horizontal mixing and transport of suspended particulate matter.