Streams or open sea? Tracing sources and effects of nutrient loadings in a shallow estuary with a 3D hydrodynamic–ecological model

The relative importance of nutrient inputs from streams and the open sea for the water quality is a key issue for an effective management of eutrophication problems in estuaries. In order to resolve this question we have developed a coupled 3D hydrodynamic–ecological model with numerical nitrogen tracking for Horsens estuary, which is a typical Danish estuary that is shallow and well ventilated with water from the adjacent open sea. The nitrogen tracking technique revealed that nitrogen discharges from local streams accounted for the major part (40\%–90\%) of the nitrogen in NO3, NH4 and detritus whereas only 15\% to 50\% of the nitrogen in phytoplankton originated from streams. Effects of nitrogen loading reductions were most pronounced for nutrient concentrations and Chl a concentrations whereas the light condition was only slightly improved. The impact of stream nitrogen reductions on nitrate concentrations, Chl a concentrations and light attenuation was respectively 7, 2 and 3 times higher than the impact of open sea nitrogen reductions. Hence, even in estuaries that are well ventilated with water from the adjacent sea significant environmental improvements can be expected after reduction in local nutrient discharges.