Danish estuaries are highly eutrophic due to high N and P inputs from local streams and a general increase in nutrient concentrations in the Baltic Sea region. Recovery plans have been implemented to reduce local loadings. A key issue for these plans is the relative importance of local sources of nutrients versus influx over the sea entrance. In order to resolve the question we have developed a coupled 3D hydrodynamic-ecological model for Horsens fjord which is a typical shallow and well ventilated Danish estuary. A numerical nutrient tracking technique was applied which allowed us to calculate the relative contribution to phytoplankton growth from nutrients originating from local sources and from the Baltic Sea outflow, respectively, which constituted the open boundary. Nutrient reduction scenarios were performed to asses the potential effects of reducing nutrient loadings from different sources (streams and open sea inputs) on phytoplankton growth and biomass. Model simulations revealed that local streams accounted for about 40% of the phytoplankton nitrogen content and that changes in local discharges and open sea nutrient concentrations almost had similar impact on phytoplankton biomass.
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ASLO Aquatic Sciences Meeting 2009. A cruice through nice waters!