A scenario analysis of effects of reduced nitrogen input on oxygen conditions in the Kattegat and the Belt Sea

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Abstract
A numerical tool has been developed for analyzing the potential effects of reduced nitrogen loading to the Kattegat and the Belt Sea. The analyzed effects relate to general trends in the occurrence of hypoxia and anoxia in the water below the pycnocline during the summer and autumn. Nitrogen is assumed to be the nutrient controlling production in these waters. The tool is a dynamic numerical model which includes the dominant hydrodynamic processes of the study area as well as the nitrogen cycle and is linked to oxygen conditions. The model has been calibrated based on the average intraannual variation of the overall hydrodynamic conditions as well as the inorganic nitrogen concentrations and the oxygen concentrations of the last decade. In addition, although data from the 1950s (here considered to be “pre-eutrophication”) are limited, the model seems to reflect the oxygen conditions found at that time. Using this model, scenarios of oxygen conditions after implementation of several different reduction plans for nitrogen loading to the area have been carried out. With the help of these scenarios, it is possible to compare different nitrogen load situations and their effect on the mass balance of biologically active nitrogen and oxygen conditions in this region.