Eutrophication in the Baltic Sea and shifts in nitrogen fixation analyzed with a 3D ecosystem model

Since the middle of the last century the Baltic Sea ecosystem has undergone a strong change. An obvious indicator is the increase of winter nutrient concentrations. This increase is attributed to increased anthropogenic nutrient loads to the Baltic Sea. With a 3D ecosystem model we made a hindcast simulation of eutrophication from 1960 to 2000. The model system was able to reproduce the main hydrographic and ecologic features of this period. However, the observed strong increase in winter nutrient concentrations was underestimated by the model. The simulated nitrogen fixation shows a pronounced interdecadal variability. Nitrogen fixation increased in the early 1990 at the same time nutrient loads to the Baltic Sea were decreasing. The changes in nutrient loads cannot fully explain the increased nitrogen fixation; in fact, the primary trigger for this increase is an intensified wind speed in winter, which is correlated with changes in the North Atlantic oscillation (NAO).