Mechanisms Controlling Shifts and Trends in Water Quality in the Baltic Sea Drainage Basin
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In the past decades, several shifts and trends in water quality were observed in the Baltic Sea which caused eutrophication of Baltic Sea water. Water quality of the Baltic Sea is dependent on the water quality of the catchments in the drainage basin as the Baltic Sea has little water exchange with the North Sea. Therefore, it is important to understand and identify mechanisms that control the water quality in the catchments surrounding the Baltic Sea, known as the Baltic Sea Drainage Basin (BSDB). The aim of this research is to quantify shifts and trends in water quality in terms of nitrogen and phosphorus in the BSDB and relate these shifts and trends to environmental, climatic and anthropogenic controls. To achieve this goal, a nutrient dataset of 117 rivers flowing into the Baltic Sea was combined with a land-use dataset and a dataset containing monthly data of temperature and precipitation. A Mann-Kendall trend test, shift-point analysis and robust factor analysis were applied to reveal and extract patterns of trends and shifts as well as the most important relationships among the variables investigated in this study. It was found that temperature increased most at the coast, probably due to warming of Baltic Sea water and reduced sea ice cover during winter. Patterns of shifts in temperature point to influence of the North Atlantic Oscillation which also caused shifts in precipitation. Both temperature and precipitation influence run-off. Increased temperature enhances evaporation thereby decreasing the flow, while increased precipitation results in more run-off thereby increasing the flow. A change in flow was observed with positive trends in the north to negative trends in the south. An east-west gradient was observed for the nitrogen and phosphorus variables. Negative trends for dissolved inorganic nitrogen (DIN) and total nitrogen (TN) were most pronounced in eastern catchments. The opposite is true for dissolved inorganic phosphorus (DIP) and total phosphorus (TP), where most of the positive trends are found in eastern catchments. The reason for this difference is explained by the different controls for nitrogen and phosphorus in the BSDB. Changes in nitrogen are mainly controlled by land-use changes or changes in agricultural practices while changes in phosphorus are mainly controlled by societal changes subsequently leading to better technologies in reducing phosphorus from point sources. The results presented in this study indicate that the controls for changes in nitrogen and phosphorus are not the same. Therefore, improving water quality in the catchments requires different approaches. Because people in the BSDB rely on many ecosystem services that are vulnerable to eutrophication, it is important to further improve the water quality in the catchments draining into the Baltic Sea. This is necessary to secure and sustain ecosystem services in the future.