Swedish National Report on Eutrophication Status in the Kattegat and the Skagerrak : OSPAR ASSESSMENT 2007

Abstract

Long time series covering the last 30 years show that with few exceptions the environmental conditions has not improved in Skagerrak and Kattegat since the 1970s. One exception is that the levels of DIP decreased during the 1990s and reached the same level as in the 1970s. However, it is difficult to evaluate the statistical significance of this trend. A general point of view which supports the conclusion about a decrease in DIP is that even though the river runoff during the period 1998 to 2000 approached the runoff found during 1985 to 1988, the DIP concentrations is not as high during 1998-2000 as compared to 1985-1988. Another parameter, which indicates a reversal of the inferior environmental conditions, is the deepwater oxygen concentration in Kattegat during the 1990s. However, the situation is far from acceptable, since this area is still severely affected by seasonal hypoxia. The monitoring data show that concentrations of DIP, DIN, deepwater oxygen and to some extent chlorophyll a co-vary with the variations in runoff from adjacent land areas, which in turn are related to the nutrient load. Periods of high runoff to the Swedish West Coast and the Baltic Sea rise the levels of the above mentioned parameters, whereas the concentrations decrease during dry periods. Typically, these periods are lasting for some years forcing the DIP, DIN, deepwater oxygen and Chlorophyll a to vary with an amplitude of approximately with 0.1 µM, 1 µM, 1 mg/l and 1 µg/l, respectively. The assessment clearly indicates that the Swedish parts of the Kattegat and Skagerrak are affected by eutrophication. Several of the investigated parameters in this assessment points towards eutrophication, such as for zoobenthos, organic carbon and nitrogen in sediments, some macroalgae, plankton, nutrients, oxygen, chlorophyll and algae toxins. The anthropogenically-derived nutrients brought to the two sea areas have origin both from domestic and transboundary sources. Due to the complex hydrographic situation in the Kattegat and Skagerrak with huge exchanges of water masses from the North Sea and the Baltic Sea as well as high atmospheric nitrogen deposition (cf. Table 7), Swedish abatement measures will only affect the Swedish coastal area. At present there are no clear signs that the eutrophication status in the two sea areas will improve in the near future.