Primary production in the Kattegat: Past and present

Abstract In recent years, symptoms of eutrophication in the form of increased frequency and intensity of oxygen depletion events in bottom waters and unusual algal blooms have been observed in the Kattegat (between the North Sea/Skagerrak and the Baltic). In general, such symptoms are believed to be responses to increases in primary production. However, for most areas, there are insufficient data to demonstrate whether or not increases in primary production have actually occurred. In this study, the evidence for increased primary production in the Kattegat is examined by comparing primary production measurements from the 1950s and measurements made in the period 1984-1993. The methods employed during the two periods differ considerably. These differences and how they may affect the validity of a comparison of the results from the studies carried out in two periods are addressed. The primary production data collected during the period 1984-1993 are calculated using the method employed in the 1950s. It is concluded that primary production in the Kattegat has increased from less than 100 g C m-2 y-1 to about 200 g C m-2 y-1 since the 1950s. This increase is not seen during the winter months when primary production is predicted to be light limited. It is, however, observable from the spring bloom and throughout the summer period when nutrients are predicted to be limiting for primary production. Finally, the primary production values recorded in the 1950s and in the period 1984-1993 are related to nitrogen loading in the two periods and compared with similar data from other marine areas.