DECLINES IN OXYGEN CONCENTRATIONS ALONG THE NORWEGIAN SKAGERRAK COAST, 1927-1993 - A SIGNAL OF ECOSYSTEM CHANGES DUE TO EUTROPHICATION

Every year since 1927 oxygen concentration, temperature, and salinity have been measured at 31 stations along the Norwegian Skagerrak coast during the latter half of September. At all analyzed depths (10 m, 30 m, and bottom water) there have been significant decreases in oxygen s aturation all along the coast. In some inner coastal areas, this has l ed to oxygen deficiency. At intermediate depths (10 and 30 m), there i s no trend in oxygen saturation until the middle of the 1960s, after w hich an almost linear decrease is observed until the 1990s. An explana tion for the decreased oxygen saturation in the intermediate layer is increased heterotrophic activity relative to primary productivity. In the bottom water, on the other hand, oxygen saturation did not change until the beginning of the 1970s, when it decreased rapidly to a signi ficantly lower level within a few years and then stabilized at this lo w level after the middle of the 1970s. The higher oxygen consumption i n the bottom water may be due to increased sedimentation of phytoplank ton and phytodetritus as a result of greater phytoplankton biomass and , in particular, to less grazing by herbivores. No corresponding chang es in meteorological or hydrographical variables were found; we theref ore conclude that the decreasing oxygen concentrations are most likely caused by increased nutrient load of the coastal waters. The present evidence suggests that the decrease in bottom-water oxygen is due to s tructural changes in the pelagic community.