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 saturation all along the coast. In some inner coastal areas, this has led to oxygen deficiency. At intermediate depths (10 and 30 m), there is no trend in oxygen saturation until the middle of the 1960s, after which an almost linear decrease is observed until the 1990s. An explanation 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 significantly lower level within a few years and then stabilized at this low level after the middle of the 1970s. The higher oxygen consumption in the bottom water may be due to increased sedimentation of phytoplankton and phytodetritus as a result of greater phytoplankton biomass and, in particular, to less grazing by herbivores. No corresponding changes in meteorological or hydrographical variables were found; we therefore 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 structural changes in the pelagic community.