Declines in oxygen concentrations along the Norwegian Skagerrak coast, 1927‐1993: A signal of ecosystem changes due to eutrophication?

Abstract

Every year since 1927 oxygen concentration, temperature, and salinity have been measured at 3 1 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 196Os, 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.

Historical changes in oxygen concentrations along the Norwegian Skagerrak coast: Reply to the comment by Gray and Abdullah

“…comm.). Consequently, when Gray and Abdullah argue that the change in methodology must have taken place between 197 1 and 1972, they are actually providing evidence that storing the sample bottles until after the cruise is not the source of error, which is in accord with our analyses (Johannessen and Dahl 1996).…”

Coastal primary productivity changes over the last millennium: a case study from the Skagerrak (North Sea)

“…The assumption of a gradual decline in oxygen concentration in the Skagerrak during the last 70 years is supported by other studies (e.g. Johannessen and Dahl, 1996).…”

Spatial patterns of the temporal dynamics of three gadoid species along the Norwegian Skagerrak coast

“…Two important environmental changes have taken place along the Norwegian Skagerrak coast during the period in which the recent decline of these 3 fish population~ has occurred: (1) a severe decline of Calanus finmarchicus , from which the nauplii and the first stages of copepodites constitute the major resource of fish larvae (Ellersten et al 1981, Skreslet 1989, Thorisson 1989, Brander & Hurley 1992; and (2) eutrophication and subsequent decline in the oxygen concentrations of the Norwegian Skagerrak coastal waters (Aure et al 1996, Johannessen & Dahl 1996.…”