Regime Shifts DataBase

Large persistent changes in ecosystem services

The North Sea is a productive, biologically rich sea. It is relatively shallow and gets water inflows from the Atlantic Ocean, Baltic Sea and from rivers and land run-off. Low oxygen contents occur naturally in the North Sea due to the thermohaline stratification; particularly shallow areas with seasonal stratification are vulnerable to oxygen depletion in the bottom layer. The North Sea ecosystems are normally capable of responding to the large natural year to year variability (e.g. heavy storms, shifts in prevailing winds, different water inflows) and maintain sufficient resilience to continue to function, but when anthropogenic stress is added to these natural phenomena, the system may drift away from its' natural state.

The North Sea, especially the southern part, is one of the most polluted seas in the world: large amounts of waste and discharges have led to eutrophication especially on the coastal areas and on the areas with restricted water exchange in combination with weak tides. Eutrophication has indirectly led to oxygen depletion. In the 80s the oxygen content, the duration and the extent of the oxygen depletion became alarming. The low oxygen concentrations of 1981-83 have been suggested to be caused by combination of meteorological and hydrographic conditions and eutrophication.

Episodic hypoxia has resulted in the death of the bottom fauna, dead fish, changes in fish size structure and fish migration away from the affected areas. It has been estimated that during severe hypoxia the fish biomass was approximately 2% of the recorded biomass of the non-hypoxic conditions. In the beginning of the 21st century, extreme oxygen deficiency occurred in fjords and estuaries of Sweden, Norway and Denmark, killing fish and invertebrates.

The countries bordering the North Sea are densely populated and highly industrialized. Humans use the sea for fishing, mineral extraction, tidal and wave energy, waste dumping etc. It is one of the most frequently traveled seas and the coastal zone is intensively used for recreation. Since the 1990s international co-operation has been done to define goals (so called Ecological Quality Objectives) for the North Sea protection and to include ecosystem approach to the management of the sea. It is suggested that the waste disposal to the sea must be managed better to combat eutrophication and consequently hypoxia.