Researchers Rutger Rosenberg of the Department of Marine Ecology, University of Gothenburg, and Robert Diaz from the Virginia Institute of Marine Science in the USA carried out a study of the soft seabeds of the world’s seas in 1995 that attracted a lot of attention. At that time the researchers found 44 zones that were so afflicted by oxygen deficiency that soft-seabed fauna and fish had been harmed.
More than ten years later they have made out a new compilation – and found that the number of dead seabeds in the world has increased to more than 400 zones.
The study, which is presented in the magazine Science, draws the conclusion that it is the
most serious threat to the health of the sea.
’There are no other variables of such great ecological significance for coastal marine ecosystems, and which have changed so drastically in such a short time, as the reduced amount of oxygen in the sea’, says Rutger Rosenberg.
According to the study by Rosenberg and Diaz at least 245,000 square kilometres of the soft seabeds of the world’s seas has been afflicted by oxygen deficiency. This is an area corresponding to the size of Great Britain. The lack of oxygen affects a number of ecological processes, and leads to an enormous quantity of food for the seabed-dwelling fish being lost.
’In the periods when the oxygen deficiency has its largest spread in Scandinavia, there can be a deficit of over three million tonnes of soft-seabed fauna, compared to the situation if the soft seabeds had been well oxygenated’, says Rutger Rosenberg.
The largest oxygen deficient zones in the world are situated in the Gulf of Mexico, the East China Sea and the Baltic Sea. All these zones are important fishing areas.
Of the around 400 afflicted zones approximately half are affected by oxygen deficiency that appears annually during a certain season. This usually results in the death of soft-seabed fauna but the areas are decolonised by fauna when the oxygen returns. In approximately one quarter of cases the lack of oxygen appears periodically and lasts only a few days or weeks. In other areas, for example the Baltic Sea, the lack of oxygen nowadays is permanent for the entire year.
’The reason for the lack of oxygen is primarily man’s increased discharge of nutrients like nitrogen and phosphorous. Before industrialisation hardly any oxygen deficiency existed at all in the soft seabeds of coastal seas’, state Rutger Rosenberg and Robert Diaz.