Baltic eutrophication


Summary by
Anders Stigebrandt
Resercher, Professor em. in oceanography
at the Department of Earth Sciences, University of Gothenburg

The external supply (source) of phosphorus (P) to the Baltic has been halved since the middle of the 1980s. In spite of this, the phosphorus content in the water column of the Baltic has continued to increase which has led to increasing areas of anoxic bottoms and increasing blooms of cyanobacteria. The increasing eutrophication is caused by the increasing internal phosphorus source emanating from anoxic bottoms. At present this source is three times greater than the external source. The internal source can only be stopped by oxygenation of anoxic bottoms. It might take generations before a successful natural oxygenation event will take place, even if one meanwhile to high costs continues to decrease the external load.

The eutrophication of the Baltic may be removed quickly by man-made oxygenation of the anoxic deepwater. Oxygenation can be done to low cost because there is a huge renewable oxygen reservoir in the so-called winter water, usually residing between 30 and 50 m depth, that each winter is cooled and saturated with oxygen by contact with the sea surface. Wind-driven pumps may transport the oxygen-saturated water down into the deepwater. An experiment in the By Fjord (the BOX experiment) showed that there is reliable inshore pumping technology that works and that the ecological effects of oxygenation throughout are quite positive. The BOX-WIN project has shown that floating wind-driven pumps, based on modern off-shore technology, will work in open sea.

As a first step toward oxygenation of the whole Baltic proper one should build an array of pumps for the Bornholm Basin where pumping of winter water (1000 m3/s) down into the deepwater would keep the deepwater oxygenated. However, before that one has to complete an environmental impact study (EIS) to investigate various ecological effects of pumping. In addition, when pumping there must be ambitious control programs recording all kinds of ecological effects. The cost to stop the internal source of phosphorus from the bottoms of the Bornholm Basin is about 100 SEK per kg P and year. The potential is enormous, ca 7500 tons P/year. After a couple of years of oxygenation of the Bornholm Basin, the effects should be evaluated before starting pumping in additional basins of the Baltic proper where the total potential at present is dizzying 100 000 tons P/year. A couple of years of oxygenation of the deepwater of the whole Baltic would eliminate the eutrophication. One may then reduce pumping to a level sufficient to keep the then oligotrophic Baltic proper oxygenated. For additional information about oxygenation of the Bornholm Basin, contact the author of this summary and/or visit from which relevant papers and reports may be downloaded.

The main weakness with the oxygenation method to remove the eutrophication of the Baltic is that it is unbelievably cheap and effective. It is hard for people to understand that this huge old problem can be solved so easily and to such a low cost.

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