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December 2012 Issue

Evaluating Geoengineering as a Method to Revive Baltic Sea Dead Zones
By Anders Stigebrandt

Narrow and shallow straits at its entrance and a large freshwater supply make the Baltic Sea strongly stratified, with a surface salinity of seven, a steep halocline starting at 60 meters depth and a maximum deepwater salinity of 12. The Baltic Sea has vast areas of dead bottoms and huge summer blooms of cyanobacteria due to a large phosphorus surplus, which is increasing despite the phosphorus output from the surrounding countries having been halved since the 1980s. Instead of relying on taxpayers to rebuild sewage treatment plants to reduce phosphorus output, large-scale oxygenation of deepwater by geoengineering could be done to resolve eutrophication.

According to the Baltic Nest Institute, the external phosphorus supply to the Baltic Sea doubled from the 1950s to the middle of the 1980s, when it peaked at about 70,000 tons per year. It has since declined and is now at the 1950s level. However, the long-term winter mean phosphorus content in the water body continued to increase by about 5,000 tons per year after the peak of the external supply. This cannot be understood from dose-response models, which predict decreasing phosphorus content after the peak. The Baltic seems to be fertilizing internally.

Anoxic bottoms have increased from 10,000 square kilometers in the 1970s to 40,000 square kilometers during the last decade, according to the Swedish Meteorological and Hydrological Institute. On both short and long timescales, the phosphorus content in the Baltic Sea is correlated to the area of anoxic bottoms, showing that eutrophication is driven by processes related to anoxic bottoms and that reducing their area could mitigate eutrophication.

Anoxic bottoms can be oxidized via geoengineering by pumping oxygen-saturated winter water from above the Balticís halocline to the deepwater. The estimated oxygen need can be met with 10,000 cubic meters per second of water containing 10 grams of oxygen per cubic meter. Without geoengineered oxygenation, one has to rely on a future natural oxygenation event that lasts long enough to radically decrease the phosphorus content to achieve a less eutrophic state. Such an event occurred from the 1980s to the early 1990s, but the oxygenation did not last long enough. Since eutrophication has worsened, an imminent natural oxygenation event is very unlikely.

The Swedish Ministry of the Environment has funded two pilot oxygenation projects. The BOX (Baltic Deepwater Oxygenation) project has oxygenated the By Fjord on the Swedish west coast, which was almost permanently anoxic inside and below the sill in the mouth and has been oxic since autumn 2010. The earlier black anoxic sediments now have an oxidized brownish top layer that has been colonized by sea worms. An evaluation of BOX and the other project, PROPPEN (controlling benthic release of phosphorus in different Baltic Sea scales), concluded that oxygenation does decrease benthic phosphorus fluxes.

However, before oxygenation can be applied to the Baltic Sea, extensive environment impact analyses must be conducted on possible harmful effects, such as triggering bottom sediments to mobilize toxic substances. The spawning volume of cod is also a concern. Cod require water salinity above 11, oxygen concentration above 2 milliliters per liter and temperatures above 1.5° C. These properties exist only in the Bornholm Basin, and critics say oxygenation will decrease codís spawning volume due to salinity changes. But preliminary investigations suggest oxygenation will increase the volume of cod-spawning water and lead to colonization of dead bottoms by benthic fauna that will become a new feed source for bottom-feeding fish like cod.

As for technological development for oxygenation, pumps must be constructed to operate in open seas at depths past 100 meters, preferably using available wind or wave energy. To get experience from a smaller regional system before building a full-scale one for the Baltic, a follow-up project, BOX-WIN (Baltic Sea Oxygenation and Floating Windpower), will construct an anchored wind-driven pump with a mean capacity of 100 cubic meters per second. The project will also analyze the ecological and biogeochemical consequences of oxygenating the small, deep basin with anoxic deepwater in the Bornholm Sea in the southwestern Baltic. Construction of the pump platform is being led by Holger Eriksson, drawing upon experience from Statoilís (Stavanger, Norway) Hywind, a floating windmill launched offshore Norway in 2009.

With geoengineering as a potential means for deepwater oxygenation, the planned rebuilding of Swedish sewage treatment plants is questionable. For instance, the Syvab plant, serving about 3 percent of Swedenís population, plans to invest 1.5 billion Swedish krona to reduce its phosphorus output by 8 tons per year, a small amount relative to the high cost of the project. This money could instead fund a system of eight pumps to oxygenate the Bornholm Sea, possibly preventing annual phosphorus leakage of several thousand tons.

Sweden is developing a trade system for nutrient removal, and geoengineered deepwater oxygenation could be included. Instead of paying for the building and running of a phosphorus removal plant, removal could be bought as a service from a company already conducting geoengineering efforts in the open Baltic Sea. Preliminary estimates suggest that phosphorus removal by deepwater oxygenation is a more cost-effective method than widespread sewage treatment plant modifications. If a trade system develops, phosphorus removal by large-scale oxygenation could also become a profitable new industry.



Anders Stigebrandt is an oceanography professor at the University of Gothenburg who has conducted research on tidal mixing in fjords and on vertical circulation in stratified systems like the Baltic Sea. He heads the Marine Systems Analysis group that maintains the BOX (Baltic Deepwater Oxygenation) and BOX-WIN (Baltic Sea Oxygenation and Floating Windpower) projects.


2013:  JAN | FEB | MARCH | APRIL | MAY | JUNE | JULY | AUG | SEPT | OCT | NOV | DEC
2012:  JAN | FEB | MARCH | APRIL | MAY | JUNE | JULY | AUG | SEPT | OCT | NOV | DEC

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