Sunday, 17. May 2009, 17:01:41
In a recent post on
pollution in the Baltic Sea I wrote about the visible effects (algal bloom and transparency) of the pollution and about cost-efficient phosphorous abatement. Today I will concentrate on the oxygen problem and the question of possible engineered remediation.
Hypoxia, the lack of oxygen in bottom waters often defined as O
2 < 2 ml/l is a growing problem worldwide and
dead zones have spread exponentially since the 1960s in coastal marine waters. The Baltic Sea is a brackish inland sea, alleged to be the largest body of brackish water in the world (other possibilities include the Black Sea). It occupies a basin formed by glacial erosion. The Baltic Proper is permanently stratified, consisting of an upper layer of brackish water with
salinities around 7−8 and a lower layer of saline waters with salinities around 11−13. It is likely that the interaction between nutrients and climate has enhanced the conditions for hypoxia to occur.
Diagram of hypoxia in the Baltic Sea from Tackling Hypoxia in the Baltic Sea: Is Engineering a Solution? by Conley et al. 2009.Saltwater enters from the Danish Straits and moves into the Baltic Proper following depth contours. As the water ages it is depleted of O
2, with anoxia occurring in the deepest basins. As a result of hypoxia the amount of sediment removal of nitrogen (N) through denitrification and anaerobic ammonium oxidation (anammox) decreases (smaller arrows) and is essentially zero in anoxic basins. A strong permanent halocline - that is a vertical zone in the water column in which salinity changes rapidly with depth - is formed at the transition zone at depths varying between ca. 60−80 m, and prevents vertical mixing of the water column and transport of more oxygenated waters to the bottom. Nitrogen removal also occurs below the permanent halocline and above the zone of hypoxia. Sediment phosphorous (P) release is highest in hypoxic area (largest arrow), low in oxic areas, and intermediate in anoxic areas. The anoxic areas tend to have low rates of phosphorous release because sediment phosphorous pools are depleted. The abundance, composition, and diversity of benthic (sea bottom) communities are also strongly influenced by O2. The amount of carbon delivered to the bottom waters of the Baltic Sea is an important control mechanism of oxygen consumption.
Significant amounts of phosphorous are currently released from sediments, an order of magnitude larger than man induced inputs. The Baltic Sea is unique for coastal marine ecosystems experiencing nitrogen losses in hypoxic waters below the halocline.
For more information about the
Hypoxia-Related Processes in the Baltic Sea I refer to the paper in Environmental Science & Technology.
In short the Baltic Sea contains the largest man induced dead zone in the world, and due to feedback loops/feedback cycles the internal acceleration of eutrophication in the Baltic is a vicious circle - so the big question is, what can we do about it? Are engineering methods possible? Before I go on I would like to stress that virtually all engineering methods proposed to date for the Baltic Sea seem unrealistic and/or not viable, I will however mention some of the methods proposed.
Large-Scale Engineering to Increase Oxygen in Bottom WatersThe total amount of O2 needed to keep the deep waters above the threshold for hypoxia varies by 2−6 million t of O2 annually. At present there is no known technology that could transfer such an enormous amount of O2 directly into bottom waters and disperse it into large hypoxic volumes. Enhanced ventilation of deep waters through additional inputs of oxygenated saltwater has been suggested as a remediation method. Enhanced saltwater input into bottom waters is, however, expected to increase stratification and thereby increase the area of hypoxia.
Use of windmills to oxygenate the waterThis includes windmills. Feasibility, costs, and further consequences for water circulation and fauna and flora so far unknown.
Chemical Removal of PhosphorusTo inactivate phosphorous by the addition of aluminium would require enormous amounts of aluminium. The method has been tried out in (smaller) freshwater lakes, but it is uncertain how aluminium will react in brackish water like that of the Baltic Sea. In the worst case it would be toxic for marine organisms, and anyway dumping of chemicals in the Sea is forbidden by an
international convention.
BiomanipulationBiomanipulation is a method to alter the biological communities by altering the abundance of specific organisms. Although some success has been achieved in freshwaters, the enormous size of the Baltic Sea adds to the uncertainty of the effectiveness of biomanipulation on such a scale.
Stop influx of Marine WaterA drastic solution would be to barricade influx of marine water and thereby change the brackish sea to a large freshwater lake. Although technically possible it would have enormous consequences. It would completely change the aquatic fauna and flora, and species dependent on saltwater, like cod, would completely disappear.
I think it is unnecessary to say that more detailed modeling efforts of the impact on physical mixing process and the response of salinity, temperature, and stratification must be made prior to any large-scale manipulation. So far engineering methods do not look promising, and the countries around the Baltic Sea must increase their efforts to reduce their loading of the Baltic Sea with nutrients like phosphorous and nitrogen. Reductions in hypoxia will not occur until nutrient loads are reduced.
References:
Hypoxia-Related Processes in the Baltic SeaConley et al.
Environ. Sci. Technol., 2009, 43 (10), pp 3412–3420
DOI: 10.1021/es802762a
Publication Date (Web): February 18, 2009
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http://pubs.acs.org/doi/abs/10.1021/es802762a Tackling Hypoxia in the Baltic Sea: Is Engineering a Solution?Conley et al.
Environ. Sci. Technol., 2009, 43 (10), pp 3407–3411
DOI: 10.1021/es8027633
Publication Date (Web): May 13, 2009
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http://pubs.acs.org/doi/abs/10.1021/es8027633 In
Danish:
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http://www.dmu.dk/Udgivelser/DMUNyt/2009/7/biobaltic.htm Some of my other posts on pollution of the Baltic Sea:
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http://my.opera.com/nielsol/blog/2009/01/15/pollution-of-the-baltic-sea •
http://my.opera.com/nielsol/blog/2009/05/12/baltic-sea-recovering •
http://my.opera.com/nielsol/blog/2009/05/13/baltic-sea-pollution-secchi