The dramatic loss of marine wildlife recorded last year in the Western Baltic Sea between Denmark, Germany and Sweden is largely the result of extreme weather conditions and an increase in man-made nutrients, according to the findings of a report recently released by the Helsinki Commission (HELCOM), to which the European Commission provided significant input. Last autumn, the two organisations joined forces to investigate exceptional oxygen depletion in the Western Baltic that had led to hundreds of dead fish being washed ashore along the east coast of Jutland, Denmark. The report reveals that the oxygen deficiency was caused in part by heavy rain and snow, leading to the run off of higher levels of nutrients from agriculture, urban wastewater and air pollution into the sea. In addition, low wind levels and high air pressure minimised exchanges between different water levels in the Baltic. The report recommends stricter controls on nutrients reaching this inland sea to prevent future oxygen depletion.
Research Commissioner Philippe Busquin said: “We must do more to reduce the level of man-made nutrients polluting the Baltic Sea and the destruction of its precious ecology. We cannot ignore nature’s alarm calls, and must ensure that our research findings help shape appropriate international policies.”
A preliminary version of the report was used in the preparatory work for the HELCOM Ministerial Meeting, which took place on 25th June 2003 in Bremen. Environment Commissioner Margot Wallström participated at this meeting on behalf of the European Commission. A wide-ranging package of measures for the protection of the Baltic marine environment was agreed upon by the Ministers. With regard to combating eutrophication , this package included the following agreements to:
Tackling a wildlife disaster
Widespread and long lasting severe oxygen depletion was observed in the Kattegat, the Sound and the Baltic Sea in late summer and autumn 2002 – amongst the worst ever recorded. In several areas, extreme oxygen deficiency led to the release of highly toxic hydrogen sulphide from marine sediments. As a result, creatures living near to the bottom of the sea died and, in October 2002, a large amount of dead marine wildlife was washed up on the Jutland coast.
Following the initiative of the HELCOM Monitoring and Assessment (MONAS) Group, an expert group was set up with Denmark, Germany, Sweden and the European Commission to analyse the development and causes of this worrying situation.
Eutrophication is still a major problem in the Baltic Sea. The symptomatic problems of eutrophication – serious oxygen deficiency, extensive algal blooms and floating mats of decaying seaweed in coastal waters – remain all too common, in spite of substantial efforts to reduce nutrient inputs over a wide area. In the EU as well, intensive agricultural methods make farmland a major source of waterborne nutrient pollution.
Analysis of the development and causes of the 2002 oxygen depletion required expertise from several disciplines, including marine biology, oceanography and satellite remote monitoring. The experts formed a Working Group to seek explanations and recommendations for decision-makers.
Identifying the cause
Comparisons between recent years marked by specific weather events in the area revealed the key roles of snow, rain, and wind and air pressure in the oxygen balance of marine bottom waters. The amount of snow and rain largely controls the nutrient loading of surrounding rivers by soil erosion. Unseasonably late rains, combined with sunlight can also indirectly enhance marine plant production in surface waters. Wind and air pressure acts on the local supply of oxygen through water exchanges with the oxygen-rich waters of the Skagerrak.
Field measurements show that in 2002 there was a higher nutrient discharge in the Baltic Sea in June and July as a result of above average rainfall. Microalgae biomass, measured by both traditional means and satellite remote sensing, showed slightly higher levels in July compared with previous years. However, 1999 levels were substantially higher than 2002, yet oxygen conditions were better.
Comparable results from three independent hydrological models (Denmark, Sweden and the Commission) provided the main explanation for particular sensitivity to oxygen consumption in 2002. August was marked by almost no water exchange at the bottom of the sea, between the Skagerak (oxygen-rich) and the Belt Sea area (oxygen-poor), and there was substantially lower inflow than usual in July and from September to November. Deep bottom oxygen depletion (at 15 to 60 metres) resulted from this particular low water exchange, the latter controlled mainly by wind and air pressure above the North and Baltic Seas. In addition, unusually low winds severely limited mixing in surface waters (down to 15 metres) and were responsible for oxygen deficiencies at shallow levels.
Reduced nutrient levels essential
While weather conditions were the main trigger of the 2002 event, investigations revealed that the Baltic Sea is particularly vulnerable to oxygen depletion. Permanent separation of water strata, minimal reaction with the sea bottom, restricted flow patterns resulting from semi-enclosed bays and estuaries and shallow bowl shapes in the sea bottom all favour the isolation of bottom water masses and therefore limit reoxygenation.
The Baltic Sea region is one of the most naturally sensitive to oxygen deficiency in Europe. Some confined regions – such as the Little Belt – were already experiencing oxygen deficiencies 100 years ago, when nutrient discharges were relatively low. For several decades the main original cause of extended oxygen deficiency has been the nutrient supply in surface marine waters.
The Commission contribution indicates that the Belt Sea area has a very limited capacity to digest the organic matter and, indirectly, to assimilate any additional supply of nutrients. Further efforts are necessary to meet the 50% nutrient reduction target set by HELCOM. But even this might turn out to be insufficient to drastically reduce the likelihood of severe oxygen depletion in terms of geographical coverage and duration in the Western Baltic.
Fabio Fabbi | European Commission
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