Water at Bothnian Bay - nutrients and metals
The amount of nutrients in the water determines the extent of the biological production. The surface water in the Bothnian Bay contains low amounts of phosphorus and the primary production in the area is thus phosphorus-limited. The nitrogen-phosphorus ratio is quite stable in the Bothnian Sea and nitrogen is the limiting nutrient for phytoplankton in the rest of the Baltic Sea. Phosphorus is often the limiting nutrient in fresh water which explains its role in the Bothnian Bay, consisting largely of fresh water. The phosphorus that could be transported from the Bothnian Sea into the Bothnian Bay, is bound to iron and exposed to sedimentation, which is why it does not reach the Bothnian Bay. Thus, even large nitrogen discharges will not have a great impact on the Bothnian Bay as long as the phosphorus concentrations remain low.
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There are no eutrophication symptoms at the open sea in the Bothnian Bay. The problems caused by eutrophication are visible only in the coastal archipe- lago and other shallow coastal waters with limited water turnover, as well as outside large urban areas and industries. No anoxia has been recorded at the bottoms in open sea during the last years. |
The impact that metals have on sea organisms depends on the chemical form of the metals, the simultaneous occurrence of other substances in the sea and the way that the organisms receive or bind metals. In general, metals are considered to be more poisonous in fresh water. Some metals can thus be more poisonous in the Bothnian Bay than in the other parts of the Baltic Sea. Furthermore, the growing season in the Bothnian Bay is very short, which means that the organisms grow slowly and therefore can accumulate relatively high amounts of environmental toxins before they are fully grown. Compared to eutrophied areas, the Bothnian Bay has a lower biomass and also lower rates of sedimentation. This results in smaller number of organisms sharing the environmental toxins, which can be seen in metal concentrations of both the sea organisms and the bottom sediments.
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Updated 04.12.2009