Oxygen-depleted area unusually wide in the Gulf of Finland, bottom conditions now worst so far in the 2000s

The research vessels Aranda and Muikku returned last week from their expeditions to the Baltic Sea. At all observation stations, in the Gulf of Finland as well as in the northern Baltic proper, there was very little or no oxygen in the bottom-near water at depths below 50-60 metres. Oxygen concentrations in the deep water were now lower than ever before in the 2000s. According to the observations made by Aranda there was hydrogen sulphide in the bottom-near water everywhere in the Gulf of Finland, including the easternmost observation points. 

Since the beginning of this year the salinity stratification has grown stronger in the Gulf of Finland. Water of higher salinity and lower oxygen concentration has flown from the Baltic proper into the Gulf of Finland. Stratification prevents the vertical mixing of the water mass and the mixing of the oxygen-rich epilimnion, i.e.the upper water, with the bottom-near water. As a consequence of the oxygen depletion, the phosphorus concentrations at the bottom have increased. The phosphorus concentrations measured this year were in most cases higher than those measured last summer.  

In the Gulf of Finland, the condition of the bottom sediment was worst so far during the coastal monitoring period of R/V Muikku which started in 1999. Also the bottom fauna monitoring gave the worst results so far. An abundant and diversified bottom fauna was now found only at 4 observation sites of 47. No less than 37 observation sites were entirely without bottom animals. The bottom fauna is a good indicator of the long-term status of the bottom and especially the changes in the oxygen regime. 

The condition of the Archipelago Sea is still considerably better than that of the Gulf of Finland. Even the more heavily loaded areas showed distinctly lower phosphorus concentrations in the bottom near water than those measured in the Gulf of Finland. The oxygen status was good, the bottom fauna was abundant and its communities were diversified.  

The blue-green algae situation in the Gulf of Finland was better than in the previous years. Surface blooms were hardly observed at all in the beginning of August, and the amounts of blue-green algae were lower, especially in the eastern part of the Gulf of Finland. During the second week of August, surface blooms were observed in the central parts of the Baltic Sea and especially in the southern parts of the Archipelago Sea. The observations are in general outline compatible with the blue-green algae bloom prognosis given in the beginning of the summer. Also toxicity studies were carried out. Aranda focused on the hepatotoxin concentrations of the water mass surrounding the blue-green algae blooms while Muikku concentrated on the toxicity of dinoflagellates. The results of these studies will be published during the autumn.

As a consequence of the poor condition of the sea floor in the Gulf of Finland, the internal phosphorus loading is launched every time the stratification, typical of the Baltic Sea, creates favourable conditions for this process. A more permanent recovery of the situation is possible only if the phosphorus and nitrogen loads are significantly reduced within the catchment of the Gulf of Finland and the entire Baltic Sea. 

More information:

Mr Harri Kankaanpää, leader of the Aranda expedition, Finnish Institute of Marine Research, phone +358 40 700 6545, harri.kankaanpaa@fimr.fi

Mr Markku Viitasalo, professor, Finnish Institute of Marine Research, phone +358 40 503 4848, markku.viitasalo@fimr.fi

Mr Seppo Knuuttila, leader of the Muikku expedition, Finnish Environment Institute, phone +358 40 760 9232, seppo.knuuttila@ymparisto.fi

Mr Jouni Lehtoranta,.research specialist, Finnish Environment Institute, phone +358 400 148 532, jouni.lehtoranta@ymparisto.fi

Source: Finnish Institute of Marine Research and Finnish Environment Institute

http://www.fimr.fi

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