This oxygen survey was completed in co-operation with the Swedish Board of Fisheries and the Baltic International Acoustic Survey (BIAS) during weeks 38-39 and 41-43 of 2010. During week 40 SMHI performed its own regular monitoring cruise in the Skagerrak, the Kattegat, Sound and Baltic Proper. SMHI has completed this extensive oxygen survey now for the past three consecutive years. Data from the Polish and Latvian BIAS cruises were also made available, which improved the results.
Together with the oxygen surveys 2008 and 2009, this was one of the most extensive oxygen surveys performed by SMHI: 121 stations were visited and the analysis is based on about 570 oxygen and hydrogen sulphide samples. A CTD equipped with an oxygen sensor was also used at each sampling occasion, complementing the water samples analyzed by Winkler titration and/or colourimetric hydrogen sulphide analysis.
Oxygen deficiency in the Baltic Sea
- The oxygen situation in the deep water of the Baltic Proper continues to be serious, says Martin Hansson, oceanographer at SMHI.
The bad conditions measured during the beginning of the 2100-century continues. About one sixth (~17%) of the bottom area in the Baltic Proper were completely oxygen free (with toxic hydrogen sulphide present), corresponding to ~10% of the volume. North of Öland, hydrogen sulphide was present already from depth exceeding 45 meters. Anoxic conditions at these shallow depths have never been observed before in this area.
Hypoxia (or acute oxygen deficiency; <2ml/l) was affecting about half ~28% of the bottom area, which is about one fifth (~20%) of the volume. This is the largest volume measured during the period 1960-2010. In the south-west, in the Arkona Basin and in parts of the Bornholm Basin, oxygen conditions were good due to some smaller inflows during August and September.
There are several likely contributory and concurrent causes to the recent development such as changes in wind speed and direction, increased loading of organic matter to the deep water due to eutrophication, altered vertical mixing and stratification, changes in precipitation and freshwater runoff and potentially some other phenomena.