The Baltic Sea is extremely stratified, with a layer of relatively fresh water sitting on top of a layer of denser, saltier, poorly oxygenated water. This stratification limits the amount of mixing, which is necessary for the transfer of oxygen to the water at the bottom of the sea. Oxygen levels in this lower layer are often so low that it causes the death of animals and plants that live on the seabed.
For a new study published in the Proceedings of the National Academy of Sciences, an international team of researchers analysed 115 year’s worth of data for water temperature, oxygen content and salinity in the Baltic. They found that over that period, the areas of oxygen depletion has increased from about 5,000 square kilometres to about 60,000 square kilometres. ‘On the basis of this analysis, we can determine that the many nutrients from the land are the main cause of the widespread oxygen depletion,’ said the study’s lead author, Professor Jacob Carstensen of Aarhus University in Denmark.
The results also indicated that during the past 20 years, climate change has played a role in the Baltic’s oxygen depletion. As the sea’s water warms, it reduces the amount of oxygen it can hold, while also increasing oxygen consumption by the organisms living in the sea.
This story was published in the May 2014 edition of Geographical Magazine