Oxygen deficiency in the Littorina sea
But this has not always been the case. After the great ice sheets of the Pleistocene geological period began to melt 16,000 years ago, the Baltic Sea has experienced alternately freshwater or brackish conditions due to crustal uplift and rising sea levels. The current period of brackish water, Littorina, began 8,500 years ago when the sea became connected to the oceans through the Sound and the Danish belts. In connection with this, periods with oxygen-free bottoms started occurring in the Baltic Sea.
The new study by Tom Jilbert and co-authors, published in the journal Geology, shows that the sills in the Åland Sea were considerably deeper from 8,000-4,000 years ago than they are today, which led to direct exchange of deep water between the Baltic Proper and the Bothnian Sea. Because of this, a long period of anoxic bottom waters affected both basins at this time, with the most severe hypoxia found in the Bothnian Sea.
Land uplift improved oxygenation in the Bothnian Sea
Land uplift probably had fundamental importance for ending this intense period of bottom water anoxia in both basins at about 4,000 years ago. When the sills at Åland became shallower this choked off the flow of saline and oxygen deficient bottom water to the Bothnian Sea.
In connection with this the bottoms of the Bothnian Sea began to act as a sink for phosphorus and iron. Thereby phosphorus levels in the Baltic Proper were also reduced and its oxygen situation improved. Decreasing temperatures during the same period may also have contributed, along with increased rainfall and a weakened stratification of the seawater.
Hypoxia in modern Baltic Sea
Periods of anoxic bottom waters have since recurred in the Baltic Proper during the so-called Medieval Climate Anomaly, and in modern times, despite continued transport of phosphorus from the Baltic Proper to the Bothnian Sea.
Therefore, argue the authors of the study, the oxygen-free bottoms in the Baltic Proper today must be sustained by eutrophication. We simply add nutrients to the Baltic Sea in such great quantities, that the natural sedimentation processes removing nutrients are overwhelmed. The authors suggest that maintaining the efforts of HELCOM to reduce nutrient loading is the best course of action to rid the Baltic Sea of hypoxia in the future.
Link to the article in Geology