Recovery from deoxygenation may be unpredictable: new study

The recovery of the oxygen-depleted bottoms in the Baltic Sea could be more unpredictable than previously estimated. New research shows that historically, the occurrence of oxygen deficiency has varied rapidly, due to an inherent instability that makes the Baltic Sea particularly sensitive to variations in climate. As the Baltic Sea recovers and approaches the goal of being unaffected by eutrophication, these sharp variations in oxygen depletion may return.

The high nutrient input to the Baltic Sea during the 20th century has caused severe eutrophication and a record extent of oxygen-depleted deep-water areas: high availability of nutrients leads to high production of algae and cyanobacteria, which, when decomposed consume oxygen. But oxygen depletion has also occurred historically in the Baltic Sea, even when nutrient inputs were much lower than today, which has puzzled scientists. A new study, in which the researchers used advanced analytical techniques to study the chemistry of sediment cores, shows that oxygen depletion has varied greatly with a periodicity of a few decades during some eras.


BSAP reductions insufficient?
Today's heavily eutrophicated Baltic Sea with high productivity and virtually permanent oxygen depletion is far from the regime in which these oscillations can occur. But the Baltic Sea Action Plan (BSAP) aims to reduce nutrient loading significantly, and this is when the implications of the study become clear.

“When the phosphorus inputs are reduced, the Baltic Sea may surprise us in the long term with a much more complicated path towards recovery than we have earlier expected”, says Bo Gustafsson.
Using the modelling tool BALTSEM, Bo Gustafsson's research group Baltic Nest Institute has also calculated what level nutrient inputs needs to be reduced in order to eventually reach a Baltic Sea unaffected by eutrophication. In the recently updated BSAP, riparian countries have agreed to curb their nutrient inputs to reach this level. Oscillations in phosphorus cycling could start to have a major impact on the prevalence of oxygen deficiency and, for example, cyanobacterial blooms as nutrient inputs are curbed and the state of the Baltic Sea eventually approaches the goals. However, the researchers emphasize that the current study is done using a box model and that more accurate modelling tools are needed to really determine if oscillations will occur as BSAP is reached.


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