The environmental state of the Baltic proper has changed with the increasing eutrophication due to increasing phosphorus (P) content in the water column. The volume of anoxic water has increased by a factor of about 7 from the period 1965 – 1999 to the period 2000 -2016. The very large inflows of new deepwater from the Kattegat in 2014 and 2015 did not lead to any substantial reduction of the volume of anoxic water in the Baltic proper. The land-based supply of P to the Baltic Sea has been halved since the 1980s. Why doesn’t the P content of the water column of the Baltic proper decrease when the land-based supply decreases?

In a recently published paper, it is shown that the development of the equilibrium P content of the water column of the Baltic proper can be explained if one considers the total P supply, which equals the land-based supply plus the internal supply from anoxic bottoms plus the (small) oceanic supply by inflowing sea water from Kattegat. Present day magnitudes, in tonnes P year^-1, of the three sources are about 30000 (land-based, NB! only about 1/3^rd of this may be removed), about 100000 (internal) and 10000 (oceanic). In the paper, it is shown that the equilibrium P content of the water column of the Baltic proper is a linear function of the total supply of P! This means that the only way to substantially decrease the P content of the water column of the Baltic proper is to decrease the internal source. This can be done by oxygenation of the at present anoxic bottoms. In the paper, it is shown that it would take 10-15 years for the Baltic proper to reach equilibrium with the total P supply. It is concluded that the Baltic proper in 10-15 years can be restored to a state that is determined by the external land-based source (which at the present is at the same level as in the 1950s) if only the internal source is eliminated by oxygenation of deep bottoms. The paper is written by Anders Stigebrandt and published in Ambio. It can be found under the tab Downloads.

Abstract

Using a time-dependent phosphorus (P) budget model for the Baltic proper, describing sources and sinks at the external borders of the water column, one may compute the e-folding time T of the adjustment of the winter surface water P concentration c1 to abruptly changed total P supply. The restoration time TR = 3T is introduced as a practical measure of the time it takes to achieve 95% of the change of c1 towards the final, equilibrium, state c1e. The P budget model, including an internal source emanating from deep anoxic bottoms, also shows that c1e is proportional to the total P supply to the water column. About 70% of present time total P supply to the Baltic proper comes from deep anoxic bottoms. If deep bottoms were kept oxygenated, this internal P supply would be turned off and the equilibrium concentration c1e would be reduced by about 70%. This should imply that the Baltic proper may be restored to a state determined by the external P supplies from land-based and oceanic sources. According to the model, restoration would take 10–15 years. Thereafter most of the equipment used for oxygenation may be shut off since also the deepwater oxygen demand by decomposition of fresh organic matter, would have decreased by about 70% implying that the deepwater would be kept oxic by the natural vertical circulation. The model presented in this paper provides a new science-based solution of the eutrophication problem of the Baltic proper, which is of great interest from a management point of view.

To follow the progress of possible colonization and the progress of chemical oxygenation and reduction processes of the bottom overlain by hypoxic water in the deeper East Gotland Basin, a sediment profile image survey similar to the July 2015 survey of Rosenberg et al. (2016) was undertaken in April 2016. Only one record of a benthic individual, Bylgides (Harmothoe) sarsi, occurred in oxygenated deepwater. At most stations, the superficial oxygenated layers had been thinner from 2015 to 2016. The results of the survey are described and discussed in a recent paper in Ambio by Anders Stigebrandt, Rutger Rosenberg, Marina Magnusson and Torsten Linders. It is concluded that colonization of deep bottoms of oxygenated layers that are covered by a thick layer of anoxia or hypoxia can probably best be performed by horizontal movement of adults or larvae carried by the inflowing new deepwater. The paper can be found under the tab Downloads.

Sediment profile images obtained along a transect with increasing water depths in July 2015 showed that a thin surface layer of earlier anoxic bottom sediments in the East Gotland Basin (EGB) had been oxidized by contact with new deepwater that arrived to EGB three months earlier, i.e. in March 2015. This is the first time re-oxygenation of surficial sediments after inflows of oxygen-rich water is shown in situ in the Baltic Sea. It was also found that the recently oxygenated bottoms had not been colonized by animals. This is described in a paper by Rutger Rosenberg, Marina Magnusson and Anders Stigebrandt published in Ambio 2016. The paper can be found under the tab Downloads.