Oxygen Survey in the Baltic Sea 2019 - Extent of Anoxia and Hypoxia, 1960-2019

Type: Report
Series: RO 67
Author: Martin Hansson, Lena Viktorsson & Lars Andersson


A climatological atlas of the oxygen situation in the deep water of the Baltic Sea was first published in 2011 in SMHI Report Oceanography No 42. Since 2011, annual updates have been made as additional data have been reported to the ICES data center. In this report the results for 2018 have been updated and the preliminary results for 2019 are presented. Oxygen data from 2019 have been collected from various sources such as international trawl survey, national monitoring programmes and research projects with contributions from Poland, Estonia, Russia, Denmark, Sweden and Finland.

For the autumn period each profile in the dataset was examined for the occurrence of hypoxia (oxygen deficiency) and anoxia (total absence of oxygen). The depths of onset of hypoxia and anoxia were then interpolated between sampling stations producing two surfaces representing the depths at which hypoxic and anoxic conditions respectively are found. The volume and area of hypoxia and anoxia were then calculated and the results transferred to maps and diagrams to visualize the annual autumn oxygen situation during the analysed period.

The updated results for 2018 and the preliminary results for 2019 show that the severe oxygen conditions in the Baltic Proper after the regime shift in 1999 continue. In 2018 the largest bottom areas and volumes affected by anoxia was recorded during the analysed period starting in 1960. Anoxic conditions affected ~24% of the bottom areas and ~33% suffered from hypoxia in 2018 and similar values just below was noted during 2019. The results from these two years could be the beginning of a new trend as the anoxia has reached another stage and new areas are affected regularly. In the southern basins of the Baltic Proper, such as the Gulf of Gdansk, Hanö Bight and in the Bornholm Basin, hypoxia has previously been found in the deep water but anoxia is now found regularly in the deep water. The hydrogen sulphide that had disappeared from the Eastern and Northern Gotland Basin due to the inflows in 2014-2016 is now steadily increasing in the deep water again. No major inflow has occurred since 2016.