Oxygen Survey in the Baltic Sea 2018 - Extent of Anoxia and Hypoxia, 1960-2018

Type: Report
Series: RO 65
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 2017 has been updated and the preliminary results for 2018 are presented. Oxygen data from 2018 have been collected from various sources such as international trawl survey, national monitoring programmes and research projects with contributions from Poland, Estonia, Latvia, 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 depth at which hypoxic and anoxic conditions respectively are found. The volume and area of hypoxia and anoxia have been calculated and the results have then been transferred to maps and diagrams to visualize the annual autumn oxygen situation during the analysed period.

The updated results for 2017 and the preliminary results for 2018 show that the severe oxygen conditions in the Baltic Proper after the regime shift in 1999 continue. Both the areal extent and the volume with anoxic conditions have, after 1999, been constantly elevated to levels only observed occasionally before the regime shift. Despite the frequent inflows to the Baltic Sea during the period 2014-2016 approximately 22% of the bottom area was affected by anoxia and 32% by hypoxia during 2018. The preliminary results indicate that this is the largest area affected by anoxia during the analysed period, starting 1960. 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.