Oxygen Survey in the Baltic Sea 2017 - Extent of Anoxia and Hypoxia, 1960-2017

Type: Report
Series: RO 63
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 ICES. In this report the results for 2016 have been updated and the preliminary results for 2017 are presented. Oxygen data from 2017 have been collected during the annual Baltic International Acoustic Survey (BIAS) and from national monitoring programmes with contributions from Sweden, Finland and Poland.

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 transformed to maps and diagrams to visualize the annual autumn oxygen situation during the analysed period.

The updated results for 2016 and the preliminary results for 2017 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 18% of the bottom area was affected by anoxia and 28% by hypoxia during 2017. The hydrogen sulphide has, due to the inflows, disappeared from the Eastern and Northern Gotland Basin. However, the oxygen concentrations in the deep water are still near zero and signs of increasing hydrogen sulphide close to the bottom have been observed during 2017.