https://phys.org/news/2018-07-oxygen-loss-coastal-baltic-sea.html

"The Baltic was strongly impacted by human nutrient inputs in the 20th century and is still experiencing the legacy of those inputs today," says Tom Jilbert, an assistant professor at the University of Helsinki, Finland, who took part in the research. But despite recent measures to reduce the release of polluting nutrients, the researchers write in the new study that they found "no evidence of recovery" from oxygen depletion in the Archipelago Sea, a coastal area between mainland Finland and Sweden that is part of the Baltic.

A reason, they say, may be climate change. Since warm waters are less effective at holding oxygen, "global warming is likely to exacerbate oxygen depletion," says Sami Jokinen, a researcher at the University of Turku, Finland, and lead-author of the Biogeosciences study. Jilbert adds: "Climate change was not the main cause of the current dead zone, but it is an important factor delaying the recovery."