Warming exacerbates oxygen depletion in the western Baltic Sea

The Baltic Sea is clearly feeling the effects of climate change, with rising water temperatures are exacerbating environmental problems. Although nutrient inputs have been reduced, oxygen minimum zones continue to expand. For their new study, researchers at GEOMAR used long-term data from the Boknis Eck time series station to investigate how environmental conditions have changed between 1991 and 2019 and how these changes affect life in the water. The study highlights significant fluctuations in temperature and oxygen levels and their influence on bacterial growth and nutrient dynamics. The results have now been published in the journal Scientific Reports.

Oxygen depletion due to higher temperatures and increased bacterial activity

The study focused on bacterial biomass production (BBP) in the southwestern Baltic Sea. BBP describes the growth of bacteria and other microorganisms that break down organic nutrients. During the summer, following the spring phytoplankton bloom, BBP rates increase significantly, leading to increased oxygen consumption, particularly in deeper water layers. Meanwhile, the warmer surface water acts as a capping top layer. 

The problem is that these layers of water mix very little, and new oxygen can only be added by strong inflows from the North Sea, often driven by storms. Over the study period, higher water temperatures extended the stratification of the water column into autumn. In some years, heat waves even reached the bottom layers, exacerbating oxygen depletion.

Nutrient reduction successes undermined by rising temperatures

Efforts to reduce nutrient loads in the coastal Baltic Sea by reducing phosphorus and nitrogen inputs have had positive effects. In recent years, nutrient inputs have been reduced by 18 to 22 percent, mainly due to advances in wastewater treatment technology. However, nutrient levels are still too high and the Baltic Sea continues to suffer from eutrophication.

“Strong seasonal variations in nitrogen compounds such as ammonium indicate that excessive amounts of these nutrients still enter the water and fuel algal blooms,” says Dr Helmke Hepach, lead author of the study and environmental scientist at GEOMAR.

Adding to the problem is the existing nutrient load: phosphorus bound in the seabed is being released back into the water column due to the increased occurrence of oxygen minima – a process that also releases ammonium. Dr Hepach explains: “This creates a feedback loop that we also observe at Boknis Eck. At present, there are no effective solutions to permanently reduce this internal load. With the increasing frequency of oxygen depletion events, the situation will get worse.”

The small successes in reducing nutrient inputs are further counteracted by rising water temperatures. Higher temperatures increase bacterial activity, which consumes oxygen as organic matter from algal blooms decomposes. Meanwhile, increased thermal stratification prevents new oxygen from reaching deeper layers.

“Increasing warming and the resulting increased bacterial activity will have serious long-term consequences for the Baltic Sea ecosystem,” says Dr Hepach. The study therefore recommends stronger efforts to reduce both inorganic and organic nutrient inputs.

Original Publication:

Hepach, H., Piontek, J., Bange, H.W., Barthelmeß, T., von Jackowski, A., & Engel, A. (2024) Enhanced warming and bacterial biomass production as key factors for coastal hypoxia in the southwestern Baltic Sea. Scientific Report 14, 29442.

https://doi.org/10.1038/s41598-024-80451-w

About: CREATE

The study is part of the CREATE project (Concepts for Reducing the Effects of Anthropogenic pressures and uses on marine Ecosystems and on Biodiversity). CREATE is part of the sustainMare research mission by the German Marine Research Alliance (DAM) and is funded by the Federal Ministry of Education and Research. The mission is now entering its second phase.