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Nachrichten, Termine, Experten
Nachrichten, Termine, Experten
4 September 2025/Kiel. Why do oxygen-depleted zones repeatedly form in the western Baltic Sea, leading to mass fish deaths? What role does the toxic hydrogen sulfide released from the sediment play? These questions are being addressed by a research team led by GEOMAR Helmholtz Centre for Ocean Research Kiel during an expedition with the German research vessel ELISABETH MANN BORGESE. The cruise runs from the Flensburg Fjord along the Schleswig-Holstein coast to Fehmarn.
Today, the expedition EMB374 set sail from Kiel into the southwestern Baltic Sea as part of the joint project PrimePrevention with the FS ELISABETH MANN BORGESE. The goal of the collaborative project is to develop a coordinated strategy among the coastal states to counter climate change. For this purpose, the expected increase in climate-related marine natural hazards, including harmful microorganisms and oxygen-depleted waters, will be investigated in more detail. The expedition focuses in particular on near-coastal oxygen-depleted zones and hydrogen sulfide on the seafloor. For the first time, GEOMAR Helmholtz Centre for Ocean Research Kiel is taking over the cruise leadership on the research vessel operated by the Leibniz Institute for Baltic Sea Research Warnemünde (IOW). The research team also includes scientists from the Universities of Hamburg and Oldenburg.
Coastal sediment as a source of toxic hydrogen sulfide
In Kiel Bay, a strong decrease in oxygen levels occurs regularly in late summer – a consequence of climate change and eutrophication. This has severe consequences for ecosystems and thus also for the regional economy. The study area of expedition EMB374 in the southwestern Baltic Sea is known for the frequent occurrence of oxygen-depleted zones in late summer. Particularly problematic is the release of toxic hydrogen sulfide (H2S) at the seafloor.
For the investigations, the ship remains close to the coast, because although coastal sediments make up only about nine percent of the seafloor, they play a central role in storing and breaking down organic material such as algae, plant, or animal remains. Under oxygen-rich conditions, the organic material can be degraded to CO₂. However, in the southwestern Baltic Sea, oxygen-poor and even oxygen-free zones occur near the seafloor in late summer. These conditions favor certain bacteria that couple the decomposition of organic material to respiration with oxygen alternatives. They use sulfate for this, which is abundant in seawater. When sulfate is reduced, hydrogen sulfide is produced. It has a characteristic smell of rotten eggs and is toxic to many marine organisms. If oxygen-poor or hydrogen sulfide-containing water rises into shallower water layers due to upwelling, it can lead to mass fish kills.
How hydrogen sulfide-containing water is formed
“We want to find out under which conditions and at which locations hydrogen sulfide is released from the sediment into bottom water. With this knowledge, we can better predict risks for marine organisms and more accurately assess the role of the Baltic Sea under the influence of climate change,” says chief scientist Prof. Dr. Mirjam Perner, Professor of Geomicrobiology at GEOMAR.
The expedition is part of the PrimePrevention project of the German Marine Research Alliance (DAM). This project investigates factors that lead to the formation of hydrogen sulfide-containing bottom waters. During the expedition, oxygen and hydrogen sulfide concentrations in the water column are measured using sensors, and geochemical and microbiological factors at the seafloor are determined. In addition, water and sediment samples are collected for laboratory analyses. All available environmental data are then incorporated into numerical models that can be used to predict the release of hydrogen sulfide. The aim is to identify particularly vulnerable regions and to assess the risk of hypoxic events for stakeholders such as tourism, fisheries, and aquaculture.
Algal blooms in the Baltic Sea
During the cruise, various systems for detecting cyanobacteria will also be tested. These organisms account for a large part of the summer algal blooms in the Baltic Sea and can produce toxins that in some cases lead to bathing bans at beaches. When they die, this also leads to increased oxygen consumption in deeper water layers.
A newly developed optical measurement system (hyperspectral module) from the University of Oldenburg will be tested during the expedition and compared with other measurements to assess its suitability for routine use on ships or measuring platforms. For comparative measurements, the HyFiVe system (modular hydrographic measuring system) is also used. It was developed at the Leibniz Institute for Baltic Sea Research Warnemünde (IOW) and at the Thünen Institute in cooperation with Hensel Elektronik GmbH, with federal funding. A newly integrated sensor can measure the amount of cyanobacteria.
With the new system from the university, fishers are also to be enabled to collect additional measurement data for marine research (projects PrimePrevention and HyFiVe-Baltic). Early detection of cyanobacteria is then to be embedded in early warning systems to protect people in coastal regions from harm. To validate and calibrate the two systems, the Oldenburg-based company AquaEcology will also take water samples, which will later be microscopically analyzed in the laboratory.
Mirjam Perner: “In the Baltic Sea, processes such as warming, acidification, and eutrophication are more pronounced and occur more rapidly than in other seas. We therefore also refer to the Baltic Sea as a time machine. This is why it is so important to already understand how the processes work that in the future will increasingly affect other marine areas as well.”
Background: PrimePrevention
The research cruise and associated work are embedded in the PrimePrevention project of the Deutsche Allianz Meeresforschung (DAM) mission mareXtreme. The project investigates ways to predict biological hazards for the ocean to prevent socio-economic impacts and is funded by the Federal Ministry for Research, Technology and Space (BMFTR). The project is coordinated by Dr. Katja Metfies at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI).
Expedition at a glance:
Name: EMB 374 PrimePrevention
Chief Scientist: Prof. Dr. Mirjam Perner (GEOMAR)
Period: 04.09.2025 – 13.09.2025
Start and end: Kiel
Cruise area: Southwestern Baltic Sea
https://www.geomar.de/forschen/fb2/fb2-mg/geomikrobiologie/bocknis-eck-langzeitr... PrimePrevention
https://www.geomar.de/n10001 Image material for download
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