Parts of the Baltic Sea seafloor are completely devoid of oxygen, rendering zones with dead bottoms. The oxygen free conditions extends, however, also into the water column and thus limits the habitats for fish and other animals in the Baltic Sea.. The phenomenon can occur in open seas and coastal zones globally but in the Baltic Sea, this is a growing problem due to eutrophication and the relatively poor ventilation of the deep water.

Stratification prevents oxygenation of water

The saline deep water, being heavier than the brackish surface water, is mostly mixed and oxygenated during specific events where large inflows of water from the Kattegat occurs. The oxygen deficiency varies spatially in the Baltic Sea throughout the year, and it is observed by continuous water sampling in several environmental monitoring programs. In a new study from the University of New Hampshire, the Swedish University of Agricultural Sciences and Stockholm University, scientists show that it is now also possible to find the hypoxic zones using active sonar systems.

- The oxygen depleted areas in the Baltic Sea has been mapped since the 1960s, by analyzing water samples. The method we developed here, can in some cases quickly, and with high-resolution, map how the hypoxic zone varies and expands, says Christian Stranne, Assistant Professor, Department of Geological Sciences at Stockholm University, who participated in the study.

Forskare i rum med datorskärmar som visar provtagning
The data from R/V Electra's multibeam, is monitored in real-time by the researchers. Photo: Jonas Hentati Sundberg

Detailed maps provide new information

In the Western Gotland Basin, northeast of Öland, where this study was conducted, the research team anticipated the halocline between the shallow and deep water to be found at the same depth as the interface of the oxygen depleted water. After ensuring this connection with ground truth measurements in the water, the researchers were able to track the varied depth of the halocline on a detailed level with the advanced sonar systems on R / V Electra.

- It turns out, it coincided very well. We found the halocline at a depth between 50 and 70 meters. At best, the difference between the acoustic information and the water samples was 10 centimeters. At most, there was a difference of 2.9 meters, says Christian Stranne.

The extensive amount of data collected can be translated into maps of expansion areas where you can easily see how water masses fluctuate over time. According to the researchers, this is an important step towards understanding the variability of the anoxic zone in space and time.

- This detailed information on how the oxygen depleted zone varies spatially and temporarily is not possible to acquire with traditional point observations, says Christian Stranne.

Efficient complement to marine monitoring

Christian Stranne and his colleagues state that this method needs to be modified for application in other areas of the Baltic Sea, but emphasize that acoustic surveys are cost-efficient methods that could be complementary to the existing marine monitoring in the Baltic Sea.

- Monitoring organizations like Helcom, have ships that are already equipped with this type of echo sounder. They could easily run acoustic measurements parallel to other sampling programmes that are conducted, Christian Stranne concludes.

Forskargrupp och besättning i gruppbild på fartyget Electra
Crew and research team on R/V Electra. From left: Calle Wiltén, Christian Stranne, Elizabeth Weidner, Thomas Strömsnäs. Front row: Jonas Hentati Sundberg.


About the publication:

The results from this study are published in ICES Journal of Marine Science: Tracking the spatiotemporal variability of the oxic–anoxic interface in the Baltic Sea with broadband acoustics

This study is a collaboration between University of New Hampshire, Swedish University of Agricultural Science and Stockholm University. The sampling campaign with R/V Electra was partially funded by BEAM, Baltic Sea Ecosystem Approach to Management och Vetenskapsrådet.