Blue-green algae: How the Baltic Sea is changing

Global warming and overfertilization are creating a vicious circle. Blue-green algae are appearing more and more frequently in the Baltic Sea. This is shown by our satellite analysis of images from the last 10 years, which we carried out for NDR.

Imagine driving to your favorite beach and finding yourself knee-deep in algae. Our exclusive satellite analysis, in which we evaluated 10 years of satellite images (2016 to 2025) for NDR, shows that blue-green algae are becoming increasingly common in the Baltic Sea. Marine biologist Anke Kremp explains to NDR that climate warming and intensive industrial agriculture are creating a vicious circle that is exacerbating the problem.

Development of blue-green algae off the Baltic Sea coast since summer 2016

For the NDR Data Team, we analyzed a total of 156 satellite images of the Baltic Sea from 2016 to 2025. We looked at the period from May to September for each year, with the evaluation for 2025 extending through July.

"Blue-green algae were particularly prevalent in the Baltic Sea near the resorts of Heringsdorf and Zinnowitz on Usedom, Binz on Rügen, and Zingst: they were visible on satellite images on more than 140 of the 156 days evaluated. However, the Baltic Sea near Fehmarn and the Bay of Lübeck also stand out due to a large number of days with blue-green algae." (NDR)

Spectacular algae patterns

The algae transform the Baltic Sea into spectacular patterns. The patterns are particularly easy to see from space. The NDR article contains further satellite images of this natural spectacle.

algae indicators

Images from the second week of August clearly show the blue-green algae in Lübeck Bay. On the left-hand slider is the satellite image, on which the patterns can be seen with the naked eye. On the right-hand side is an evaluation of the algae indicators. Areas with particularly strong coloring indicate a particularly high occurrence of algae.

Method

The images were taken by the Sentinel-3 satellite, which can only capture usable images where there are no clouds in the way. We therefore limited our investigation to cloud-free days over the sea in order to ensure comparability.

In addition to purely optical images, Sentinel-3 also provides information on specific color ranges. The European Space Agency uses algorithms and AI models to calculate two algae indicators (OC4Me, CHL_NN) from this data. We used this and other color information to measure the leaf pigment chlorophyll in order to determine algae blooms.

It should be noted that the Sentinel-3 satellite has a resolution of 300 meters. Therefore, in individual cases, 300x300m tiles may overlap with the land area.

Link to the NDR article