Toxin-forming blue-green algae confirmed in Baltic Sea off Rostock

Confirmation and scope

The Rostock health office has confirmed the presence of toxin-forming cyanobacteria, commonly known as blue-green algae, in the Baltic Sea off Mecklenburg-Vorpommern. The confirmation followed an initial suspicion raised over the weekend, when the Rostock health office had already reported an algae carpet west of Warnemünde. The affected area is primarily between beach sections 30 and 36 near Warnemünde, according to the NDR report. A city spokesperson said that not only the Baltic Sea off Rostock, including Warnemünde, was potentially affected, but wide areas of the coast. Where the organisms ultimately drift ashore depends heavily on weather conditions. No swimming ban has been issued.

Health risks and official guidance

The Landesamt für Gesundheit und Soziales (Lagus) states that cyanobacteria can produce toxins capable of causing allergic reactions, skin and mucous membrane irritation upon contact, and nausea and vomiting if larger quantities of contaminated water are swallowed. The NDR report adds that contact can also cause conjunctivitis, ear pain, and inflammation of the throat and upper respiratory tract from inhaled droplets. In high concentrations, the toxins can lead to acute liver damage. Fatal poisonings have been recorded in fish, birds, and domestic and farm animals.

The city issued specific guidance for identifying contaminated water.

“If the water is so turbid that feet are no longer visible at knee depth, or if streaks or noticeable discoloration are visible in or on the water, swimming should be avoided there.”

Skin contact with such accumulations should also be avoided. Instructions and warnings from lifeguards, beach wardens, and the local tourism centre should be followed.

Vulnerable groups and pets

Particular caution is urged for children, people with chronic illnesses, those with weakened immune systems, toddlers, and pregnant women, according to the health office and the NDR report.

“Particularly children and people with chronic illnesses should be careful and refrain from swimming when the water is noticeably unusual.”

Dogs face heightened danger because they frequently drink from bodies of water. The city warned that dogs should never drink noticeably discoloured or cyanobacteria-contaminated water, as swallowing larger amounts can pose significant health risks to animals. Owners should keep dogs away from potentially contaminated lakes and ponds and prevent contact with blue-green algae.

Why blue-green algae thrive now

Summer weather along the Baltic coast provides ideal conditions for blue-green algae. Kristin Beck from the Leibnitz-Institut für Ostseeforschung Warnemünde explained that despite their name, the formations are not classic algae but cyanobacteria. These organisms multiply particularly well in the warm, sunny weather and nutrient-rich waters of the Baltic Sea, collecting at the water surface as visible carpets. Wind and currents can shift or disperse the accumulations at short notice, meaning affected areas can change rapidly. Individual cyanobacteria are not visible to the naked eye, but when they appear en masse they become visible as streaks or discoloration in the water. The city noted that depending on wind direction and currents, accumulations can form in swimming areas or dissolve again, making the situation highly variable.