Warning to bathers: Blue-green algae and cercaria thrive in hot weather

Although no Berlin swimming spots currently show signs of blue-green algae, Lageso warms that algal blooms can explode within a few hours, depending on wind and currents. Blue-green streaks on the water’s surface could be an indication of a high concentration of blue-green algae, according to the health department.

In biological terms, blue-green algae are not actually algae but cyanobacteria. They produce toxins which, in higher quantities, can cause symptoms such as vomiting, diarrhoea, rashes, eye irritation, coughing, sore throat, and headache.

Hot weather can also foster the growth of cercaria in lakes. The tiny bloodsucking larvae are more common in shore areas where aquatic plants and birds are found. The worm-like creatures can cause skin inflammation in humans. Lageso says skin irritations reported by swimmers at Tegeler See in the north of the city that may have been caused by cercaria.

Lageso  added that, for safety reasons, swimming is prohibited in canals, in the Havel River between Eiswerder Island to Pichelsdorfer Gmünd, including the bays around the Schildhorn peninsula, in harbours and locks, near bridges, piers and ferries.

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