In the Max-Eyth-See in Stuttgart, blue-green algae have been found in high concentrations.
Skin contact with the water should be avoided at all costs and dogs should be kept away from the water, the city announced on Friday.
The blue-green algae could cause nausea, vomiting, diarrhea and skin rashes in people.
At the end of September, blue-green algae were also detected in higher concentrations in Bärensee, Neuer See and Pfaffensee in Stuttgart.
Stuttgart - According to the information, the bacteria are generally found in bodies of water, but their concentration often increases sharply in warm weather.
dpa