Prague hygiene station urges PET bottle test to detect cyanobacteria

Helena Mikulášová of the Prague hygiene station explained a simple method for swimmers to check water for cyanobacteria. By filling a clear PET bottle with water from an unmonitored bathing spot and leaving it in direct sunlight for about 20 minutes, a green ring will appear at the bottle’s neck if cyanobacteria are present; cloudy water throughout the bottle indicates algae instead.

The latest measurement at the Džbán reservoir confirmed a health‑hazardous cyanobacterial bloom, so a bathing ban remains in force. Similar monitoring is carried out at nine locations in the capital, including the Vltava River at Braník and several ponds. Authorities warned that cyanobacteria can cause skin rashes, eye irritation and gastrointestinal issues, especially for children, pregnant women, the elderly and people with weakened immune systems. The public can also view water‑quality data for monitored sites on the hygiene station’s website.