Number of lakes declared unsafe for swimming due to cyanobacteria

The current hot and dry weather is causing many problems - from increased fire hazard to crops withering in the fields. Those who can use the opportunity to cool down in the country's numerous lakes. But the quality of the water there can also be gravely affected by the heat-wave, which increases the growth of micro-organisms known as cyanobacteria. They produce poisons which can be dangerous for people and therefore a number of lakes have been closed to visitors.

Photo: Štěpánka Budková

Ten locations around the country, including Hostivar lake in Prague and the Brno Dam, have been declared unsafe for swimming by the health authorities as the concentration of cyanobacteria, also known as "blue-green algae" or "pond scum", has reached critical levels.

Petr Pumann from the State Health Institute is an expert on the quality of water.

"The biggest risk occurs when the so-called surface water bloom develops. The bacteria have special cells filled with gas and that's why they float. If water bloom is not present, only sensitive people need to be cautious."

Cyanobacteria thrive in shallow, warm, slow-moving or still water. Besides being unsightly, they produce certain toxins which can cause headaches, fever, diarrhoea, nausea and vomiting. In some people allergic reactions may occur, such as itchy and irritated eyes and skin.

"Cyanobacteria produce a wide spectrum of substances affecting human health, the most dangerous ones being so-called cyanotoxins which can affect the liver or the nervous system, and also allergens."

Photo: Barbora Kmentová

Petr Pumann says that the growth of cyanobacteria is greatly encouraged by increased levels of phosphorus coming from sewage waters and unless the phosphorus is removed, any measures to get rid of the organisms work short-term. He recommends that people should not swim in water which looks like pea soup. If you are not sure, you can perform a simple test: leave the water in a plastic bottle for half an hour and see if the scum rises to the surface. If so, then the water most likely contains cyanobacteria.

In the summer months, all swimming areas in the Czech Republic are being monitored by the health authorities who take samples to be analysed in a laboratory before a body of water can be declared safe.