Every year, usually at the start of summer, Swiss police release a statement warning swimmers and dog walkers about blue-green algae in lakes. The green-tinged blooms may seem harmless at first, but if ingested, the toxic bacteria can cause serious health issues in humans and animals.
Despite the name, blue-green algae isn’t actually a species of algae but a cyanobacterium. During the summer, nutrient-rich and sunny lakes can attract “explosive blooms” of blue-green algae, especially when the weather is hot.
There are around 2.000 different species of blue-green algae, covering all of the world's freshwater lakes and rivers. It remains very common in Swiss lakes and can cause big problems, especially for our canine friends.
When blue-green algae develop an explosive bloom, some species, including those in Switzerland, can release toxic substances into the water. At a high enough concentration (around 100.000 bacterial cells per millimetre), drinking this water can cause breathing problems, flu-like symptoms, diarrhoea and nausea.
That is why particular caution has to be paid to children and animals, especially dogs, who can consume a large amount of lake water when they jump in and can lick their bacteria-covered coats after swimming. This was the reason why two dogs died in a hospital in Zurich in May 2022, after consuming too much of the toxic substance when swimming in the Greifensee, a popular swimming place just outside the city.
Every canton in Switzerland keeps a close watch over the water quality of its lakes, and the police will alert the public through social media as to when algal blooms occur. You will also often see warning signs dotted around the lake during the summer. The bacteria typically spawn when poor weather is followed by hot, clear weather, growing rapidly over the first few days. Blue-green algae prefer water with little current, making lakes the most likely place to encounter them.
Algal blooms can be easily recognised by their greenish, oily tinge on the water’s surface. They also tend to smell musty and earthy. A good rule of thumb is that if you are knee-deep in algal water and cannot see your own feet, you should exercise caution and avoid the water.
People with sensitive skin and small children are advised not to go swimming in blue-green algae. While you can swim in algal water - provided you don't drink any - if you do come into contact with a lot of the bacteria, be sure to take a good shower and dry off.
Dogs should not drink the water and should be washed if they do come into contact with blue-green algae. If you think your dog has swallowed any lake water with blue-green algae in it, call a vet immediately.
As the climate warms, blue-green algal blooms will become increasingly common. While it shouldn’t put people off swimming in Swiss lakes, it is important to keep in mind in order to stay safe.