While Swiss lakes are usually known for their cleanliness and beauty, there are occasions when crystal blue water can transform into a sea of red. That was the case for the lake near Zug, which turned a shade of blood red around Christmas. Here’s why the lake has turned red and why the police have issued a warning.

Lake Zug turns burgundy-red during Christmas

During the Christmas period, according to Blick, residents of the Swiss city were shocked to find that Lake Zug (Zugersee) had turned a bloody shade of red. The water around Landsgemeindeplatz and Badi Hünenberg was covered with a thick layer of burgundy-red sludge.

Quickly, locals took to the internet to report the horrific red colour, with many worrying that the pigment was the result of an environmental disaster or industrial accident. In response, authorities said that the red colouring was not man-made or pollution-related, but the result of a natural phenomenon.

Swiss police warn dog owners about algae in lakes

According to the emergency services, the red in the lake was caused by a bloom of burgundy algae, also known as blood algae. Experts told Blick that the planktothrix that makes up the algal cloud adopts different colours based on the depth of the water and their habitat, which in this case made the bloom a vibrant shade of red.

While the algae poses little harm to humans, police in Canton Zug warned against taking animals like dogs into the water. Much like blue-green algae in Switzerland, drinking water with red algae in it can be dangerous for pets and humans. This was demonstrated at an incident during the summer on Lake Greifensee, Canton Zurich when two dogs died after they drank water with algae in it.