Due to the mild autumn/winter, there is currently an increased occurrence of so-called Burgundy blood algae. Blue-green algae are actually cyanobacteria, not algae. High nutrient concentrations and plenty of sunlight are perfect conditions for blue-green algae. This leads to explosive growth of blue-green algae in lakes and water bodies. They can be yellow, green, brown, or even red in color. Here are the recommendations for dealing with blue-green algae from the Health Department of the Canton of Zurich:

Blue-green algae can be hazardous to health when present in high concentrations!

• Do not swim if the density of algae in knee-deep water is so high that you cannot see your feet.
• Avoid areas with visibly colored water surfaces, streaks, and clumps.
• People with sensitive skin, including young children, should only swim in clear water.
• Avoid swallowing water.
• Always shower thoroughly and dry off after swimming.
• Limit playing and training with dogs to suitable shoreline areas with clear water.
• Do not let dogs drink colored water and wash their fur after contact, as bacteria can be ingested through licking.

The fact sheet can be found here: