A little reference: blue-green algae (cyanobacteria) are photo-synthesizing bacteria that appear as soon as the water temperature in the pond becomes favorable for reproduction. These algae are very aggressive, multiplying extremely quickly, flooding the entire pond.
The danger of cyanobacteria
About half of them release toxins that can cause poisoning in pets and humans. Different types of cyanobacterial toxins affect the liver (hepatotoxins), the nervous system (neurotoxins), the skin (dermatotoxins), and have carcinogenic and mutagenic properties.
Contact with green water may cause dermatitis on the skin, and if swallowed, nausea, vomiting, diarrhea. Some may notice an external effect - upon contact with such water, the hair acquires a greenish tint, which is especially noticeable in blondes.
