“In the last days we’ve had nice calm weather,” Helena Höglander, a marine biologist at the Baltic information centre, told AFP.
“Due to this, the algae have floated up to the surface.”
The algae, technically known as cyanobacteria, “start to decompose when they are very concentrated at the shore line,” she said. “It makes their cells burst and their toxins are released in the water.”
Adults would need to drink a lot to fall ill but might suffer stomach sickness, headaches and other effects as the toxin was a liver poison.
“People should be careful when they swim. I wouldn’t recommend swimming where the water is green,” Höglander said.
Small children were more vulnerable and “they shouldn’t be bathing when you see it.”
The algae could also cause allergic reactions.
“They are surrounding the whole island of Gotland and are in most parts of the Upper Baltic sea, from the Stockholm archipelago to Gdansk in Poland,” she said.
“If it gets windy and colder, the algae will (…) probably disappear.”
Höglander said that though the bloom was very common in the summertime in the Baltic, this year it was present in greater quantities and earlier than usual.