Outbreaks of the algae, which by this time last year had turned much of the Baltic into a green, soupy mush, have been noticed in the water north of Gotland.
So far this year the algae outbreaks have not been as bad as last year’s, but the cyano bacteria that cause the algae are now starting to multiply thanks to the warm water and the light winds.
But scientists say that the algae is unlikely to get as bad this year as last year. The explosive spread of the cyano bacteria was caused by high levels of phosphorous in the water.
The Helsinki Commission, which monitors the Baltic Sea environment, says levels of phosphorous in the water are lower this year than last. Phosphorous is introduced into the water from fertilizers used in agriculture and from other sources such as washing powders.
The highest risk of algae outbreaks is in southern parts of the Baltic. Bathing in water where algae is blooming can be dangerous, causing stomach upsets, vomiting and nausea.
Algae-infested water can be fatal for dogs and other animals. It is impossible to differentiate between the dangerous blue-green algae and other common harmless substances except in lab tests, so experts are warning bathers to avoid swimming in any water that looks suspect.
Scientists believe that the severity of the 2005 outbreak was partly down to the effect of fierce storms during the preceding winter, which they say stirred up phosphates lying on the sea bed.
Last year algal bloom was the worst in the history of the Baltic.