A satellite image has revealed the scale of a vast algal bloom spreading in the Baltic Sea.
The potentially toxic bloom, covering 377,000 sq km, could pose a risk to marine life in the region, warn scientists.
They added that a lack of wind and prolonged high temperatures had triggered the largest bloom since 2005.
The affected area stretches from Finland in the north to parts of Germany and Poland in the south.
The image, captured earlier this month, was recorded by a camera on the European Space Agency’s Envisat satellite.
How algae turns the tide toxic
Researchers monitoring the spread of the blue-green algae said such blooms had spread over the Baltic Sea each summer for decades.
They added that fertilizers from surrounding agricultural land were being washed into the sea and exacerbating the problem.
This has led to a process called eutrophication, in which the additional nutrients stimulate rapid growth of phytoplankton – microscopic free-floating marine plants.
This accelerated growth also reduces the amount of oxygen available to other plant and animal species in the affected area; raising fears that it could destabilise fragile marine ecosystems.
As well threatening certain species, blue-green algae can also pose a risk to human health, and officials are advising people not to bathe in areas where the algae is visible.
However, researchers said the current bloom would quickly break up with the arrival of strong winds, as the resulting waves would disperse the algae.