Algae is one of Denmark's worst summer paradoxes. As soon as the sea gets warm enough to swim in, there's a risk that the crystal clear water will turn into a thick, greenish soup.

Poisonous cyanobacteria, commonly known as blue-green algae, is about to bloom in the northern Baltic Sea and may spoil the fun at the beaches of Bornholm and Zealand if the summer weather stays hot, Videnskab reports.

Øjvind Moestrup, a biology professor at the University of Copenhagen, said the hot weather and sunlight will make the blue-green algae grow and multiply. 

"Right now there is a large blossoming of blue-green algae near the Gulf of Finland down along the south coast to Poland. If it keeps developing like this, there's a major risk that we will get problems in Denmark," he told Videnskab.

Dogs get killed
Bathers are advised to stay out of the water when the blue-green algae hits the shores. While humans often react to blue-green algae by getting ill, there have been several cases in which dogs have died after drinking algae-infested waters.

However, it's still to early to predict how bad it is going to be.

"It depends on the wind, when the algae will reach Denmark. If the wind blows from the east, it will be here in a short time, but if it blows from the west, we may even avoid it," Moestrup said.