If you cannot see your own feet when standing in knee-deep water along coasts or in lakes, you should not bathe warns the National Environmental Research Institute (NERI). This rule of thumb might be common practice for beachgoers the next few weeks as the amount of harmful algae increases.
The number of toxic blue-green algae, also known as cyanobacteria, is on the increase this summer due to the unseasonable warm weather, causing swimming bans at several lakes along the coast in Denmark.
NERI is worried how algae will develop especially along the coasts of Lolland-Falster, the Great Belt, Little Belt, Øresund and southern Kattegat.
'It is due to calm weather. The best condition for algae occurs when the water is not too salty. That is why they thrive especially well in the Baltic Sea where levels of salt are low,' Bo Rieman, head of research at NERI, told daily newspaper Jyllands-Posten Friday.
While being a nuisance for holiday beach bums, the effects on the environment are even worse. Blue-green algae consume nitrogen from the atmosphere to stay alive, which in turn increases the number of algae even further. When the algae die, they rot and fall to the sea floor using a lot of the oxygen found in water. The rotting algae also increase the amount of phosphorus released from the seafloor.
Although the situation is already unpleasant, we have only seen the tip of the iceberg, Rieman warned. If the winds turn to the east, water quality will worsen because algae from the Baltic Sea would set course for Denmark.
Denmark has invested a double-digit amount of billions to bring down the discharge of phosphorous and nitrogen from farming, industry and private households since the end of the 1980s.