Bornholm particularly affected: Severe blue-green algae bloom affects large parts of the Baltic Sea

​In places, the surface of the water between Denmark and Rügen resembles a greenish-brownish broth. Massive mats of blue-green algae are currently spreading across the southern and western Baltic Sea.

According to reports, the ports of Hasle and Hammerhavn on the Danish island of Bornholm have been particularly hard hit. The Danish regional broadcaster TV2 reports that a foul-smelling mass is spreading across the entire harbour basin. Consistency and density would vary from day to day, depending on the wind, weather and water temperature.

Even off the coast of the island, between Bornholm and Rügen, sailors are encountering the sludge. In a video sent to YACHT by a reader, a boat can be seen battling its way through a thick, greenish-brown mass.

​Institute confirms spread

​The blooms are not confined to the region around Bornholm. They have also spread to the German Baltic Sea coast, from Schleswig-Holstein to Mecklenburg-Western Pomerania. ​The Swedish Meteorological and Hydrological Institute (SMHI) confirms this unusual spread: According to the institute, surface blooms of cyanobacteria have been observed from the south-western Baltic Sea as far as the northern Baltic Sea – as well as in the Gulf of Bothnia, the Sound, the Belts and the Archipelago Sea. In the southern Baltic Sea, the blooms are drifting predominantly westwards.

​You can find a map from the SMHI and further information on the current situation here.

​Blue-green algae are not algae

​Although the term ‘blue-green algae’ is often used colloquially, these organisms are actually cyanobacteria. They owe their colloquial name to the blue-green pigment phycocyanin, which is found within their cells.

Some types of cyanobacteria produce toxins which dissolve in the water and can be dangerous to humans and animals. Typical symptoms following contact include skin irritation, conjunctivitis and earache. If even the smallest amounts of contaminated water are swallowed whilst swimming, this can lead to nausea, diarrhoea and vomiting.

​And they bring with them a secondary problem: vibrio bacteria. These feed on dying blue-green algae, which is why increased concentrations of vibrio bacteria in the water are to be expected following a severe algal bloom.

• ​Deadly danger: Scientists develop an early-warning system for vibrio bacteria
• ​The Baltic Sea: How dangerous are vibrio bacteria?

​What sailors should bear in mind

​For sailors, these algal blooms also have consequences if large quantities of blue-green algae enter the engine’s seawater cooling circuit. There, they can block filters and interrupt the flow of cooling water – in the worst-case scenario, this could lead to the engine overheating.

The extent of the risk depends on where the water is being drawn in along the hull and how thick the algae mats actually are. As a rule, these are only a few centimetres thick, which is barely deep enough to reach below the water’s surface. Nevertheless, it is advisable to check the seawater filter regularly when passing through dense patches of algae and to clean it if necessary.

Falling temperatures help

​Stable, calm weather conditions and high water temperatures keep the mats on the surface. If the wind changes direction or temperatures drop, the mats may be driven out of the harbours or sink. It is not possible to predict this with any certainty.