Risk of cyanobacterial blooms in seas seems low this summer

The risk of significant and widespread cyanobacterial blooms this summer for the eastern Gulf of Finland and the main basin of the Baltic Sea is lower than that in summer 2019, said the Finnish Environment Institute (SYKE) in a press release.

In average summer weather conditions, the risk of algae is moderate in the northern part of the main basin of the Baltic Sea, a major part of the Archipelago Sea and in the southern part of the Bothnian Sea this summer.

In the Bay of Bothnia and the eastern part of the Gulf of Finland, the risk is low.

The realisation of the risk is determined by the summer weather conditions, as the nutrient situation in the northern part of the main basin of the Baltic Sea is similar to that of 2018, which was very favourable for the birth of cyanobacterial rafts.

The risk assessment of cyanobacterial blooms in the sea areas near Finland is based on the amounts of nitrogen and phosphorus available for the algae measured during the previous winter.

SYKE and the Swedish Meteorological and Hydrological Institute (SMHI) have measured nutrient levels very comprehensively during winter monitoring trips. SYKE and the Finnish Meteorological Institute assess the risk of algal blooms using the Baltic Sea ecosystem model. In addition to winter nutrient levels, the assessment took into account the development of nutrient concentrations during spring 2020.

The winter nutrient situation provides a regional overview of nutrient availability during the following summer. However, summer weather conditions also determine the actual algae situation in the summer in the Finnish sea areas.

The cyanobacterial species that bloom in the Baltic Sea thrive in warm water and require plenty of light for their growth. The first algae rafts may appear after a couple of beautiful summer weeks, and continuous high-pressure weather, as in 2018, which can lead to very significant and long-lasting blooms.

“The risk forecast issued at the beginning of June is a reminder of potential cyanobacterial bloom areas, but more detailed information of the algal situation can be found in the weekly bulletins and with your own eyes,” said Finnish Environment Institute Group Leader Harri Kuosa.

“We have found that the cyanobacterial blooms in the Bothnian Sea have expanded further north. Elsewhere in Finland, the amount of phosphorus is stable, but in the Bothnian Sea it has doubled in the last 20 years.”

Cyanobacterial blooming usually begins after midsummer at the earliest. Blooming peaks in the Finnish sea areas are usually in July and early August. Algae rafts can still be seen during autumn, but the wide blooms are then usually over.