Higher cyanobacterial blooms risk in sea forecast in summer

The risk of significant and widespread cyanobacterial blooms this summer in the eastern Gulf of Finland and the main basin of the Baltic Sea is slightly higher than in summer 2020, said the Finnish Environment Institute (SYKE) in a press release on Thursday.

In average summer weather conditions, the risk of algae is considerable in the northern part of the main basin of the Baltic Sea and in the Archipelago Sea.

The risk of algae is moderate in the eastern part of the Gulf of Finland and almost the entire Bothnian Sea, and low in the Bay of Bothnia.

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 has not changed in recent years.

Blooms in the Bothnian Sea have increased in recent years. Particularly favourable weather conditions, meaning warm and sunny weather, can cause significant cyanobacterial blooms; correspondingly, unfavourable weather causes considerably less 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. Nutrient levels are mapped before algae start to take advantage of them in the spring. SYKE and the Swedish Meteorological and Hydrological Institute (SMHI) measure nutrient levels very comprehensively in the entire Baltic sea area during winter monitoring cruises. SYKE, together with the Finnish Meteorological Institute, assesses the risk of algae blooms using the Baltic Sea ecosystem model. In addition to the nutrient levels in the winter, the assessment takes into account the development of nutrient concentrations during the spring as spring water movements may change the overall picture.

”The purpose of the assessment at the beginning of June is to remind particularly people moving in the sea and coastal areas of the start of the cyanobacterial blooming season. The regional risk assessment has remained fairly stable from year to year. An up-to-date picture of the algae situation can be obtained from SYKE's weekly algae reviews and by observing the situation yourself,” said Harri Kuosa, leading researcher at the Finnish Environment Institute.

”In particular, we have found that the cyanobacterial blooms in the Bothnian Sea have become more common. Phosphorus levels in the Bothnian Sea have increased considerably. Therefore, it is also important to make observations of cyanobacterial blooms in the Gulf of Bothnia," Kuosa added.

Early summer weather will also have a big influence on the onset of cyanobacterial blooms.

Generally, the amount of cyanobacteria usually increases after midsummer at the earliest, and blooming peaks in the Finnish sea areas are usually in July and early August.

Algae rafts can still be seen during the autumn, but the wide blooms are then usually over. In the autumn, significant amounts of cyanobacterial blooms have occasionally been observed, especially in shallow bays. However, autumn cyanobacterial blooms that dye the shorelines occur usually only locally.

Winter rainfall and mild weather cause nutrient load from river waters to coastal areas. All nutrient loads are linked to eutrophication in the Baltic Sea.

However, the impact of one winter on the summer cyanobacterial blooms is difficult to assess as phytoplankton consumes nutrients already in the spring. In spring, algae production is based on species other than cyanobacteria. One characteristic of spring algae species is their rapid descent to the seabed, where the decomposition of plankton mass increases oxygen consumption and thus phosphorus release. Thus, the nutrient load during winter also indirectly increases cyanobacterial blooms and maintains the general poor ecological status in Finnish sea areas.