Different cyanobacteria in the microscope image. The ability to bind nitrogen gives blue-green algae species Aphanizomenon, Dolichospermum and Nodularia a competitive advantage compared to other phytoplankton species. Photo: Sirpa Lehtinen / Finnish Environment Institute


The Finnish Environment Institute has issued a warning about the high risk of extensive blue-green algae blooms in the Baltic Sea regions surrounding Finland. The areas of greatest concern include the Gulf of Finland, the Archipelago Sea, and the southern and central parts of the Bothnian Sea.

Despite efforts to mitigate algae blooms, the nutrient levels in the seawater remain largely unchanged, continuing to support the growth of blue-green algae.

This summer, the risk remains significant due to the nutrient situation combined with seasonal weather conditions.

Algae bloom risk 2024. The white area has not been evaluated because no winter nutrient data are available for the area. Finnish Environment Institute

The blooms are influenced by several factors including nutrient availability, temperature, and wind conditions. The risk in the Bothnian Bay remains low compared to other regions.

The Baltic Sea's main basin contains high levels of phosphorus, which, during winter, mixes with surface layers due to water circulation. This nutrient-rich water, coupled with increased light in spring, triggers the growth of phytoplankton algae, known as the spring bloom. Although visually impactful, this bloom does not pose a health risk and differs from the more problematic blue-green algae blooms.

As spring progresses, the nitrogen levels deplete, leaving excess phosphorus that blue-green algae can utilize. These algae, such as Aphanizomenon, Dolichospermum, and Nodularia, thrive due to their ability to use nitrogen from the air, giving them a competitive advantage during summer.

Warm, sunny, and calm weather further accelerates the growth of blue-green algae, forming large rafts visible from satellites. These blooms consist mainly of old algae cells and can release toxins that irritate the skin and eyes, making them hazardous for human contact.

The risk of blue-green algae blooms in the Bothnian Sea has significantly increased over the past decade, while other regions have seen little change. The prevalence of blooms is closely tied to summer weather conditions, with local variations occurring more rapidly along the coast and in the archipelago compared to the open sea.

The Finnish Environment Institute's forecast relies on data from extensive monitoring cruises, satellite observations, weather data, and automated sampling from merchant ships. AI-based modeling methods enhance the accuracy of these predictions.

Starting from Midsummer week, the Finnish Environment Institute will provide weekly updates on the blue-green algae situation, from June 20 to August 8, 2024. These reports will be published every Thursday at 13:00.

Given the potential health risks, including skin and eye irritation from blue-green algae toxins, the public is advised to exercise caution. Local municipalities will monitor and report on the algae conditions of bathing waters throughout the summer.

By staying informed and cautious, residents and visitors can enjoy the summer safely while mitigating the risks posed by blue-green algae blooms.