Blue-Green algae plague the Baltic Sea – WWF experts explain how eutrophication can be controlled

Original article by Heidi Volotinen, WWF Finland

The sun is scorching, but swimming is not an option. Large amounts of harmful blue-green algae have been detected in both sea and inland water areas, posing risks to humans and animals. Due to warm weather conditions, there have been more blue-green algae observations than usual for this time in summer. The increase in blue-green algae is influenced, among other factors, by water eutrophication and climate change.

Blue-green algae, or cyanobacteria are naturally occurring, and blue-green algae blooms are part of the annual cycle of the Baltic Sea. Blue-green algae require light, warmth, and nutrients to grow. During calm weather, they form massive mats on the water’s surface. The prevalence and growth of blue-green algae in the Baltic Sea is linked to human activities like nutrient loading, and climate change. Climate change causes blue-green algae to be present in the Baltic Sea for longer periods, the blooms are also larger and more dense than before.

Apart from eutrophication, the Baltic Sea is burdened by overfishing, the increase in invasive species, oil transportation, and intensified shipping. Plastic waste ending up in the sea also poses a threat to the marine ecosystem. Plastic pollution is a problem in the Baltic Sea with Finland’s shores being considered some of the most littered in the region. WWF’s conservation expert, Iiris Kokkonen, emphasizes that each individual can help the Baltic Sea by ensuring proper waste disposal.

“Typical items found in coastal plastic waste include food and beverage packaging, as well as cigarette butts,” Kokkonen explains.

What causes eutrophication in the Baltic Sea?

The root cause of eutrophication is nutrient loading from human activities. Nutrients such as phosphorus and nitrogen have been flowing into the Baltic Sea for decades. The majority of external nutrient loading comes from agriculture and forestry but also occurs from industrial discharges, domestic wastewater, and airborne deposition from traffic. The effects of climate change, such as droughts and increased rainfall, further increase the amount of nutrients carried from land to sea.

Natural processes in the sea intensify the effects of eutrophication. To begin with, eutrophication causes an increased abundance of algae and aquatic vegetation, and the water becomes turbid (cloudy), which deteriorates oxygen conditions at the sea bottom. Internal nutrient loading occurs in these oxygen-depleted conditions. When algae and aquatic plants die, they further accumulate phosphorus on the bottom reinforcing the eutrophication process unless external nutrient loading is reduced.

Eutrophication threatens the biodiversity of water ecosystems, as many species suffer from water turbidity and benthic animals (bottom-dwelling animals) and fish suffer from oxygen depletion in the seabed.

The sensitive Baltic Sea and its species are affected by eutrophication

The Baltic Sea is a small and shallow sea, where slowly exchanging water is a mixture of freshwater from rivers and saltwater from the ocean. Only a few species thrive in the brackish water of the Baltic Sea, which is too salty for many freshwater species and too fresh for oceanic species. Many Baltic Sea species suffer from deteriorating habitats and the impacts of climate change, as warmer seawater and low salinity alter the living conditions for underwater vegetation and fish. One threatened vegetation is bladderwrack, which is one of the key species in the Baltic Sea, forming meadows on the seabed that shelters many species.

How eutrophication affects Baltic Sea fish stocks

Eutrophication and warming of the waters have both direct and indirect effects on Baltic Sea fish stocks. According to WWF’s conservation expert Matti Ovaska, direct effects include the silting of fish spawning and habitat areas, while indirect effects include changes in food webs and decreasing water salinity.

Water turbidity, the amount of plant plankton, and changes in benthic vegetation have altered the natural habitats of fish. Shoaling fish species, such as roaches and three-spined sticklebacks, benefit from eutrophication and water warming-induced changes. On the other hand, species that prefer cooler and oxygen-rich waters, like whitefish, suffer. Herring and cod suffer from the combined effects of eutrophication and warming, as their required spawning areas change due to these factors.

“A significant portion of the important spawning and habitat areas for Baltic cod has already been lost due to oxygen depletion in deep-sea areas,” Ovaska explains.

Many economically and ecologically important species have significantly declined, according to Ovaska. Cod stocks are at a record low, and the herring stocks situation is concerning. The herring stocks in the main basin of the Baltic Sea, the Gulf of Finland, and the Gulf of Bothnia have fallen below target levels.

“Human-induced changes have significant, even irreversible effects on the species living in the brackish waters of the Baltic Sea, especially those species already living on the edge of their comfort zone,” says Ovaska.

The future of fish stocks affected by eutrophication and water warming also depends on fishing regulations and the protection of aquatic biodiversity.

Eutrophication can be prevented through water management – WWF Finland has already implemented 25 water management sites in Western Uusimaa. One way to reduce eutrophication in the Baltic Sea is through water protection that reduces nutrient loading from the land, such as from agriculture and forestry. The increasing occurrence of heavy rains has contributed to more frequent flooding. In agricultural and forested areas, floods readily wash nutrients from the soil into ditches and rivers, ultimately reaching the Baltic Sea.

“Water management will be even more important in the future, as heavy winter rains increase due to climate change. When water management is improved sufficiently, nutrient loading from land to the Baltic Sea will decrease,” says Viivi Kaasonen.

The straightening of ditches and waterways to improve land drainage over the last decades has resulted in waters and their accompanying sediments and nutrients being more efficiently transported from fields, forests, and residential areas to rivers and, ultimately, the Baltic Sea. Water management aims to restore catchment areas to a more natural state, enhancing water retention capacity and slowing water flow. Flooding of fields can be prevented, for example, by constructing two-level ditches that allow water to rise onto flood terraces instead of flooding fields. Constructing wetlands is also an effective way to prevent nutrients from reaching the Baltic Sea.

Over the past five years, WWF Finland has implemented approximately 25 water management projects in Western Uusimaa. Water management efforts reduce water loads in the region’s river basins and, therefore, prevent nutrients from reaching the Baltic Sea.

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