Risk of eutrophication problems in the northern Baltic Sea
Whilst large parts of the Baltic Sea have been severely affected by eutrophication, the Gulf of Bothnia has been relatively spared. But now something is beginning to change – 30 years of Swedish and Finnish environmental monitoring data show that the nutrient balance in the Bothnian Sea has shifted, increasing the risk of algal blooms and other effects of eutrophication. Will the problems spread even further north to the Bothnian Bay? This is what researchers at Umeå University are investigating with support from BalticWaters.
Phytoplankton require both nitrogen and phosphorus for their growth. Historically, phosphorus has been the nutrient that has limited algal growth in the northern Baltic Sea. But as the amount of available phosphorus is now increasing, the balance is shifting. This particularly favours cyanobacteria, which have the ability to extract nitrogen directly from the air and therefore have an advantage when phosphorus is abundant but nitrogen is scarce. Cyanobacterial blooms are common in the southern basins of the Baltic Sea, where, on warm and calm summer days, they can cause severe blooms. When they subsequently die and sink to the bottom, decomposition begins – a process that causes oxygen depletion.
The nutrient balance is changing in the north
In a new study, researchers have found that the nutrient balance in the Bothnian Sea has changed – the availability of phosphorus has increased, which favours cyanobacteria. It is not yet clear where the increasing amount of phosphorus is coming from – one explanatory model is that phosphorus-rich water is transported northwards between the various basins of the Baltic Sea. Without the long time series of data from the northern Baltic Sea, this change would not have been detected at an early stage. This underlines the importance of environmental monitoring and the continued allocation of funds to data collection in order to detect changes in the marine environment.
Figure 1. Samples have been taken at eleven stations in the Kvarken area on five occasions.
Will the Bothnian Bay be hit by eutrophication?
But will phosphorus levels also rise in the Bothnian Bay? There are signs that this is the case. Professor Agneta Andersson and her team at Umeå University are therefore investigating the path of phosphorus from the Bothnian Sea across the Kvarken area to the Bothnian Bay, and whether and when the Bothnian Bay will be hit by eutrophication. The investigations in the Kvarken area began in June 2025 and are scheduled to run until December 2026. To gain an understanding of nutrient transport, the research team has measured both water currents and nutrient levels across the Kvarken (Fig. 1).
Tracing the water’s path northwards
The concentration of nutrients in the water is at its highest during the winter – this provides an indication of both the quantity being transported northwards and whether the water can be classified as eutrophic. Researchers know the ratio of nitrogen to phosphorus in the Bothnian Sea – this acts as a sort of marker for the water mass and enables them to track the water’s path from the Bothnian Sea through the Kvarken area and into the Bothnian Bay.
Figure 2: Water chemistry analyses and CTD sampling have been carried out in the Kvarken area at 11 stations on 5 occasions, evenly spread throughout the year. A CTD is an instrument used to measure the physical properties of the water, such as salinity, oxygen and temperature. Photo: Marléne Johansson
Why the results are important
Analysis of the collected data is currently underway, and initial findings indicate large differences in nutrient status between some of the stations in the study. This means that the transport of nutrients over the sills in the Northern Kvarken is crucial to how the situation in the northern Baltic Sea may change in the future.
It is important to identify changes in the nutrient balance at an early stage so that measures can be implemented in good time. The management of the Bothnian Bay needs to be adapted to a changing sea – where measures take into account the relationship between the nutrients nitrogen and phosphorus.
The final report from the project is due in spring 2027 – at which point we will return with a new article on the situation in the Bothnian Bay.