NEWS The Gulf of Bothnia seems to be heading towards eutrophication. It is not only the increasing blooms of cyanobacteria that indicate this. An analysis of many years of monitoring data shows that phosphorus concentrations are increasing and the ratio of nitrogen to phosphorus is decreasing. A research group from the Umeå Marine Sciences Centre at Umeå University has analysed nutrient dynamics and how human activity has affected the situation.
The Gulf of Bothnia, with its two sea basins the Bothnian Sea and the Bothnian Bay, has long been considered to be spared from eutrophication. At present, for example, the requirements for nitrogen removal only apply to the treatment plants located south of the Gulf of Bothnia. In 2022, a revised wastewater directive was issued in which the rules were tightened and will apply to all of Sweden's coastal areas. This has raised the question of where the situation in the Gulf of Bothnia is heading, and what we really know about it. There has been criticism against that nitrogen removal should also be required in the Gulf of Bothnia, but the results from the recently published study show that it is relevant to re-evaluate the issue of nitrogen removal in the Gulf of Bothnia as well.
Water sampling from the research vessel KBV 181. Sampling is carried out all year round in the Gulf of Bothnia, as part of the national environmental monitoring.
ImageMarlene JohanssonThe issue of eutrophication is very much about the balance between the amounts of nitrogen and phosphorus. If the amounts of phosphorus increase, the system will become nitrogen-sensitive, which means that the production of, for example, phytoplankton will increase if more nitrogen is added. In a nitrogen-sensitive system, the risks for problems with eutrophication increase significantly. One effect may be that potentially toxic algal blooms increase, as the imbalance between nitrogen and phosphorus favours nitrogen-fixing cyanobacteria. The study clearly shows that nitrogen sensitivity is increasing in both the Bothnian Sea and the Bothnian Bay, although the situation has not yet reached as far in the Bothnian Bay. In the Bothnian Sea, cyanobacteria blooms have already increased.
Why are we seeing this development in the Gulf of Bothnia? The explanation that may seem obvious is that the wastewater treatment plants' emissions have caused the problems. Therefore, a review of measurement data from water both upstream and downstream treatment plants was made at a large number of premises along the coast. It was not possible to trace the cause of the change in nutrient balance to the treatment plants. Does this show that the emissions from the treatment plants have no significance?
Cyanobacteria blooms increase when marine areas become nitrogen-sensitive. The photo is taken in Ulvöhamn in the Bothnian Sea in summer 2024.
ImageKristina Viklund"Probably not," says Joakim Ahlgren, environmental analyst at Umeå Marine Research Centre and one of the researchers behind the study. "The measurement programs at the treatment plants are not designed to answer that question. In addition, there is a dilution effect from the large rivers, which further complicates the analysis."
What can be shown, however, is that some of the problems stem from the Baltic Proper. Through the passage around Åland, phosphorus-rich water flows in from the Baltic Proper, causing the balance between nitrogen and phosphorus to change in the Bothnian Sea. Water also flows northwards in the area between the Bothnian Sea and the Bothnian Bay, the northern Quark. The already known problems in the Baltic Proper are thus spreading to the entire Gulf of Bothnia.
"The question is no longer IF the Gulf of Bothnia will have problems with eutrophication, but rather WHEN. Knowledge about nutrient conditions and ecosystem responses in the Gulf of Bothnia has so far been very limited. By compiling data from both Sweden and Finland, we have gained a clearer picture of how the situation has been, is and how it can be expected to develop," says Joakim.
The report contains a lot of concrete advice for marine management and arouses great interest among the authorities responsible for the marine environment in the Gulf of Bothnia.
"The report will provide input throughout the chain from mapping the conditions to planning measures linked to eutrophication. The fact that there is now nitrogen limitation in the Bothnian Sea is an important finding they have been able to make, together with the conclusion that the supply of nutrients from the Baltic Proper plays a major role in the nutrient situation in the entire Gulf of Bothnia. We will take this with us when we start the work of planning measures in the autumn. " says Anneli Sedin, environmental officer at the County Administrative Board of Västerbotten.
The researchers will now deepen their knowledge further through detailed studies of what is happening in the southern and northern Quark.
"This is where it all happens. By examining the water flows over these passages, we can get an even clearer picture of the development in these sea areas," says Agneta Andersson, project manager and professor at the Department of Ecology and Environmental Sciences, Umeå University.
Report Naturvårdsverket 2025 (in Swedish, summary in English)
