The Bothnian Bay has been spared from the eutrophication that has long affected the southern Baltic Sea. But now there are signs that something is changing in the northern parts of our inland sea. Is there a risk that the Bothnian Bay will also be affected by eutrophication – and if so, what are the causes? This will be investigated in a research project at Umeå University, funded by BalticWaters.
Eutrophication is one of the Baltic Sea’s biggest environmental problems, resulting in algal blooms, oxygen depletion and ecosystem changes. But in the Bothnian Bay, the northernmost part of the Baltic Sea, conditions have been different.
‘The aquatic environment here has been relatively unaffected so far, which may have led to the assumption that the Bothnian Bay is spared from human impact, but that is not the case,’ says Agneta Andersson, professor at the Department of Ecology, Environment and Geosciences at Umeå University.
Agneta has been following developments in connection with the annual environmental monitoring, where she has seen phosphorus levels rising – both in the Bothnian Bay and the Bothnian Sea. This is a worrying development that has raised new questions.
‘If the trend continues, the Bothnian Bay also risks suffering from the same eutrophication problems we are seeing further south. We don’t yet know exactly what is causing the increase,’ she says.
In a new research project funded by BalticWaters, Agneta and her colleagues will investigate how phosphorus is transported to the Bothnian Bay – and whether or when the Bothnian Bay may suffer from eutrophication.
The nutrient pathway through the Northern Quark
To understand how nutrients are transported, researchers are focusing their attention on the Northern Quark – the narrow sea passage connecting the Bothnian Sea with the Bothnian Bay. Here, water flows through two channels, one on each side of the Holmö Islands. Researchers suspect that nutrients enter the sea through these channels.
‘We know that the increase in phosphorus in the Bothnian Bay is not due to inflows from rivers. Instead, there are many indications that it comes from the south, from the Bothnian Sea, where phosphorus levels are higher,’ says Agneta.
Water samples are taken at several stations in the channels on either side of the Holmö Islands to determine the levels of phosphorus and nitrogen. At the same time, water currents are measured using flow meters to understand how nutrients move through the area. Sensors also record temperature, salinity, oxygen content and other basic parameters.
‘By combining measurements of nutrient levels with data on water currents, we can get a clear picture of how much phosphorus is actually transported through the Northern Quark to the Bothnian Bay,’ says Agneta.
Translation: Bottenviken = The Bothnian Bay; Norra Kvarken = The Northern Quark; Bottenhavet = The Bothnian Sea.
Water sampling aboard the Umeå Marine Research Centre’s vessel. Photo: Marléne Johansson
Future risks of eutrophication are modelled
Mapping phosphorus inflows to the Bothnian Bay is just the beginning. In the second part of the project, measurement data will be transformed into understanding – and into possible future scenarios. Using oceanographic models, researchers will simulate how phosphorus moves through the Northern Quark and further into the Bothnian Bay, both between water layers and along the seabed.
‘The models will help us understand not only what the situation is like right now, but also what might happen if conditions change,’ Agneta explains.
By combining recent field data with historical measurement series, researchers can see both patterns and trends – and test different future scenarios based on how climate and environmental conditions are expected to change in the future. What happens if the water temperature rises? If the salinity decreases? Or if phosphorus levels in the Bothnian Sea continue to increase?
‘We want to understand what is behind the changes and be able to assess whether, and if so when, the Bothnian Bay is at risk of eutrophication. This knowledge is crucial in order to be able to influence developments in the right direction,’ says Agneta.
Basis for future decisions on wastewater treatment
With better knowledge of nutrient flows through the Northern Quark and how the future may develop, we will not only gain a clearer picture of the current state of the Bothnian Bay, but also a scientific basis for making the right decisions at the right time.
‘It is crucial that we understand nutrient inflows and identify risks in advance so that we can act before eutrophication becomes a reality,’ says Agneta.
The project’s results have a direct bearing on, among other things, the EU’s new Waste Water Directive, which came into force on 1 January 2025. The directive sets stricter requirements for reducing nutrient emissions to water, but does not currently cover the Gulf of Bothnia because the area is not considered nitrogen-limited – i.e. it is not nitrogen deficiency that controls how much phytoplankton grows, but phosphorus.
‘If the results show that phosphorus is being released through the Northern Quark, the Bothnian Bay will become nitrogen-sensitive in the foreseeable future. This knowledge would have a direct impact on the implementation of the Wastewater Directive and mean stricter requirements for nitrogen treatment in wastewater,’ explains Agneta.
About the project
The project When will eutrophication problems affect the Bothnian Bay? is being carried out by Umeå University. Through BalticWater’s Program to fund research projects and pre-studies, the project has been granted funding of SEK 912.480. You can read more about the other three funded projects in the article New projects for a living sea.
