Major ecological change in the Gulf of Bothnia – new study

One of the world’s lowest salinity marine waters, the Gulf of Bothnia, faces major challenges with an increased risk of algal blooms. The reason is that phosphorus levels in the shallow inland sea are rising. This means that the delicate balance between phosphorus and nitrogen is being disrupted, which in turn affects which organisms thrive in the sea. The new study, led by researchers at Umeå University, shows that a fundamental change in the marine environment is about to take place.

^{Text: Marika Griehsel}

´We are seeing a clear change in the Bothnian Sea. The nutrient balance that characterised the area a few decades ago no longer exists. This is important because the limiting nutrient affects how the entire ecosystem functions,´says Siv Huseby, a researcher at the Umeå Marine Research Centre and lead author of the study.

The research findings are based on 30 years of Swedish and Finnish environmental monitoring data and have been published in *Scientific Reports*. The study provides the first comprehensive assessment of how the nutrient balance in the Gulf of Bothnia has changed over time.

In the Baltic Proper, the ratio of phosphorus to nitrogen is higher than in the Gulf of Bothnia. It is usually said that the Baltic Proper is therefore nitrogen-limited.

Previously, the Bothnian Sea was affected by nitrogen limitation, and now it is the Gulf of Bothnia’s turn. One reason for this is that phosphorus-rich water is moving northwards from the Baltic Proper. As the water becomes more nitrogen-limited, conditions may favour nitrogen-fixing cyanobacteria, which can cause extensive algal blooms during the summer.

A marked change in the nutrient balance

Researchers have noted that phosphorus levels have increased significantly over the last three decades, whilst nitrogen levels have remained stable or declined. This has altered the balance between the two nutrients and, consequently, which nutrient limits biological production in the sea. (See fact box on biological production in the sea.)

Rising phosphorus levels may also help to create conditions that favour nitrogen-fixing cyanobacteria. Such blooms are already on the rise in parts of the northern Baltic Sea and are often associated with eutrophication problems, including cloudier water, oxygen-depleted seabeds and disruptions to marine ecosystems that can affect everything from small organisms to fish stocks.

´I was surprised by the marked change in the balance between nitrogen and phosphorus in the seawater, particularly in the Gulf of Bothnia. If this trend continues, the area could become nitrogen-limited within a few decades. Nitrogen limitation favours filamentous cyanobacteria over other phytoplankton, which can disrupt ecosystems and increase the risk of harmful algal blooms. We are already seeing more of these cyanobacteria in the Bothnian Sea, and they may become common in the Bothnian Bay too,´ says Siv Huseby.

Environmental management may need to be reviewed

The researchers believe that environmental conditions in the various parts of the Baltic Sea are closely interlinked.

´The northern parts of the Baltic Sea, such as the Bothnian Sea and the Bothnian Bay, have not been the focus of attention when models and action plans have been drawn up for this severely stressed sea.´

Perhaps this is because the population is smaller in the north, and there have been no requirements for sewage treatment plants to remove nitrogen from effluent.

The research findings have implications for environmental management. Current strategies are based on long-standing assumptions about which nutrients limit productivity in the northern Baltic Sea. According to the researchers, these assumptions may need to be reassessed as conditions change.

´To reduce eutrophication in the Gulf of Bothnia, efforts must continue across the entire Baltic Sea. A large proportion of the increase in phosphorus originates from the Baltic Proper, which makes regional cooperation within HELCOM – the joint marine environment commission of the Baltic Sea states – crucial. Reducing dead seabeds, limiting the input of nutrients and tackling climate change are all important measures for reducing extensive cyanobacterial blooms, says Siv Huseby.´

Link to article: Rapid change of limiting nutrient in the Gulf of Bothnia, northern Baltic Sea | Scientific Reports

^{Cover image: Cyanobacteria from the Gulf of Bothnia under a microscope.}

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