This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: For decades, scientists have regarded phosphorus as the main nutrient limiting biological production in the Gulf of Bothnia. Now, a new study led by researchers at Umeå University shows that a fundamental change is underway.
The Bothnian Sea has shifted to nitrogen limitation, while the Bothnian Bay is rapidly moving in the same direction. The balance between nitrogen and phosphorus affects which organisms thrive in the sea and how the ecosystem functions. When waters become increasingly nitrogen-limited, the conditions can favor nitrogen-fixing cyanobacteria, some of which form large blooms during summer.
The findings are based on 30 years of Swedish and Finnish monitoring data and have been published in Scientific Reports. The study provides the first comprehensive assessment of how nutrient balances have changed across the Gulf of Bothnia over time. Historically, the northern Baltic Sea has been considered largely phosphorus-limited.
However, the researchers found that phosphorus concentrations have increased substantially during the past three decades, while nitrogen levels have remained stable or declined. This has altered the balance between the two nutrients and changed which nutrient limits biological production in the sea. "We can see a clear shift in the Bothnian Sea.
The nutrient balance that characterized the area a few decades ago no longer exists. This is important because the limiting nutrient influences how the entire ecosystem functions," says Siv Huseby, a researcher at Umeå Marine Sciences Centre and lead author of the study. The researchers found that the Bothnian Sea transitioned from phosphorus limitation to nitrogen limitation shortly after the turn of the millennium.
The Bothnian Bay remains phosphorus-limited, but the same trend is visible there, suggesting that similar changes could occur in the future. Increasing phosphorus concentrations may also contribute to greater primary production and create conditions that favor nitrogen-fixing cyanobacteria. Such blooms are already increasing in parts of the northern Baltic Sea and are often associated with eutrophication problems, including murkier water, oxygen-poor seabeds and disruptions to marine ecosystems that can affect everything from small organisms to fish populations.
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