Decreasing eutrophication


Successful long-term measures

Despite algal blooms and dead zones - the overall eutrophication in the Baltic Sea have in fact decreased. According to a new study by Swedish, Danish and Finnish researchers, the improvement is a result of long-term measures to reduce nutrient inputs from land.

Text: Henrik Hamrén

35 years ago bathing bans were seen in many locations along the Baltic coast. The water was so heavily polluted that it was considered a health hazard.

Today the situation is different. The new study shows that although the Baltic Sea is still a heavily impacted sea, the state has improved thanks to many years of international cooperation on measures to reduce nutrient inputs from land.

- We note improvements in most open areas of the Baltic Sea, says Jesper Andersen, head of research at NIVA Denmark and one of the authors of the study.

Integrated assessment of eutrophication in the Baltic Sea 1901–2012, combining all 621 individual classifications of eutrophication status into a single assessment

During the last century, the Baltic Sea was classified as "not affected" (green) by eutrophication. During the 1950s the load from land dramatically increased. A clear peak was reached in the early 1980s. Since then, the external nutrient load has gradually been reduced.

Trends during 111 years

The study "Long-term temporal and spatial trends in the eutrophication status of the Baltic Sea", recently published in the scientific journal Biological Reviews, gives a unique overview of how eutrophication have developed in different parts of the Baltic Sea in the last century, from 1901 to 2012.

- Our study documents the very first signs of recovery in the Baltic Sea. It is gratifying to see that in recent decades, efforts to reduce nitrogen and phosphorus inputs from the countries around the Baltic Sea are now beginning to take effect.

The most obvious improvement are seen in Kattegat. But even in areas south of the Baltic appear the same positive trend.

Differences between surface and bottom waters

Baltic Sea eutrophication is most often discussed in far bleaker terms, not least in the media. The spread of oxygen-free so-called dead zones are at record levels. And no one has been able to escape the summer's abundant blooms.

So is it certain that eutrophication of the Baltic Sea has been reduced?

- Yes! From the data available we are sure, says Jesper Andersen. Simplified, one can say that the indicators in the pelagic parts of the sea has been greatly improved, while the situation closer to the seabed has deteriorated in some areas. But overall, the improvements exceed the deterioration.

Keep the environmental monitoring

The study also evaluated the environmental monitoring and access to data, which is the basis for reliable assessments. In the 1960s, the national environmental monitoring programs were introduced. But since then, and especially in recent decades, the access to environmental data has deteriorated.

According to Jesper Andersen, a further dismantling of environmental monitoring could have serious implications for scientists and the ability to document long-term trends and understand the future large-scale changes in the Baltic Sea ecology.

- Billions are spent on reducing nutrient loads in order to combat eutrophication. It would be strange if society did not also focus on documenting the real impact of these investments.