Over a hundred years ago, the Baltic Sea could be classified as "not affected" by eutrophication. Eutrophication largely begun in the 1950s, when the land-based load of nutrients increased dramatically. A clear peak was reached in the early 1980s. Since then, the external nutrient load has gradually been reduced, largely due to modernized wastewater treatment and measures in the agriculture in the countries around the Baltic Sea.

Published: 2015-11-10

Eutrophication decreases according to new study

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

For 35 years ago bathing was prohibited 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. Although the Baltic Sea is still a polluted sea, the condition has been inproved by many years of international cooperation on measures to reduce nutrient inputs from land, the new study shows.

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

Development over 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 various parts of the Baltic Sea during the last century, from 1901 to 2012.

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

The most clear improvements are found in the Kattegat. But the same positive trend is also observed in the Southern Baltic Proper.

Larger improvements in surface layers than deterioration at the bottom

The eutrophication in the Baltic Sea is often discussed in much gloomier terms, not least in the media. The prevalence of anoxic, so-called dead zones, are at record levels. And no one can have missed the summer's abundant cyanobacterial blooms.

So is it correct that the eutrophication has actually decreased in the Baltic Sea?

- Yes! According to available data, Jesper Andersen says. In a somewhat simplified matter, one can say that the indicators in the pelagic parts of the ocean have been greatly improved, while the situation closer to the seabed has deteriorated in some areas. But overall, the improvements we observe are greater than the deterioration.

Important to maintain environmental monitoring

The study also evaluated the environmental monitoring and access to data, which is the basis for reliable assessments. National environmental monitoring program were introduced during the 1960s. 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 may have serious implications for the scientists ability to document the long-term trends and understand future large-scale changes in the Baltic Sea ecology.

- Billion Swedish crowns are spent on reducing nutrient loads and thereby try to reduce eutrophication. It would be strange if we at the same time also did not focus on documenting the real impact of these investments.

Link to the study:

Long-term temporal and spatial trends in the eutrophication status of the Baltic Sea

Facts: How the study was performed

The scientists have collected large amounts of historical environmental data from national monitoring. An important source was the database DAS at Baltic Nest Institute at Stockholm University Baltic Centre. Based on a set of agreed indicators and by using the indicator-based assessment tool HEAT 3.0, the changing eutrophication status was mapped and classified over time in different parts of the Baltic Sea.


  • Denmark: Jesper Andersen, NIVA Denmark, +45 20 31 32 21
  • Finland: Alf Norkko, Helsinki University, + 358 50 568 67 66
  • Sweden: Bo Gustafsson, Stockholm University, + 46 8 674 75 93
  • International: Daniel J. Conley, Lund University, +46 70 749 43 41
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Web editor: Marmar Nekoro

Updated: 2015-11-12
Baltic Nest Institute Sweden
Baltic Sea Centre, Stockholm University
SE-106 91 Stockholm, Sweden, +46-8-16 37 18
Baltic Nest Institute Denmark
Aarhus University, Fredriksborgsvej 399
DK-4000 Roskilde, Denmark, +45 4630 1200
Baltic Nest Institute Finland
Finnish Environment Institute, P.O. Box 140
FI-00251 Helsinki, Finland, + 358 20 610 123