How the Baltic Sea escaped disaster
The Baltic Sea is highly eutrophic, with anoxic seabeds and large algal blooms. But if the countries surrounding the sea had not reduced nutrient discharges, the situation would have been much worse. “Thanks to the measures taken, the Baltic Sea has avoided an ecological disaster”, says researcher Eva Ehrnstén.
It has been several decades since the eutrophication of the Baltic Sea was recognised as a major environmental problem and the countries in the basin agreed to reduce nutrient inputs to the sea. Improved wastewater treatment, reduced air emissions and agricultural measures have significantly reduced nutrient inputs to the sea, with nitrogen inputs to the Baltic Sea as a whole reduced by about a third and phosphorus inputs by more than half since the 1980s.
In some coastal areas the environment has improved, but in the Baltic Proper the situation is worse than ever, with large algal blooms and anoxic bottoms. This is mainly due to internal processes in the sea and the fact that it takes a long time for the water in the Baltic to be replaced.
"It is very frustrating for many people that the state of the sea has not improved, even though a lot has been done to reduce emissions," says Eva Ehrnstén, a researcher at the Baltic Sea Centre and lead author of a new modelling study. "That's why we wanted to find out what would have happened if we hadn't taken all these measures."
The researchers used modelling to investigate what the environmental state of the central parts, the Baltic Proper, would have been like if no action had been taken and the nutrient inputs had remained at 1980s levels. The study showed that in such a scenario, nitrogen concentrations in the Baltic Proper would have been twice as high as today, and phosphorus levels would have been almost 60 per cent higher than today.
"We would also have had significantly more algal blooms than today, and the completely oxygen-free seabed would have been more than 80 per cent larger than it is today," says Eva Ehrnstén.
Coastal effects
While the researchers' modelling only covers the open sea, the high nutrient inputs would have also affected the coastal areas of the Baltic Sea. The environmental condition would have been particularly bad in areas heavily influenced by land-based discharges, such as a failing sewage treatment plant or extensive agriculture. In the 1980s, nutrient-rich water from the land was mixed with nutrient-poor water from the open sea. If the discharges had continued for several more decades, and nutrient levels in the open sea had increased as modelled, the situation in these areas would have deteriorated further.
"We already have a situation where people do not want to go swimming in the sea on certain days in late summer because of algal blooms", says Eva Ehrnstén. "Without measures to reduce emissions, there might have been twice as many such days."
Continued high nutrient inputs, resulting in even more algal blooms and even greater oxygen depletion, would probably also have had a major impact on the Baltic Sea flora and fauna.
"It is quite likely that we would have lost some species and habitats that are still in poor condition today, such as eelgrass beds and some fish populations. Some coastal areas would probably have been completely dead”, says Eva Ehrnstén.
“The modelling shows, for example, that cod would have had a much harder time reproducing in the Bornholm Basin”, adds Bo Gustafsson, co-author of the study.
Towards a brighter future
So what does the future hold for the Baltic Sea? The new study could shed some light on this aspect as well, as the researchers also modelled how the environmental state of the sea will evolve if nutrient inputs remain at current levels.
According to the modelling, nutrient levels in the Baltic Proper have peaked and will decline in the coming decades. By 2050, algal blooms are expected to decrease significantly and oxygen levels are expected to increase.
"The future looks brighter for the Baltic Sea, but to meet the targets agreed by the countries, nutrient inputs must be further reduced", concludes Eva Ehrnstén.
The researchers emphasise that new study shows the importance of responding quickly to environmental warning signals.
"The slow response of the Baltic Sea to improvements is something we cannot influence, but because the countries around the Baltic Sea started to reduce emissions in time, an ecological disaster was avoided", says Eva Ehrnstén.
"Hopefully, people in other areas where nutrient inputs are still increasing will learn from this and take action as soon as possible", says Bo Gustafsson. "The longer we wait, the worse the situation will get and the more difficult and expensive it will be to combat eutrophication."
About the study
The study ‘Disaster avoided: current state of the Baltic Sea without human intervention to reduce nutrient loads’ was conducted by Eva Ehrnstén, Christoph Humborg, Erik Gustafsson and Bo Gustafsson at the Stockholm University Baltic Sea Centre.
The researchers used the Baltic Sea Long-Term Large-Scale Eutrophication Model (BALTSEM) to simulate what the state of the Baltic Sea would have been if nutrient inputs had continued at 1980s levels (a so-called ‘no reduction’ scenario). They also simulated what the Baltic Sea might look like in 2050 if nutrient inputs continued at current levels.
Dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) concentrations were 104 and 58 per cent higher, respectively, in the ‘no reduction’ scenario compared to the situation today. Algal blooms are represented in the model as levels of chlorophyll a and primary production (grams of carbon per square metre), which would have been more than 60 per cent higher than today if nothing had been done. The oxygen-free (anoxic) bottoms would be 82 per cent larger than today.
If the inputs remain at current levels, significant reductions in anoxic bottoms and algal blooms are expected due to reduced nutrient levels, especially of phosphorus.
Text: Lisa Bergqvist
Last updated: November 5, 2024
Source: Östersjöcentrum