The loads of nitrogen and phosphorus into the Baltic Sea has decreased significantly since the peak of emissions in the 1980s, and in parts of the sea the nutrient loads are now at or below the 1950s’ level. Despite this, the measured concentrations of nitrogen and phosphorus in the water have not decreased to any great extent in the major basins.

– The inertia of the system makes the environmental state of the Baltic Sea reflect the cumulative effect of many decades of nutrient loads. Therefore, the high concentrations we see in the sea today are a result of the high nutrient loads of the past, explains Bo Gustafsson.

However, modeling done by the researchers at the Baltic Sea Center using the Baltsem model shows that the development is going in the right direction. Even with the current loads on the Baltic Sea continuing, nutrient concentrations in the sea will decrease in the future. However, in order to achieve good ecological status, further reductions in the loads are required.

– The measures that are implemented will have an effect in the sea, but it will take time, says Bo Gustafsson.

Bo Gustafsson at Baltic Nest Institute, Stockholm University Baltic Sea Centre, has modeled what the state of the Baltic Sea had been if there has been no action taken to limit eutrophication.

Significantly higher levels without reductions

In new studies, the researchers have also examined what the Baltic Sea had looked like if the comprehensive measures to reduce the nutrient load had not been implemented. The results were presented at the stakeholder conference organized by HELCOM last week. The conference is part of the preparatory work on the update of the Baltic Sea Action Plan, which will be launched next year.

The modeling shows that if the loads had continued at the level of the 1980s, winter concentrations of dissolved inorganic nitrogen (DIN) in the Baltic Sea would have been around 7 micromolar today. This is 75 percent higher than today's actual level; about 4 micromolar. Phosphorus concentrations would also have been higher without nutrient reductions, with winter concentrations of dissolved inorganic phosphorus (DIP) of about 1.1 micromolar, almost 40 percent more than today's 0.8 micromolar.

– It was surprising that nitrogen concentrations turned out so high, while the level of phosphorus concentrations where about what could be expected, says Bo Gustafsson. Further, these high nutrient concentrations would most probably significantly deteriorated the state of the coastal areas favoring opportunistic green algae and deteriorating oxygen concentrations.

Dissolved inorganic phosphorus in the Baltic Proper surface water. The black graph shows a scenario with real load and the red graph shows the no reduction scenario.

Dissolved inorganic nitrogen in the Baltic Proper surface water. The black graph shows a scenario with real load and the red graph shows the no reduction scenario.

Higher primary production and less oxygen

The high levels of nitrogen and phosphorus in the scenario would have led to a greatly increased amount of phytoplankton and the primary production in the Baltic Proper would have been about 50 percent higher than today, according to the model.

–  It is a very big difference compared to the situation today and it would also have had great effects on the environment along the coasts, says Bo Gustafsson.

The higher primary production would have led to increased sedimentation with significant deterioriation of the oxygen conditions in the sea and on the seabed. For example, today, the oxygen content of the water at 80 meters depth in the Baltic Proper is about 1 ml per liter of water, which is considered low in oxygen – hypoxic. Without reductions, the environment at the same depth would have been completely oxygen-free – anoxic – and even with significant amounts of hydrogen sulfide (shown as negative oxygen in the graph).

Oxygen level at 80 meters depth in the Baltic Proper. The black graph shows a scenario with real load and the red graph shows the no reduction scenario.

On the other hand, the spatial extent of oxygen-poor or oxygen-free bottoms in the open Baltic Sea would not have changed significantly – these areas are already in principle as large as they can be as the hypoxia reach almost to the surface layer that is supplied with oxygen from the atmosphere. The amount of cyanobacteria in the sea would have been somewhat higher than today in the scenario, but the high nitrogen concentrations favor other phytoplankton.

– But with such high primary production and so low oxygen levels, we had locked the Baltic Sea in a significantly worse situation than the one we have today, says Bo Gustafsson.

– If we had not reduced the loads in time, the situation in the Baltic Sea would have continued to deteriorate. Instead, we are heading towards reduced eutrophication in the long run.

Text: Lisa Bergqvist