There are recurring reports of dead zones and hypoxia in the Baltic Sea. Eutrophication – when too many nutrients are released into the sea – is one cause of this. Emissions from agriculture, industry, waste water and forestry increase the injection of nutrients from the areas of land surrounding the Baltic Sea. This increases the growth of algae. When algae sinks to the seabed and is broken down, large amounts of oxygen are used. This can lead to hypoxia and what are known as “dead zones”.
Nutrients stay in the coastal zone
Now research has shown that the coastal zone can act as a filter for nutrients that are brought in from the land. In a study of the Stockholm archipelago, researchers have shown that approximately 70 percent of nutrients remain in the area, rather than continue out to the Baltic Sea.
“In shallow areas, a larger portion of the organic material reaches the seabed. The filter processes that are there remove the nutrients from circulation,” explains Elin Almroth-Rosell, researcher at SMHI with special focus on the coastal zone.
In the shallow coastal area, the nutrients become buried in the sediment of the seabed and a process then turns nitrogen into nitrogen gas which is not primarily used for the growth of algae. In deeper sea areas, more of the organic material can be broken down and release nutrients into the water before it sinks to the bottom.
Long-term changes
When the supply of nutrients from the land changes, this affects the level of nutrients available in the sea area. A model study has shown that a temporary reduction in the supply of nutrients can result in a deterioration of the archipelago’s filtering effects, but this will increase again. Calculations show that with today's supply of nutrients, in the long-term the Stockholm archipelago will be able to take care of the phosphorus that arrives from the land and simultaneously filter the phosphorus that is brought in with water from the open seas.
“With phosphorous it takes around 40 years for the seabed in the Stockholm archipelago to reach a balance with the supply of nutrients from the land. This is why it will take time before we see the effects, so the reduction in nutrients must be looked at from a long-term perspective,” says Elin Almroth-Rosell.
The studies of the nutrients and the importance of the coastal zone were conducted by researchers at SMHI within the framework for the BONUS project COCOA. This is an international project that studies the coastal zone's significance for the transportation of nutrients between land and sea.