Alkalinity affects the potential of seawater to take up carbon dioxide from the atmosphere. The more alkalinity produced in the sea or delivered by river runoff, the more carbon dioxide can be taken up by the sea.

Important sink for atmospheric carbon dioxide

- We found that there are large sources within the Baltic Sea that generate about half of the total alkalinity, says researcher Erik Gustafsson at the Baltic Nest Institute, Stockholm University Baltic Sea Centre. We believe these sources are mainly denitrification and sulfate reduction by bacteria. That is, bacteria generate alkalinity as a by-product in their energy metabolism that make the sea act as a sink for atmospheric carbon dioxide, he further explains.

Researchers at Baltic Nest Institute at the Baltic Sea Centre, Department of Applied Environmental Science and Department of Geological Sciences at Stockholm University used monitoring data from 82 major rivers entering the Baltic Sea during a five-year period to determine the alkalinity of riverine inputs to the sea. The river loads were then used in the BALTSEM model developed by the Baltic Nest Institute to estimate total alkalinity of the Baltic Sea.

What is alkalinity?

- Alkalinity can be seen as the buffering capacity of the sea, explains Erik Gustafsson. According to the definition, alkalinity is a measure of how much acid a solution, or in this case, the Baltic Sea water, can neutralise without changing its pH and becoming more acidic. Ocean acidification is an environmental concern because it harms marine organisms by affecting their survival, growth and ability to form shells.

Bacteria, algae, and plankton affect alkalinity

A number of biogeochemical processes affect the alkalinity of seawater. Algal and phytoplankton growth can either increase or decrease alkalinity depending on what nitrogen sources that are used for growth; mineralization under oxygenated conditions decreases alkalinity; bacterial use of sulfate reduction for energy production during oxygen-deficient conditions and bacterial conversion of nitrate to gaseous nitrogen (denitrification) increases alkalinity.

- Alkalinity is produced in all marine environments, but the quantity is difficult to determine. As a result, estimates of alkalinity generated on a global scale vary widely. This is why we choose to study alkalinity sources and sinks in the semi-enclosed Baltic Sea, says Erik Gustafsson. Our results illustrate how internal alkalinity sources in coastal and estuarine systems generate a considerable carbon sink, enhancing the absorption of atmospheric carbon dioxide, Erik Gustafsson concludes.

Link to the article in Global Biogeochemical Cycles:

For further information
Erik Gustafsson, Researcher, Baltic Nest Institute, Baltic Sea Centre, Stockholm University, Telephone +46 (0)8 674 71 25, Cell +46 (0)73 461 25 34,

Christoph Humborg, Scientific supervisor of the Baltic Sea Centre, Stockholm University, Telephone +46 (0)8 674 76 68,

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Author: Marie Löf