Sheltered bays are significant carbon sinks in the Baltic Sea
Coastal zones are identified as important natural carbon sinks globally, especially habitats with abundant vegetation such as seagrass meadows and mangrove areas. In brackish environments like the Baltic Sea, knowledge of carbon sequestration in coastal habitats is limited, but a new study shows that shallow and partly enclosed Baltic bays are significant sinks for carbon and nutrients.
2025-03-12
Ecosystems and habitats that help reduce carbon dioxide in both the ocean and the atmosphere are receiving increasing attention when discussing climate action and nature restoration. A new study in Nature Scientific Reports shows that the sediments of sheltered bays in the Baltic Sea can sequester and store significant amounts of carbon and nutrients.
“We see that these areas play a similar role to seagrass meadows in other marine areas,” says Sofia Wikström, marine ecologist at the Baltic Sea Centre and lead author of the study.
Narrow openings make the difference
Bays that are protected from the surrounding sea by a narrow and shallow sill have good conditions for capturing and storing a lot of organic material. In the archipelagos of the Baltic Sea, such bays are common, with some being more or less cut off from the sea. The study examined carbon and nutrient content in the sediments of nine archipelagic bays and found that levels were consistently high. The uppermost sediment layers (down to 25 cm) contained between 2.5-4.6 kg of organic carbon per square metre of the seafloor.
”These levels are comparable to the higher end of carbon levels recorded in seagrass meadow sediments in neighbouring areas,” Sofia Wikström presents.
The researchers also investigated whether the openness of the bays to the sea or the land use in the bay catchments could explain differences in the amount of stored carbon and nutrients. The most important factor was found to be the openness of the bays. Bays with a very narrow and shallow mouth had a very high proportion of organic matter in the bottom sediment.
”In these bays, water exchange is slow and the bottom sediments are not much affected by waves and currents. This allows organic matter to settle and accumulate,” explains Sofia Wikström.
Bays with a large area of open land in the catchment and bays with a lot of underwater plants also had, on average, higher proportions of carbon and nitrogen in the sediment. So, all in all, several factors seem to contribute to the ability to store carbon and nutrients.
Easily disturbed sediments
The study is part of a larger research effort to better understand the importance of the Baltic Sea's coastal areas for the ocean's uptake and release of climate gases. This knowledge can contribute both to climate measures and to planning the protection and restoration of coastal ecosystems. One important question is how carbon storage is affected when the ecosystem is disturbed in various ways.
”In this study, we have not investigated how disturbance can affect the sediment of the bays, but we see from other studies that the sediment in the most enclosed bays is easily stirred up by boat traffic, for example. That could cause a risk of stirring and releasing carbon and nutrients,” says Sofia Wikström.
“Dredging to open up the mouths of bays can also affect water circulation and lead to increased erosion of bottom sediments.”
There are also studies suggesting that eutrophication can impair the ability of coastal environments to absorb carbon dioxide and lead to increased emissions of greenhouse gases. The researchers argue that management needs to take carbon sequestration into account when planning protection or restoration measures.
”Coastal restoration is often framed as a joint measure for climate mitigation and biodiversity conservation. This has led to a large focus on restoration of seagrass meadows, but this study shows that also other coastal habitats need to be considered when targeting restoration measures in the Baltic Sea.” Sofia Wikström concludes.
Text: Isabell Stenson
Read the scientific paper: Influence of landscape characteristics and submerged aquatic vegetation on sediment carbon and nitrogen storage in shallow brackish water habitats
Learn more about this study's leading projects Thriving bays and CLIM-SCAPE and the collaborating partner CoastClim.
Last updated: March 12, 2025
Source: Baltic Sea Centre