Carbon cycling in the Baltic Sea - the fate of allochthonous organic carbon and its impact on air-sea CO2 exchange
Journal of Marine Systems
Highlights of the article
  • The BALTSEM model (read more about the Nest model here) is used to calculate carbon fluxes in the Baltic Sea.
  • About 50% of the supplied terrestrial organic carbon is mineralized in the system.
  • Some sub-basins in the system are net sources of CO2 to the atmosphere.
  • Results imply that the Baltic Sea as a whole is a net sink for atmospheric CO2.

In relation to surface area, coastal and marginal seas play a disproportionally large role in the cycling of carbon and nutrients. Substantial riverine and atmospheric loads to such areas contribute to high production, mineralization, and sequestration of organic material, as well as large CO2 transports between air and sea.

In addition to issues such as nutrient loads, eutrophication, and spread of dead zones, the fate of organic carbon supplied from terrestrial sources is a mounting concern.

Modelling carbon cycling in the Baltic
The Baltic Sea can be considered an ideal area for model experiments on the dynamics of organic carbon.

Researchers at the Baltic Nest Institute in Sweden have used the coupled physical—biogeochemical model (BALTSEM) (read more about BALTSEM here) to estimate carbon fluxes in the Baltic Sea over the period 1980—2006.

Organic carbon in the Baltic Sea
Budget calculations for organic carbon indicate that of the total allochthonous organic carbon (TOCT) supplied to the system, on average 56% is mineralized, 36% is exported out of the system, and the remainder is buried. River discharge is the main source of dissolved inorganic carbon (DIC) to the Baltic Sea.

However, model results indicate that in the Gulf of Bothnia (northern Baltic Sea), the contribution to the DIC stock by TOCT mineralization is of the same order as direct river input of DIC. In the Kattegat and Danish Straits (south-western Baltic Sea) on the other hand, net uptake of atmospheric CO2 comprises the major DIC source.

Baltic Sea as a net sink for atmospheric CO2
Despite large variations within the system, with net outgassing from some sub-basins and net absorption in others, the Baltic Sea as a whole was estimated to be a net sink for atmospheric CO2.

Mineralization of allochthonous dissolved organic carbon (DOCT) influences air—sea CO2 exchange. A sensitivity study indicates that depending on the labile fraction of DOCT, the contribution from CO2 absorption to total external DIC sources can amount to 10—25%.

Read more
Download the Baltic Nest Institute Technical Report Series, No. 7 "BALTSEM - a marine model for decision support within the Baltic Sea region"here


In press-version available online 18 July 2013. In print January 2014.

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Updated: 2014-02-05
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