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Organic carbon dynamics in the Baltic Sea: A modelling perspective
Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).ORCID-id: 0000-0001-8280-4018
2018 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

Coastal seas constitute a link between land and the open ocean, and therefore play an important role in the global carbon cycle. Large amounts of carbon, of both terrestrial and marine origin, transit and are transformed in these waters, which belong to the more productive areas of the oceans. Despite much research has been done on the subject, there are still many unknown factors in the coastal sea carbon cycling. 

This doctoral thesis investigates the carbon dynamics in the Baltic Sea, with a focus on the production and fate of marine and terrestrial organic carbon and its influence on the air-sea CO2 exchange in its northernmost part, the Gulf of Bothnia. The main approach is the use of a coupled 3D physical-biogeochemical model, in combination with a long series of measurements of physical and biogeochemical parameters. 

A new coupled 3D physical-biogeochemical model, which includes the stoichiometric flexibility of plankton and organic matter, is set up for the Gulf of Bothnia. It is found that phytoplankton stoichiometric flexibility in particular, with non-Redfieldian dynamics, is key to explaining seasonal pCO2, dissolved organic carbon (DOC), and nutrient dynamics. If the Redfield ratio is instead used to predict organic carbon production, as done in most biogeochemical models currently in use, the uptake of atmospheric CO2 is reduced by half. Furthermore, it is shown that the organic carbon production needed to reproduce the summer pCO2 drawdown is larger than measured estimates of primary production. This discrepancy is attributed to a substantial production of extracellular DOC, which seems not to be captured by measurements. 

The dynamics of terrestrial dissolved organic carbon (tDOC) is studied by the use of a passive tracer released from rivers into the physical model of the Baltic Sea. It is found that 80% of the tDOC released in the Baltic Sea is removed, and the rest is exported to the North Sea. Two different parameterisations of tDOC removal are tested. In the first one a decay rate with a timescale of 1 year applied to 80% of the tDOC, and the remaining 20% is assumed to be refractory. In the second one a decay rate with a timescale of 10 years applied to 100% of the tDOC. Trying these parameterisations in a full biogeochemical model shows that only the one with the faster decay is able to reproduce observations of pCO2 in the low-salinity region. A removal rate of one year agrees well with calculated removal rates from bacterial incubation experiments, indicating that bacteria have the potential to cause this remineralisation. It is not only remineralisation of tDOC that affects the pCO2; it is also suggested that a strong tDOC induced light extinction is needed to prevent a too large pCO2 drawdown by phytoplankton in the low salinity region. 

sted, utgiver, år, opplag, sider
Stockholm: Department of Meteorology, Stockholm University , 2018. , s. 36
Emneord [en]
carbon cycle, organic carbon, primary production, remineralisation, terrestrial DOC, extracellular DOC, flexible stoichiometry, Redfield ratio, air-sea CO2 exchange, pCO2, coupled 3D physical-biogeochemical modelling, Gulf of Bothnia, Baltic Sea
HSV kategori
Forskningsprogram
atmosfärvetenskap och oceanografi
Identifikatorer
URN: urn:nbn:se:su:diva-151724ISBN: 978-91-7797-124-5 (tryckt)ISBN: 978-91-7797-125-2 (digital)OAI: oai:DiVA.org:su-151724DiVA, id: diva2:1175372
Disputas
2018-03-22, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (engelsk)
Opponent
Veileder
Merknad

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Submitted.

Tilgjengelig fra: 2018-02-07 Laget: 2018-01-17 Sist oppdatert: 2018-01-31bibliografisk kontrollert
Delarbeid
1. Tracing terrestrial DOC in the Baltic Sea - a 3-D model study
Åpne denne publikasjonen i ny fane eller vindu >>Tracing terrestrial DOC in the Baltic Sea - a 3-D model study
Vise andre…
2016 (engelsk)Inngår i: Global Biogeochemical Cycles, ISSN 0886-6236, E-ISSN 1944-9224, Vol. 30, nr 2, s. 134-148Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

The fate of terrestrial organic matter brought to the coastal seas by rivers, and its role in the global carbon cycle, are still not very well known. Here the degradation rate of terrestrial dissolved organic carbon (DOCter) is studied in the Baltic Sea, a subarctic semi-enclosed sea, by releasing it as a tracer in a 3-D circulation model and applying linear decay constants. A good agreement with available observational data is obtained by parameterizing the degradation in two rather different ways; one by applying a decay time on the order of 10 years to the whole pool of DOCter, and one by dividing the DOCter into one refractory pool and one pool subject to a decay time on the order of 1 year. The choice of parameterization has a significant effect on where in the Baltic Sea the removal takes place, which can be of importance when modeling the full carbon cycle and the CO2 exchange with the atmosphere. In both cases the biogeochemical decay operates on time scales less than the water residence time. Therefore only a minor fraction of the DOCter reaches the North Sea, whereas approximately 80% is removed by internal sinks within the Baltic Sea. This further implies that DOCter mineralization is an important link in land-sea-atmosphere cycling of carbon in coastal- and shelf seas that are heavily influenced by riverine DOC.

HSV kategori
Forskningsprogram
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-124885 (URN)10.1002/2014GB005078 (DOI)000372963900004 ()
Forskningsfinansiär
Baltic Ecosystem Adaptive Management (BEAM)
Tilgjengelig fra: 2016-01-07 Laget: 2016-01-05 Sist oppdatert: 2019-12-17bibliografisk kontrollert
2. Non-Redfieldian Dynamics Explain Seasonal pCO2 Drawdown in the Gulf of Bothnia
Åpne denne publikasjonen i ny fane eller vindu >>Non-Redfieldian Dynamics Explain Seasonal pCO2 Drawdown in the Gulf of Bothnia
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2018 (engelsk)Inngår i: Journal of Geophysical Research - Oceans, ISSN 2169-9275, E-ISSN 2169-9291, Vol. 123, nr 1, s. 166-188Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

High inputs of nutrients and organic matter make coastal seas places of intense air‐sea CO2 exchange. Due to their complexity, the role of coastal seas in the global air‐sea CO2 exchange is, however, still uncertain. Here, we investigate the role of phytoplankton stoichiometric flexibility and extracellular DOC production for the seasonal nutrient and CO2 partial pressure (pCO2) dynamics in the Gulf of Bothnia, Northern Baltic Sea. A 3‐D ocean biogeochemical‐physical model with variable phytoplankton stoichiometry is for the first time implemented in the area and validated against observations. By simulating non‐Redfieldian internal phytoplankton stoichiometry, and a relatively large production of extracellular dissolved organic carbon (DOC), the model adequately reproduces observed seasonal cycles in macronutrients and pCO2. The uptake of atmospheric CO2 is underestimated by 50% if instead using the Redfield ratio to determine the carbon assimilation, as in other Baltic Sea models currently in use. The model further suggests, based on the observed drawdown of pCO2, that observational estimates of organic carbon production in the Gulf of Bothnia, derived with the method, may be heavily underestimated. We conclude that stoichiometric variability and uncoupling of carbon and nutrient assimilation have to be considered in order to better understand the carbon cycle in coastal seas.

Emneord
pCO2 drawdown, modeling, primary production, coastal sea, DOC, stoichiometry
HSV kategori
Forskningsprogram
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-151423 (URN)10.1002/2017JC013019 (DOI)000425589800011 ()
Tilgjengelig fra: 2018-01-11 Laget: 2018-01-11 Sist oppdatert: 2018-07-26bibliografisk kontrollert
3. Remineralization rate of terrestrial DOC as inferred from CO2 supersaturated coastal waters
Åpne denne publikasjonen i ny fane eller vindu >>Remineralization rate of terrestrial DOC as inferred from CO2 supersaturated coastal waters
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(engelsk)Manuskript (preprint) (Annet vitenskapelig)
HSV kategori
Forskningsprogram
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-151718 (URN)
Tilgjengelig fra: 2018-01-17 Laget: 2018-01-17 Sist oppdatert: 2019-12-17bibliografisk kontrollert
4. Nemo-Nordic: A NEMO based ocean model for Baltic & North Seas, research and operational applications
Åpne denne publikasjonen i ny fane eller vindu >>Nemo-Nordic: A NEMO based ocean model for Baltic & North Seas, research and operational applications
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(engelsk)Inngår i: Geoscientific Model Development Discussions, ISSN 1991-9611, E-ISSN 1991-962XArtikkel i tidsskrift (Fagfellevurdert) Submitted
HSV kategori
Forskningsprogram
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-151510 (URN)
Tilgjengelig fra: 2018-01-15 Laget: 2018-01-15 Sist oppdatert: 2018-01-23bibliografisk kontrollert

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