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  • 1.
    Fransner, Filippa
    Stockholm University, Faculty of Science, Department of Meteorology .
    Organic carbon dynamics in the Baltic Sea: A modelling perspective2018Doctoral thesis, comprehensive summary (Other academic)
    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. 

  • 2.
    Fransner, Filippa
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Fransson, Agneta
    Humborg, Christoph
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute.
    Gustafsson, Erik
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute.
    Tedesco, Letizia
    Hordoir, Robinson
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Remineralization rate of terrestrial DOC as inferred from CO2 supersaturated coastal watersManuscript (preprint) (Other academic)
  • 3.
    Fransner, Filippa
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Fransson, Agneta
    Humborg, Christoph
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre. University of Helsinki, Finland.
    Gustafsson, Erik
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute.
    Tedesco, Letizia
    Hordoir, Robinson
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Remineralization rate of terrestrial DOC as inferred from CO2 supersaturated coastal waters2019In: Biogeosciences Discussions, ISSN 1810-6277, E-ISSN 1810-6285, Vol. 16, no 3, p. 863-879Article in journal (Refereed)
    Abstract [en]

    Coastal seas receive large amounts of terrestrially derived organic carbon (OC). The fate of this carbon, and its impact on the marine environment, is however poorly understood. Here we combine underway CO2 partial pressure (pCO2) measurements with coupled 3D hydrodynamical-biogeochemical modelling to investigate whether remineralization of terrestrial dissolved organic carbon (tDOC) can explain CO2 supersaturated surface waters in the Gulf of Bothnia, a subarctic estuary. We find that a substantial remineralization of tDOC, and that a strong tDOC induced light attenuation dampening the primary production, is required to reproduce the observed CO2 supersaturated waters in the nearshore areas. A removal rate of tDOC of the order of one year, estimated in a previous modelling study in the same area, gives a good agreement between modelled and observed pCO2. The remineralization rate is on the same order as bacterial degradation rates calculated from published incubation experiments, suggesting that this remineralization could be caused by bacterial degradation. Furthermore, the observed high pCO2 values during the ice covered season argues against photochemical degradation as the main removal mechanism. All of the remineralized tDOC is outgassed to the atmosphere in the model, turning the northernmost part of the Gulf of Bothnia to a source of atmospheric CO2.

  • 4.
    Fransner, Filippa
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Gustafsson, Erik
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute.
    Tedesco, Letizia
    Vichi, Marcello
    Hordoir, Robinson
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Spilling, Kristian
    Kuznetsov, Ivan
    Eilola, Kari
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Humborg, Christoph
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute. University of Helsinki, Finland.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Non-Redfieldian Dynamics Explain Seasonal pCO2 Drawdown in the Gulf of Bothnia2018In: Journal of Geophysical Research - Oceans, ISSN 2169-9275, E-ISSN 2169-9291, Vol. 123, no 1, p. 166-188Article in journal (Refereed)
    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.

  • 5.
    Fransner, Filippa
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute. Stockholm University, Faculty of Science, Department of Geological Sciences.
    Humborg, Christoph
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute. Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Meier, H. E. Markus
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    Hordoir, Robinson
    Gustafsson, Erik
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute.
    Deutsch, Barbara
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute. Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Tracing terrestrial DOC in the Baltic Sea - a 3-D model study2016In: Global Biogeochemical Cycles, ISSN 0886-6236, E-ISSN 1944-9224, Vol. 30, no 2, p. 134-148Article in journal (Refereed)
    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.

  • 6. Hordoir, Robinson
    et al.
    Axell, Lars
    Höglund, Anders
    Dieterich, Christian
    Fransner, Filippa
    Stockholm University, Faculty of Science, Department of Meteorology . Bergen University, Norway; Bjerknes Centre for Climate Research, Norway.
    Gröger, Matthias
    Liu, Ye
    Pemberton, Per
    Schimanke, Semjon
    Andersson, Helen
    Ljungemyr, Patrik
    Nygren, Petter
    Falahat, Saeed
    Nord, Adam
    Jönsson, Anette
    Lake, Iréne
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Hieronymus, Magnus
    Dietze, Heiner
    Löptien, Ulrike
    Kuznetsov, Ivan
    Westerlund, Antti
    Tuomi, Laura
    Haapala, Jari
    Nemo-Nordic 1.0: a NEMO-based ocean model for the Baltic and North seas - research and operational applications2019In: Geoscientific Model Development, ISSN 1991-959X, E-ISSN 1991-9603, Vol. 12, no 1, p. 363-386Article in journal (Refereed)
    Abstract [en]

    We present Nemo-Nordic, a Baltic and North Sea model based on the NEMO ocean engine. Surrounded by highly industrialized countries, the Baltic and North seas and their assets associated with shipping, fishing and tourism are vulnerable to anthropogenic pressure and climate change. Ocean models providing reliable forecasts and enabling climatic studies are important tools for the shipping infrastructure and to get a better understanding of the effects of climate change on the marine ecosystems. Nemo-Nordic is intended to be a tool for both short-term and long-term simulations and to be used for ocean forecasting as well as process and climatic studies. Here, the scientific and technical choices within Nemo-Nordic are introduced, and the reasons behind the design of the model and its domain and the inclusion of the two seas are explained. The model's ability to represent barotropic and baroclinic dynamics, as well as the vertical structure of the water column, is presented. Biases are shown and discussed. The short-term capabilities of the model are presented, especially its capabilities to represent sea level on an hourly timescale with a high degree of accuracy. We also show that the model can represent longer timescales, with a focus on the major Baltic inflows and the variability in deep-water salinity in the Baltic Sea.

  • 7. Hordoir, Robinson
    et al.
    Axell, Lars
    Höglund, Anders
    Dieterich, Christian
    Fransner, Filippa
    Stockholm University, Faculty of Science, Department of Meteorology .
    Gröger, Matthias
    Liu, Ye
    Pemberton, Per
    Schimanke, Semjon
    Andersson, Helen
    Ljungemyr, Patrik
    Nygren, Petter
    Falahat, Saeed
    Nord, Adam
    Jönsson, Anette
    Lake, Iréne
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Hieronymus, Magnus
    Stockholm University, Faculty of Science, Department of Meteorology .
    Dietze, Heiner
    Löptien, Ulrike
    Kuznetsov, Ivan
    Westerlund, Antti
    Tuomi, Laura
    Haapala, Jari
    Nemo-Nordic: A NEMO based ocean model for Baltic & North Seas, research and operational applicationsIn: Geoscientific Model Development Discussions, ISSN 1991-9611, E-ISSN 1991-962XArticle in journal (Refereed)
1 - 7 of 7
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