The mechanisms driving the air-sea exchange of carbon dioxide (CO2) in the North Sea are investigated using the three-dimensional coupled physical-biogeochemical model ECOHAM (ECOlogical-model, HAMburg). We validate our simulations using field data for the years 2001-2002 and identify the controls of the air-sea CO2 flux for two locations representative for the North Sea's biogeochemical provinces. In the seasonally stratified northern region, net CO2 uptake is high (2: 06 mol m-2 a-1) due to high net community production (NCP) in the surface water. Overflow production releasing semi-labile dissolved organic carbon needs to be considered for a realistic simulation of the low dissolved inorganic carbon (DIC) concentrations observed during summer. This biologically driven carbon drawdown outcompetes the temperature-driven rise in CO2 partial pressure (pCO2) during the productive season. In contrast, the permanently mixed southern region is a weak net CO2 source (0:78 mol m-2 a-1). NCP is generally low except for the spring bloom because remineralization parallels primary production. Here, the pCO2 appears to be controlled by temperature.
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