Mechanisms controlling the air–sea flux in the North Sea

The mechanisms driving the air–sea exchange of carbon dioxide ( CO 2 ) 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 CO 2 flux for two locations representative for the North Seaʹs biogeochemical provinces. In the seasonally stratified northern region, net CO 2 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 CO 2 partial pressure ( pCO 2 ) during the productive season. In contrast, the permanently mixed southern region is a weak net CO 2 source ( 0.78 mol m - 2 a - 1 ). NCP is generally low except for the spring bloom because remineralization parallels primary production. Here, the pCO 2 appears to be controlled by temperature.