Assessment of the processes controlling seasonal variations of dissolved inorganic carbon in the North Sea

We used a seasonal North Sea data set comprising dissolved inorganic carbon (DIC), partial pressure of CO2 (pCO2)), and inorganic nutrients to assess the abiotic and biological processes governing the monthly variations of DIC. During winter, advection and air-sea exchange of CO2 control and increase the DIC content in the surface and deeper layers of the northern and central North Sea, with the atmosphere supplying CO2 on the order of 0.2 mol C m-2 month-1 to these areas. From February to July, net community production (NCP) controls the seasonal variations of DIC in the surface waters of the entire North Sea, with a net uptake ranging from 0.5 to 1.4 mol C m-2 month-1. During the August-December period, NCP controls the seasonal variations of DIC in the southern North Sea, with a net release ranging from 0.5 to 0.8 mol C m-2 month-1. Similarly, during the April-August period in the deeper layer of the northern North Sea, the NCP was the main factor controlling DIC concentrations, with a net release ranging from 0.5 to 5.5 mol C m-2 month-1. In the surface layer of the North Sea, NCP on the basis of DIC was 4.3 +/- 0.4 mol C m-2 yr-1, whereas, NCP on the basis of nitrate was 1.6 +/- 0.2 mol C m-2 yr-1. Under nutrient-depleted conditions, preferential recycling (extracellular) of nutrients and intracellular mechanisms occurred and were responsible for the non-Redfield uptake of DIC versus nitrate and phosphate