Assessment of the processes controlling the 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 m22 month21 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 m22 month21. 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 m22 month21. 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 m22 month21. In the surface layer of the North
Sea, NCP on the basis of DIC was 4.3 6 0.4 mol C m22 yr21, whereas, NCP on the basis of nitrate was 1.6 6
0.2 mol C m22 yr21. 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.