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 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. Coastal and marginal seas play a key role in the global carbon cycle by linking terrestrial, oceanic, and atmospheric reservoirs (Walsh 1991; Mackenzie et al. 2004). They occupy only 7% of the global ocean surface area but house 10–30% of the global marine primary production (Gattuso et al. 1998). Recent investigations have underlined the importance of coastal seas in the global carbon cycle and the necessity of introducing them into realistic models, notably to quantify the air–sea exchange of CO2 (Chen 2004; Thomas et al. 2004; Muller-Karger et al. 2005). However, even the latest global estimates of the oceanic uptake of anthropogenic CO2 (Sabine et al. 2004) do not include coastal seas in the calculations because of the smallscale variability observed within each marginal sea and between all marginal seas worldwide and because of the difficulty in increasing the spatial resolution of global numerical models. Despite growing interest and debate on the role of all coastal seas in the global carbon cycle, robust estimates of the CO2 air–sea fluxes are only available for a few individual coastal ecosystems. The air–sea exchange of CO2 has been more intensively investigated in temperate marginal seas. Marginal seas account for a relatively large portion of the CO2 uptake of the entire coastal ocean because they cover 55.6% of the total surface area of the coastal ocean (see for overview Borges 2005; Borges et al. 2005; Bozec et al. 2005). The links between air–sea exchange of CO2, biological processes, and physical forcing have also been investigated to determine the net community production (NCP) in coastal seas and open shelves, notably for the Baltic Sea and Bering Chukchi sea shelf (Thomas 1 To whom correspondence should be addressed. Present address: Scripps Institution of Oceanography, Geophysical Research Division, 9500 Gilman Drive, University of California San Diego, La Jolla, California 92093-0244 ([email protected]). Acknowledgments We thank the captains and crews of RV Pelagia for their excellent cooperation during the cruises. The constructive comments by two anonymous reviewers, which helped improve the manuscript significantly, are gratefully acknowledged. This study has been supported by the Netherlands Organization for Scientific Research (NWO), grants 810.33.004 and 014.27.001; the Dutch–German bilateral cooperation NEBROC; and the Belgium Federal Office for Scientific, Technical, and Cultural Affairs (CANOPY project, EV/03/20). This study has been encouraged by and contributes to the LOICZ core project of the IGBP and to CARBO-OCEAN, an integrated project of the European Union (contract 511176-2). Limnol. Oceanogr., 51(6), 2006, 2746–2762 E 2006, by the American Society of Limnology and Oceanography, Inc.