Balch et al.  evaluated from remote sensing data the contemporary global calcification related to coccolithophores to 1.6 ± 0.3 Pg PIC yr−1 (1 Pg = 1015 g; PIC, particulate inorganic carbon). Other estimates of contemporary global pelagic calcification range between 0.7 Pg PIC yr−1, based on accumulation rates and sediment trap data [Milliman et al., 1999], and 1.4 Pg PIC yr−1, based on the seasonal cycle of total alkalinity (TA) in the euphotic zone [Lee, 2001]. Each of these estimates of global pelagic calcification suffers from specific shortcomings. For instance, the estimate of Milliman et al.  might be underestimated due to supralysoclinal dissolution of calcium carbonate (CaCO3) [e.g., Wollast and Chou, 1998; Beaufort et al., 2007; Berelson et al., 2007], and the TA analysis of Lee  could be affected by other processes than calcification such as nutrient uptake and release or mixing of water masses. The estimate of Balch et al.  is probably less biased by the sparseness of data coverage compared to the two other approaches, although strongly dependent on the predictive capability of the algorithm used to process the remote sensing data. Three-dimensional circulation models mostly tuned to reproduce the observed TA distributions also provide a wide range of global pelagic calcification values from 0.6 to 1.8 Pg PIC yr−1 [Bacastow and Maier-Reimer, 1990; Yamanaka and Tajika, 1996; Archer et al., 1998; Murnane et al., 1999; Heinze et al., 2003; Moore et al., 2004; Jin et al., 2006; Gehlen et al., 2007; Hofmann and Schellnhuber, 2009]. The fact that the estimate of Balch et al.  of contemporary global pelagic calcification related to coccolithophores is comparable to the other estimates would imply that coccolithophores are the most important pelagic calcifiers in the oceans.
 The development of coccolithophore blooms affects the seawater carbonate chemistry, and air-sea CO2 fluxes, through the organic carbon pump and the carbonate counterpump. The organic carbon pump relies on organic carbon production by photosynthesis and leads to an uptake of CO2 from surface waters, according to:
 The carbonate counterpump relies on the production of CaCO3, leading to a thermodynamic shift of HCO3− to CO2, hence, a release of CO2 to surrounding surface waters, according to:
 The ratio between calcification (carbonate counterpump), and organic carbon production (organic carbon pump), the C:P ratio, depends on the life cycle (bloom development) and growth conditions of coccolithophores [Fernández et al., 1993; Paasche and Brubak, 1994; Paasche, 2002; Delille et al., 2005]. At the onset of the coccolithophore bloom, when nutrients are available for growth, organic carbon production dominates over calcification (C:P ≪ 1, the so-called organic phase). At the end of the bloom, in nutrient-depleted conditions and high irradiances (due to stronger stratification), organic carbon production decreases and calcification increases (C:P ≤ 1, the so-called inorganic phase).
 The accumulation of anthropogenic CO2 in the oceans [e.g., Sabine et al., 2004] has altered carbonate chemistry in surface waters (ocean acidification) since preindustrial times, and is expected to continue to do so in the coming centuries [e.g., Caldeira and Wickett, 2003; Orr et al., 2005; Cao et al., 2007; McNeil and Matear, 2007]. Changes of the carbonate chemistry of surface waters related to ocean acidification can alter the rates and fates of primary production and calcification of numerous marine organisms and communities [as reviewed by Raven et al., 2005; Kleypas et al., 2006; Fabry et al., 2008; Doney et al., 2009]. Such changes can provide either positive or negative feedbacks on increasing atmospheric CO2 by modifying the flux of CO2 between the ocean and the atmosphere.
 Several manipulative experiments to test the effect of ocean acidification on coccolithophores have shown that while calcification would decrease, the export of organic carbon would increase mainly through increasing production of transparent exopolymer particles (TEP) [Riebesell et al., 2000; Engel et al., 2004a, 2004b; Delille et al., 2005; Riebesell et al., 2007]. On the other hand, the reduction of pelagic calcification due to ocean acidification could also lead to a reduction of carbon export due to the decrease of the ballast effect of CaCO3 on marine particles [e.g., Armstrong et al., 2002; Klaas and Archer, 2002; Barker et al., 2003; Hofmann and Schellnhuber, 2009]. The modelling study of Hofmann and Schellnhuber  suggested that the positive feedback on increasing atmospheric CO2 related to the decrease of carbon export from the reduction of ballast effect of CaCO3 on marine particles would represent ∼40% of the negative feedback related to the decrease of the CO2 emission to the atmosphere due to the reduction of pelagic calcification. For a credible implementation in mathematical models of such feedback mechanisms to allow the projection of a future evolution of marine carbon biogeochemistry under global change, it is required to understand present day biogeochemistry and ecology of naturally occurring pelagic calcifying communities. In particular, the overall effect of phytoplankton communities on the C:P ratio, carbonate chemistry, and air-sea CO2 fluxes.
 In the northwest European continental margin, blooms of the coccolithophore Emiliania huxleyi have been frequently reported [Holligan et al., 1983; Garcia-Soto et al., 1995; Wollast and Chou, 1998, 2001; Godoi et al., 2009; Harlay et al., 2009, 2010]. Here, we report a data set of carbonate chemistry in surface waters obtained during three cruises in the northern Bay of Biscay (Figure 1). We evaluate the relative effect of calcification and organic carbon production on seawater carbonate chemistry and air-sea CO2 fluxes.