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a b s t r a c tThe present paper synthesizes data obtained during a multidisciplinary cruise carried out in June 2004 at the continental margin of the northern Bay of Biscay. The data-set allows to describe the different stages of a coccolithophore bloom dominated by Emiliania huxleyi. The cruise was carried out after the main spring phytoplankton bloom that started in mid-April and peaked in mid-May. Consequently, low phosphate (PO 4 < 0.2 lM) and silicate (DSi < 2.0 lM) concentrations, low partial pressure of carbon dioxide (pCO 2 ) and high calcite saturation degree in surface waters combined with thermal stratification, probably favoured the blooming of coccolithophores. During the period of the year our cruise was carried out, internal tides induce enhanced vertical mixing at the continental shelf break leading to the injection of inorganic nutrients to surface waters that probably trigger the bloom. The bloom developed as the water-column stratified and as the water mass was advected over the continental shelf, following the general residual circulation in the area. The most developed phase of the bloom was sampled in a remote sensed high reflectance (HR) patch over the continental shelf that was characterized by low chlorophyll-a Bacterial protein production (BPP) measurements in surface waters (0.3-0.7 lg C L À1 h À1 ) were much higher than those reported during early phases of coccolithophore blooms in natural conditions, but similar to those during peak and declining coocolithophorid blooms reported in mesocosms. Total alkalinity anomalies with respect to conservative mixing (DTA) down to À49 lmol kg À1 are consistent with the occurrence of biogenic precipitation of calcite, while pCO 2 remained 15-107 latm lower than atmospheric equilibrium (372 latm). The correlation between DTA and pCO 2 suggested that pCO 2 increased in part due to calcification, but this increase was insufficient to overcome the background under-saturation of CO 2 . This is related to the biogeochemical history of the water masses due to net carbon fixation by the successive phytoplankton blooms in the area prior to the cruise, hence, the investigated area remained a sink for atmospheric CO 2 despite calcification.

The method used consists of adding highly radioactive material (40 ~Ci/ 1 NaH14COs) to sea water with its natural concentrations of zoo-and phytoplankton, incubating this water in the light, separating zoo-from phytoplankton a~er 1 h or at the most 2 h and measuring the radioactivity of both. Under such conditions, the concentration of the tracer in phytoplankton can be simplified as a linear function of time, and that of the zooplankton as a parabolic function of time. This simplification leads to an overestimation of grazing of at most 2-3 %. Comparisons with the Coulter Counter method are given and discussed.

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