Biogeochemical study of a coccolithophore bloom in the northern Bay of Biscay (NE Atlantic Ocean) in June 2004

The 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 (PO₄ < 0.2 μM) and silicate (DSi < 2.0 μM) concentrations, low partial pressure of carbon dioxide (pCO₂) 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 (Chl-a) concentration in surface waters (<1.0 μg L^?1), high particulate inorganic carbon (PIC) concentration (～8 μmol L^?1) and coccolithophore abundance up to 57 × 10⁶ cells L^?1. Transparent exopolymer particles (TEP) concentrations ranged between 15 and 75 μg C L^?1 and carbon content of TEP represented up to 26% of the particulate organic carbon (POC; maximum concentration of 15.5 μmol L^?1 in the upper 40 m). Integrated primary production (PP) ranged between 210 and 680 mg C m^?2 d^?1 and integrated calcification (CAL) ranged between 14 and 140 mg C m^?2 d^?1, within the range of PP and CAL values previously reported during coccolithophore blooms in open and shelf waters of the North Atlantic Ocean. Bacterial protein production (BPP) measurements in surface waters (0.3–0.7 μg C L^?1 h^?1) 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 (ΔTA) down to ?49 μmol kg^?1 are consistent with the occurrence of biogenic precipitation of calcite, while pCO₂ remained 15–107 μatm lower than atmospheric equilibrium (372 μatm). The correlation between ΔTA and pCO₂ suggested that pCO₂ increased in part due to calcification, but this increase was insufficient to overcome the background under-saturation of CO₂. 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 CO₂ despite calcification.