PRODUCTION OF ORGANIC AND INORGANIC CARBON WITHIN A LARGE-SCALE COCCOLITHOPHORE BLOOM IN THE NORTHEAST ATLANTIC-OCEAN

An extensive bloom (250 000 km2) of the coccolithophore Emiliania huxleyi developed in the northeast Atlantic in June 1991. At the time of the observations in late June, the centre of the bloom was characterised by low concentrations of chlorophyll a (< 1 mg m-3) and particulate organic carbon (< 200 mg m-3) and high particulate inorganic carbon (PIC) (> 300 mg m-3) values. Photosynthesis-irradiance experiments carried out along the cruise track showed the existence of a negative exponential relationship between assimilation number (P(max)B) and PIC. Significant rates of inorganic carbon incorporation were only measurable in surface waters at the periphery of the bloom and in subsurface waters north of 61-degrees-N. Calcification rates of up to 1.5 mg C m-3 h-1 were measured. In general, a direct relationship was found between calcification rates and P(max)B or chlorophyll a-normalized photosynthesis. Calcification-irradiance curves revealed that significant amounts of carbon were incorporated into coccoliths in darkness and also that, on some occasions, calcification in subsurface populations saturated at higher irradiances than in surface ones. An empirical model based on the biomass of E. huxleyi and Coccolithus pelagicus and the beam attenuation coefficient accounted for a large proportion (> 70 %) of the variability in surface rates of calcification. Estimates of calcification rates derived from the model are consistent with previously reported data and compatible with the measured stocks of PIC and with the duration of the bloom as revealed by satellite imagery.