THE 1991 COCCOLITHOPHORE BLOOM IN THE CENTRAL NORTH-ATLANTIC .1. RELATING OPTICS TO COCCOLITH CONCENTRATION

This study summarizes the relationships between various biological and optical properties of a mesoscale coccolithophore bloom observed in t he North Atlantic during June 1991. Backscattering and coccolith conce ntration were positively correlated. Backscattering and concentration of suspended calcite were even better correlated because atomic absorp tion analyses of calcite calcium were equally accurate whether calcite was attached or detached from cells, whereas it was difficult to enum erate the numbers of coccoliths attached to cells by means of microsco py. As the bloom aged, the ratio of detached coccoliths to plated cell s increased. Dilution experiments provided the most precise relationsh ips between coccolith backscattering and coccolith abundance. The calc ite-specific scattering coefficient was estimated from measurements of beam attenuation, absorption, and calcite concentration. The contribu tion of coccolith backscattering to total scattering was modeled as a function of coccolith concentration and chlorophyll concentration. Eve n outside the coccolithophore bloom, coccoliths were responsible for 5 -30% of the total backscattering. Anomalous diffraction theory was use d to show that calcite-specific scattering is the highest for 1-3-mu m spheres, which correspond to the diameters of Emiliania huxleyi cocco liths (this prediction was close to the observed values). The calcite- specific scattering coefficients of larger calcite particles (e.g. pla ted coccolithophore cells, foraminifera, pteropods) would be expected to be considerably lower. These data were used to test an approach for predicting coccolith concentration from water-leaving radiance in the blue and green wavelengths.