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 the North Atlantic during June 1991. Backscattering and coccolith concentration were positively correlated. Backscattering and concentration of suspended calcite were even better correlated because atomic absorption analyses of calcite calcium were equally accurate whether calcite was attached or detached from cells, whereas it was difficult to enumerate the numbers of coccoliths attached to cells by means of microscopy. As the bloom aged, the ratio of detached coccoliths to plated cells increased. Dilution experiments provided the most precise relationships between coccolith backscattering and coccolith abundance. The calcite-specific scattering coefficient was estimated from measurements of beam attenuation, absorption, and calcite concentration. The contribution of coccolith backscattering to total scattering was modeled as a function of coccolith concentration and chlorophyll concentration. Even outside the coccolithophore bloom, coccoliths were responsible for 5-30% of the total backscattering. Anomalous diffraction theory was used to show that calcite-specific scattering is the highest for 1-3-mu m spheres, which correspond to the diameters of Emiliania huxleyi coccoliths (this prediction was close to the observed values). The calcite-specific scattering coefficients of larger calcite particles (e.g. plated 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.