THE 1991 COCCOLITHOPHORE BLOOM IN THE CENTRAL NORTH-ATLANTIC .1. OPTICAL-PROPERTIES AND FACTORS AFFECTING THEIR DISTRIBUTION

Optical scattering and absorption were measured in the central North A tlantic Ocean during it mesoscale bloom of the coccolithophorid Emilia nia huxleyi. The chlorophyll-specific absorption was similar to previo usly measured levels for this species in laboratory cultures. Suspende d coccoliths were responsible for similar to 80% of the total backscat tering in the center of the bloom, and the greatest calcite-dependent backscattering was observed just below the base of the mixed layer. Ar eal maps of calcite-dependent backscattering and reflectance were simi lar because of the dominance of backscattering over absorption. Calcul ated reflectance at 440 and 550 nm reached similar to 24%, which is sl ightly less than what has been observed previously in Gulf of Maine bl ooms. Total scattering (b) was also calculated as the difference betwe en beam attenuation and absorption. The ratio <(b)over tilde (b)> (bac kscattering divided by total scattering) was similar to 0.01-0.02 at 4 40 and 550 nm at the most turbid parts of the coccolithophore bloom (b = 1-3 m(-1)). As total scattering decreased below 1 m(-1), <(b)over t ilde (b)> increased. The behavior of <(b)over tilde (b)> was compared for coccolith-dominated vs. chlorophyll-dominated waters. Vertical pro files of calcite-dependent scattering, combined with satellite remote sensing data, were used to assess the factors responsible for vertical transport of calcite. The subsurface peak in calcite-dependent scatte ring did not result from detached coccoliths sinking but resulted eith er from plated coccolithophores sinking and then detaching their plate s or from deep coccolithophores producing and detaching their plates i n situ.