The 1991 coccolithophore bloom in the central North Atlantic. 1. Optical properties and factors affecting their distribution

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Abstract
Optical scattering and absorption were measured in the central North Atlantic Ocean during a mesoscale bloom of the coccolithophorid Emiliania huxleyi. The chlorophyll‐specific absorption was similar to previously measured levels for this species in laboratory cultures. Suspended coccoliths were responsible for ∼80% of the total backscattering in the center of the bloom, and the greatest calcite‐dependent backscattering was observed just below the base of the mixed layer. Areal maps of calcite‐dependent backscattering and reflectance were similar because of the dominance of backscattering over absorption. Calculated reflectance at 440 and 550 nm reached ∼24%, which is slightly less than what has been observed previously in Gulf of Maine blooms. Total (b) was also calculated as the difference between beam attenuation and absorption. The ratio b̃b (backscattering divided by total scattering) was ∼0.01–0.02 at 440 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, b̃b increased. The behavior of b̃b was compared for coccolith‐dominated vs. chlorophyll‐dominated waters. Vertical profiles 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 scattering did not result from detached coccoliths sinking but resulted either from plated coccolithophores sinking and then detaching their plates or from deep coccolithophores producing and detaching their plates in situ.