Optical impacts of oceanic coccolithophore blooms

Blooms of coccolithophores, particularly those of the species Emiliania hux leyi, cause light in the surface ocean to behave in an unusual fashion, pro ducing distinctive bright "white waters," apparent from ships and readily d etected by remote sensing. The brightness is caused by scattering of light from calcium carbonate platelets (coccoliths). Here we present the results of a modeling study, giving precise calculations of how the coccolith light scattering changes the behavior of light in the water. The results from a Monte Carlo optical model are closely compared to data from the CD60 cruise for a coccolithophore bloom south of Iceland in 1991 [Holligan et al., 199 3], and the model is then used to extrapolate from the observational data t o predict diverse optical properties that were not measured. Model. perform ance was also tested by comparison of results with those from other, more e stablished optical models. The model results demonstrate clearly that cocco liths cause (1) an increase in the emergent flux (the water-leaving radianc e), (2) brighter, more intensely heated water in the top few meters, and (3 ) darker, less intensely heated water deeper down. Implications of these ef fects for phytoplankton productivity and for climatology are discussed. Coc colith light scattering is estimated to contribute to global annually avera ged planetary albedo by a maximum of similar to 0.13%, equivalent to only a small globally averaged radiative forcing of similar to 0.22 W m(-2).