Coccolithophorids are an abundant and widely distributed component of the marine phytoplankton that are thought to play an important role in the oceanic carbon and sulfur cycles through their production of coccoliths, composed of calcium carbonate, and dimethyl sulfide (DMS). The biogeochemical influence of coccolithophorids is probably most pronounced when they occur in "bloom" proportion. Yet little is known of how prevalent these blooms are in the ocean.

To ascertain the prevalence of coccolithophorid blooms and estimate their CaCO3 and DMS production on a regional and global scale, their distribution pattern was mapped in the western North Atlantic and the global ocean. Mapping was accomplished by classifying pixels of coastal zone color scanner imagery into bloom and nonbloom classes based on their mean normalized water-leaving radiances using a supervised, multispectral scheme. Coccolithophorid blooms can be distinguished from most other water conditions in visible satellite imagery because of their unique spectral signature characterized by high water-leaving radiance at all visible wavelengths.

In the western North Atlantic, surface waters with the spectral signature of coccolithophorid blooms annually covered an average of 300,000 km^2 in the Gulf of Maine, the Gulf of St. Lawrence and on the Nova Scotian Shelf and Slope, the Grand Bank and the shelf off northeastern Newfoundland and southeastern Labrador.

Globally, surface waters with the spectral signature of coccolithophorid blooms annually covered an average of 1.4 x 10^6 km^2 in the world oceans from 1979 to 1985, with the subpolar latitudes - particularly the North Atlantic - accounting for 71% of this surface area. The blooms were most extensive in the subarctic North Atlantic. Large expanses of the bloom signal were also detected in numerous lower latitude marginal seas, though the condition(s) responsible for this signal is equivocal. The greatest spatial extent of classified blooms in subpolar oceanic regions occurred in the months of summer - early autumn, while those in lower latitude marginal seas occurred in mid winter - early spring.

Classified blooms are presumed to be composed primarily of the coccolithophorid Emiliania huxleyi (Lohmann) Hay et Mohler. This presumption was substantiated in the western North Atlantic by noting the presence of E. huxleyi coccospheres and/or detached coccoliths at high concentrations in or near the vicinity of high reflectance patches observed in contemporaneous visible AVHRR (Advanced Very High Resolution Radiometer) imagery. However, the presence of conditions that could mimic the spectral characteristics of coccolithophorid blooms renders the determination of actual bloom presence equivocal in some geographic areas, particularly on shelf regions at lower latitudes.

Standing stock estimates suggest that the presumed Emiliani huxleyi (Lohmann) Hay et Mohler blooms act as a significant source of calcite carbon and DMS sulfur on a regional basis. On a global scale, however, satellite-detected coccolithophorid blooms are estimated to play only a minor role in the annual flux of calcite and DMS from the surface mixed layer to depth and the atmosphere, respectively.