Interannual variability of the prevalent early summer coccolithophore blooms within surface waters of the Black Sea was studied by means of satellite-based bio-optical observations. Two coccolith detection algorithms, tested for the Black Sea conditions, were found to provide comparable spatial coccolith patterns consistent with the corresponding true color images. Reliability of the algorithms is also supported by several sets of time-series measurements in different parts of the basin. An analysis of 6 year-long series of OCTS and SeaWiFS images from 1997 onwards shows major phytoplankton bloom activity every year in early summer. These blooms are often dominated by dense populations of coccolithophores but with some year-to-year variations in their spatial coverage, intensity, duration and timing of their maximum expanse over the basin. Moreover, limited coccolith coverage noted in the historical CZCS data may imply an increasing contribution of coccolithophores to the summer phytoplankton community from the early 1980s to 1990s. It is consistent with almost an order of magnitude increase in phytoplankton production, and the current view of dramatic shifts in taxonomic composition from diatoms to coccolithophores and dinoflagellates, as a part of transformations that took place in the Black Sea biogeochemistry and ecosystem structure under changing anthropogenic and climate forcing. (C) 2004 Elsevier Ltd. All rights reserved.