Interannual variability of the early summer coccolithophore blooms in the Black Sea: impacts of climatic factors

Abstract

Interannual variability of the early summer (May-June) coccolithophore blooms within surface waters of the Black Sea was studied by means of satellite-based bio-optical observations. The performance of two coccolithophore detection algorithms were tested for Black Sea conditions, and were found to provide comparable spatial patterns consistent with the corresponding true color images. An analysis of six, year-long OCTS and SeaWiFS imagery from 1997 onwards points to the presence of a major phytoplankton bloom in every early summer season. Blooms are dominated by densely populated coccolithophore algae within the entire basin, except during 2001. In the early summer of 2001, the coccolithophore activity was limited to the northeastern coastal zone, and the bloom in the rest of the basin was formed by non-coccolithophore groups, as suggested by their relatively strong chlorophyll signature. More coccolithophore over, limited coccolithophore abundance noted in the historical CZCS data suggests substantial differences in terms of spatial coverage and total biomass from the early 1980s to the late 90s. The increasing contribution of coccolithophores to the early summer phytoplankton community structure during the last decade is also consistent with the current view of dramatic shifts in taxonomic composition from diatoms to coccolithhophores and flagellates, as a part of transformations that took place in the Black Sea biogeochemistry and ecosystem structure under changing anthropogenic and climate forcing during the 1980s and 1990s, respectively.