Winter coccolithophore blooms in the Black Sea: Interannual variability and driving factors

The spatial and temporal variability of winter coccolithophore blooms in the Black Sea is studied using satellite measurements for 2003–2017 and Bio-Argo buoys data. Analysis shows that such blooms can occupy the entire upper 60-m layer with satellite-derived estimates of surface cell concentration reaching in some years 3.0·106 cells l−1. On average, winter coccolithophore blooms start in November–December, peak in the first decade of January, and terminate in February. They are characterized by significant interannual variability. Blooms in different years can be “basin-wide” covering most of the sea (2004–2005, 2005–2006, 2006–2007, 2012–2013) and “local” when they are observed only in certain areas of the Black Sea, most often in its southern part. A detailed analysis of the evolution of winter blooms has shown that they are usually triggered by intense storms, which are followed after 1–2 weeks by the initial growth of coccolithophores in the zone of storm-driven upwelling. Cross-shelf nutrients exchange may contribute to the coccolithophore growth over the continental slope. Intense winter coccolithophore blooms are observed in years with low autumn concentrations of chlorophyll-a, i.e., during periods of weakening of the typical diatom blooming autumn-winter. The strongest correlation on annual time scales is found between bloom intensity and decrease of the autumn-winter wind curl. We suggest that low wind curl causes a weakening of the basin-scale cyclonic circulation and intensifies the cross-shelf exchange, impacting the Black Sea trophic structure in the preceding summer period and may promote the winter bloom of coccolithophores in such years.