Following a summer bloom of coccolithophore Emiliania huxleyi (Lohmann) Hay & Mohler, 1967, in 2003, a winter bloom was observed for the first time between late December 2003 and early January 2004 in the Dardanelles. Microscopic observations showed that the cell dimensions of E. huxleyi (Ehux) varied from 9.85 to 13.50 μm in diameter (mean: 11.20±1.38 μm). While Ehux revealed a relatively small population density (1.60 × 104 cells L−1) in early December 2003, the bloom started in middle December 2003 (7.86 × 106 cells L−1) and then peaked in early January 2004 (5.03 × 107 cells L−1) in the superficial layer. The peak dramatically decreased in late January 2004 (7.50 × 106 cells L−1). Ehux was the dominant species and represented about 90.0% of the phytoplankton assemblage. The bloom started flourishing after the diatom and dinoflagellate blooms under nitrogen depletion and moderate light, temperature and salinity conditions. Water temperature (10.31±1.14°C) and salinity values (27.05±0.88 ppt) were usually stabile. Surface chlorophyll-a concentrations ranged from 1.23 to 2.32 μg L−1 during the bloom. The ratios of N:P (mean: 4.12±2.22) and Si:P (40.35±16.25) of the bloom period were lower than those of the non-bloom periods.

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