The Black Sea is characterized by powerful summer coccolithophore blooms with strong interannual variability. In this paper, we used satellite and Bio-Argo measurements to show that the emergence of extremely strong blooms in several years was caused by intense vertical entrainment of nutrients from deep isopycnal layers during winter convection. In the highly stratified Black Sea, the depths of maximum nutrient concentrations are tightly associated with certain isopycnals. Due to this, the density of the winter upper mixed layer can be used as an indicator of the amount of newly entrained nutrients penetrating into the euphotic layer. In years with the weak bloom of coccolithophores, the average density in the mixed layer during February–March was less than 1013.9–1014.2 kg m⁻³. In years with moderately strong blooms (2006, 2008, and 2019) the density reached 1014.25–1014.3 kg m⁻³, while with the strongest blooms (2012 and 2017) it was 1014.35–1014.45 kg m⁻³. Such differences in density corresponded to a twofold and threefold difference in the newly entrained amount of phosphate, which is known to be the main nutrient that stimulates the growth of coccolithophores in the Black Sea. Despite the long-term rise of average winter temperature in the Black Sea, an increase in density has been observed in the upper layer in the recent period caused by the rise of its salinity, the sharpest since 2014. The increase in salinity of upper layers weakens the vertical haline stratification and intensifies vertical mixing and related nutrient fluxes, which can explain the extremely strong coccolithophore blooms observed in the recent period, particularly in a moderately cold 2017.

黑海的特征是夏季强大的球石藻绽放，年际变化强烈。在本文中，我们使用卫星和生物Argo的测量结果表明，几年中极强水华的出现是由于冬季对流过程中深深等深层营养物质的强烈垂直夹带引起的。在高度分层的黑海中，最大营养物浓度的深度与某些等位面紧密相关。因此，冬季上层混合层的密度可以用作指示新夹带的营养物渗透到富营养层的量的指标。在球藻科植物的花开较弱的年份，2-3月混合层中的平均密度小于1013.9-1014.2 kg m ^-3。在水华中等的年份（2006年，2008年和2019年），密度达到1014.25–1014.3 kg m ^-3，而水华最强的年份（2012年和2017年）则为1014.35–1014.45 kg m ^-3。密度的这种差异对应于新夹带的磷酸盐量的两倍和三倍的差异，磷酸盐是已知的刺激黑海球藻生长的主要营养物质。尽管黑海冬季平均温度已长期升高，但最近一段时间以来，由于其盐度的上升（2014年以来的最高水平），观测到上层的密度有所增加。上层盐度的上升减弱了盐卤的垂​​直分层，并增强了垂直混合和相关的养分通量，这可以解释最近一段时间（尤其是在2017年中度寒冷）观察到的极其强烈的球石藻水华。