Extremely strong coccolithophore blooms in the Black Sea: The decisive role of winter vertical entrainment of deep water

Abstract

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.