Warmer winter causes deepening and intensification of summer subsurface bloom in the Black Sea: the role of convection and self-shading mechanism

Abstract. Large differences in the vertical distribution of chlorophyll-a concentration (Chl) in a year with cold and warm winter are observed in the Black Sea on the base of Bio-Argo data. Stronger winter nutrient flux from deeper isopycnal layer in cold 2017 caused an increase of Chl in the upper 40-meter layer observed throughout the whole year – from February to October, with a maximum exceeding 1.3 mg/m³ in February-May of 2017. In warm 2016 with weaker winter convection maximum of Chl during winter-spring in this layer was only about 0.8–0.9 mg/m³. However, the increase of Chl in 2017 led to strong light attenuation in the upper layer and a decrease of euphotic layer depth due to the self-shading mechanism. In 2016 with weaker bloom irradiance penetrated to a 40–70 m layer, below the maximum winter mixed layer depth (40–50 m) and reached the upper layer of nitroclyne, which was not affected by winter mixing. As a result, in warm 2016 the subsurface chlorophyll maximum deepens and Chl in deeper layers was on 0.2–0.6 mg/m³ higher than in 2017. The maximum difference (0.6 mg/m³) was observed during a summer seasonal peak of irradiance due to the largest increase of light attenuation in 2017. As a result, the column-averaged yearly values of Chl in warm 2016 and cold 2017 were comparable. These results demonstrate that the effect of self-shading largely compensates the role of winter convective entrainment of nutrients and causes the deepening of Chl subsurface maximum in warmer years.