Anomalous summer-autumn phytoplankton bloom in 2015 in the Black Sea caused by several strong wind events

Abstract

Abstract Four consecutive strong wind events in August 2015 caused an anomalous phytoplankton bloom in the Black Sea, which occupied almost the entire eastern basin. Maximal surface chlorophyll A concentration (Chl-A) in the bloom area exceeded 5 mg/m3 in comparison with 0.6 mg/m3 in the surrounding waters. The anomalous bloom was observed for over 3 months, from August till November. In this study, the physical mechanisms producing this long-lasting bloom are investigated using, satellite-derived optical, infrared, and altimetry measurements and in situ Bio-Argo data. The bloom resulted from the interaction of four physical factors: wind-induced turbulent entrainment; diapycnal mixing caused probably by the vertical shear of inertial currents; local Ekman upwelling at the left wind jet periphery; Ekman pumping of the basinscale cyclonic circulation. Wind induced vertical mixing deepened the mixed layer and entrained nutrients and Chl-A from the subsurface layers. At the same time, the maximum response of Chl-A occurred in a thin layer (~5 m) near the lower boundary of the subsurface chlorophyll peak. This feature results from diapycnal mixing, manifested in the weakening of the stratification in the upper 100 m, which caused the injection of nutrients into the euphotic layer. The mountains surrounding the enclosed Black Sea have an impact on the wind spatial structure generating the zones of wind jets and wind shadows. The wind gradients between areas of strong and weak winds produce local Ekman upwelling along the left periphery of the wind jet, with cyclonic shear, which strongly modify bloom spatial structure. At larger spatial scales, Ekman pumping alters the dynamical structure, producing a transition from “mesoscale” to “basin-scale” circulation regime. As a result, the Rim Current accelerates, uplifting the pycno-halocline and chemocline in the central basin. In the areas of upwelling, the elevated chemocline is eroded more intensely and turbulent entrainment provides nutrients and Chl-A to the surface. The most intense bloom occurred in the central region of the east cyclonic gyre and in mesoscale cyclones; blooms were absent in mesoscale anticyclones. Orbital velocities in both anticyclones and cyclones promoted the horizontal advection of nutrients and Chl-A on more than 200 km to the western part of the basin from the bloom centre.