Sinking velocity of small particles in the Black Sea: Vertical distribution and seasonal variability from continuous Bio-Argo measurements of backscattering

Abstract

High-resolution Bio-Argo measurements of backscattering (bbp) were used to study the vertical distribution and time variability of the sinking of small, highly reflective particles in the Black Sea in the 0–200 m layer. The evolution of particles is divided into three stages: formation, sinking, and removal. The formation of small, highly reflective particles is mainly related to early summer and early winter coccolithophore blooms and the formation of suspended oxidized manganese in the suboxic zone. The removal rate of the particles was estimated from the vertical distribution of bbp in relation to its values at the layers of particle formation. Further, contour analysis of the time-depth diagram of bbp anomalies was used to automatically estimate the vertical distribution and seasonal and interannual variability of sinking velocity (w). The average value of w is ~0.8 m/day, which agrees with laboratory estimates of coccolith sinking (Fritz and Balch, 1996). Physical processes significantly affect w: mixed layer deepening in late autumn-early winter increase w up to 1–1.5 m/day; warming and a decrease in density of upper layers promote coccolithophores deepening in summer (w = 0.9–1.1 m/day). Minimum sinking velocity is observed in the warm period of a year in the strongly stratified layers of the main pycnocline at 40–80 m. On interannual time scales, we observed an increase in w during the strongest coccolithophore blooms, which is probably related to the enhanced particle aggregation. Sinking particles after winter coccolithophore blooms are traced up to the suboxic zone, where it probably affects the settling of manganese-containing particles and related biochemical processes.