Spectral features of particulate light absorption in the Black Sea in winter

Abstract

The vertical distribution of chlorophyll a concentration (Ca) was uniform in the upper mixed layer (UML). In UML the Ca values varied between stations from 0.8 to 1.8 mg m-3 and decreased to 0.06-0.25 mg m-3 in the layer below maximum temperature gradient. The values of phytoplankton light absorption coefficient varied more than in twice in accordance with Ca variability. The shapes of phytoplankton light absorption spectra slightly changed. The characteristics of phytoplankton light absorption in UML were typical for the winter phytoplankton community which is dominated by Bacillariophyceae and Dinophyceae. In the layer below thermocline, phytoplankton light absorption spectra with a pronounced local absorption maximum at wavelength ~ 550 nm were obtained. The local maximum at ~ 550 nm likely indicates the domination of C-phycoerythrin-containing cyanobacteria Synechococcus spp. in the biomass of the phytoplankton community below the thermocline. The contribution of non-algal particles to total particulate light absorption at wavelength ~ 438 in the layer below thermocline was more than 2 times higher than in the UML.