Light absorption properties and absorption budget of the Black Sea

Abstract

This study is focused on bio-optical data collected from deep parts of the eastern and western Black Sea and the Crimean shelf waters during nineteen cruises of RV “Professor Vodyanitsky” carried out in different seasons in 2015–2022. Based on a long-term data set, a parameterization of light absorption by phytoplankton ($a_{\mathrm{ph}}\left(\lambda \right)$), non-algal particles ($a_{\mathrm{NAP}}\left(\lambda \right)$), and colored dissolved organic matter $a_{\mathrm{CDOM}}\left(\lambda \right)$ was performed. Parameterization coefficients for phytoplankton absorption were revealed for all seasons. A two-fold difference was observed in the chlorophyll “a” specific light absorption coefficient of phytoplankton in the Black Sea between summer and winter seasons. The difference was related to the “pigment package effect” resulting in the self-shading of pigments caused by changes in intracellular pigment concentration and phytoplankton cell size due to adaptation of phytoplankton to the varying environmental factors. The use of seasonal-specific coefficients of parameterization of phytoplankton light absorption provides more accurate retrievals of $a_{\mathrm{ph}}\left(\lambda \right)$ spectra based on the chlorophyll “a” concentration in modelling the light field and primary production using a spectral approach. The parameterization of CDOM light absorption and the revealed relationship between $a_{\mathrm{CDOM}}\left(\lambda \right)$ and spectral slope (S_CDOM) allow one to correctly retrieve the $a_{\mathrm{CDOM}}\left(\lambda \right)$ based on the CDOM light absorption coefficient at particular wavelength. This is applicable both for data from submersible probes and for satellite data. Accurately retrieved CDOM spectra allow more correct modelling of the light field in the sea, which is crucial for waters, where CDOM is the main optically active component. Empirically assessed spectral slope of NAP absorption and share of NAP in particulate absorption allow to differentiate CDOM and NAP based on values of their summary absorption, which can be applicable for remote sensing data interpretation. The light absorption budget reveals CDOM as the main optically active component in the Black Sea upper layer (first optical depth). The parameterization of light absorption by all optically active components can be used to refine regional models for assessing water quality and productivity indicators based on remote sensing information.