Standard NASA ocean color algorithm OC4 was developed on the basis of ocean optical data and while appropriate for Case 1 oceanic waters could not be adequately applied for the Black Sea waters due to its different bio-optical properties. OC4 algorithm is shown to overestimate chlorophyll concentration (Chl-a) in summer and underestimate Chl-a during early spring phytoplankton blooms in the Black Sea. For correct conversion of satellite data to Chl-a, primary production and other indicators regional algorithms should be developed taking into account bio-optical properties of the Black Sea waters. Light absorption by phytoplankton pigments – a_ph (λ) have been measured in open sea and shelf Black Sea waters in different seasons since 1998. It was shown that the first optical depth was located within the upper mixed layer (UML) for most of the year with the exception of the spring when seasonal stratification was developing. As a result spectral features of water leaving radiance were determined by optical properties of the UML. Significant seasonal differences in Chl-a specific light absorption coefficients of phytoplankton within UML have been revealed. These differences were caused by adaptive changes of composition and intracellular pigment concentration due to variable environment conditions – mainly light intensity. Empirical relationships between a_ph (λ) and Chl-a were derived by least squares fitting to power functions for different seasons. Incorporation of these results will refine the regional ocean color models and provide improved and seasonally adjusted estimates of chlorophyll a concentration, downwelling radiance and primary production in the Black Sea based on satellite data.

Bio-optical data, obtained during six cruises in the Black Sea carried out during periods of seasonal stratification in years between 1996 and 2016, have been used to parametrize phytoplankton light absorption ( $a_{ph}\left(\lambda \right)$) in the deep chlorophyll maximum (DCM) layer located near the bottom of euphotic zone. Relationships between $a_{ph}\left(\lambda \right)$ and the sum of chlorophyll-a and phaeopigment concentrations (Chl-a) differed from those for the summertime upper mixed layer (UML). Notably, chlorophyll a specific absorption coefficients ( $a_{ph}^{\ast }\left(\lambda \right)$) were lower in the DCM and more comparable with $a_{ph}^{\ast }\left(\lambda \right)$ values typical for winter phytoplankton in the Black Sea. The $a_{ph}\left(\lambda \right)$ spectral shapes in the DCM differed markedly from those in winter and in the summer UML, due to a shoulder at ~490 nm and a local maximum at ~550 nm corresponding to the absorption bands of phycourobilin and phycoerythrobilin. Light absorbing properties of phytoplankton in the DCM (amplitude and spectral shape of $a_{ph}^{\ast }\left(\lambda \right)$) reflected physiological acclimation to local conditions on the cellular level and population shifts leading to changes in the biomass-dominant species, with Synechococcus spp. domination in the DCM. The parameterization of phytoplankton absorption in the DCM will enable refined spectral models of the downwelling radiance and primary production in the Black Sea.

Climate affects the characteristics of the Southern Ocean ecosystem, including bio-optical properties. Remote sensing is a suitable approach for monitoring a rapidly changing ecosystem. Correct remote assessment can be implemented based on a regional satellite algorithm, which requires parameterization of light absorption by all optically active components. The aim of this study is to analyse variability in total chlorophyll a concentration (TChl-a), light absorption by phytoplankton, non-algal particles (NAP), coloured dissolved organic matter (CDOM), and coloured detrital matter (CDM = CDOM+NAP), to parameterize absorption by all components. Bio-optical properties were measured in the austral summer of 2020 according to NASA Protocols (2018). High variability (1–2 orders of magnitude) in TChl-a, absorption of phytoplankton, NAP, CDOM, and CDM was revealed. High variability in both CDOM absorption (uncorrelated with TChl-a) and CDOM share in total non-water absorption, resulting in a shift from phytoplankton to CDOM dominance, caused approximately twofold chlorophyll underestimation by global bio-optical algorithms. The light absorption of phytoplankton (for the visible domain in 1 nm steps), NAP, CDOM, and CDM were parametrized. Relationships between the spectral slope coefficient (S_CDOM/S_CDM) and CDOM (CDM) absorption were revealed. These results can be useful for the development of regional algorithms for Chl-a, CDM, and CDOM monitoring in the Southern Ocean.

The research was carried out based on the results of measuring the concentration of chlorophyll-a (TChl-a), spectral coefficient of light absorption by phytoplankton, non-algal particles (NAP) and colored dissolved organic matter (CDOM) in the southern part of the Sea of Azov in different seasons from 2016 to 2020. New data for the Sea of Azov on the variability of the spectral coefficient of light absorption by all optically active components have been obtained. A high (more than an order of magnitude) variability of all studied parameters was shown. The seasonal dynamics of TChl-a shows maxima in winter and summer. A relationship between the phytoplankton absorption and TChl-a has been revealed, which is described by a power function. Significant seasonal differences (two or more times) between the coefficients in the equation for parametrization of light absorption by phytoplankton were revealed. The NAP and CDOM light absorption were parametrized, the ranges of variability of the absorption coefficient and the exponential slope were revealed, and their seasonal dynamics was shown. Annual variability of absorption coefficient of NAP and CDOM was opposite to the annual cycle of TChl-a. The NAP and CDOM absorption coefficients increased after phytoplankton blooms, when TChl-a decreased. The relative contribution of phytoplankton, NAP, and CDOM to the total absorption by particles and dissolved matter at 440 nm varied significantly over the year. The share of phytoplankton was maximum in winter and summer, while in spring and autumn the maximum contribution to absorption was made by CDOM and NAP.