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.

In this paper, we propose to analyze the values of the “blue” color index for further use in additional atmospheric correction of Level 2 remote sensing reflectance data for the waters of the Black Sea. Regardless of seasonal phenomena, atmospheric conditions, and the type of water, the average color index in the short-wave region, according to in situ measurements CI(412/443), varies from 0.77 to 0.83. The most frequently observed value is 0.8. In turn, the values of the “blue” color index CI(412/443) according to the satellite data of MODIS Aqua/Terra, VIIRS SNPP, and OLCI Sentinel 3A scanners showed a large scatter in values based on the standard deviation of the sample. The paper proposes to introduce the value of the minimum allowable threshold CI(412/443) > 0.59 based on the small variance found from in situ measurements, as well as on the basis of a theoretical estimate of the possible values of the index CI(412/443) when varying the backscattering exponent and the exponent for the absorption approximation. The quality check of the remote sensing data showed that, according to this selection criterion, 15% of data are physically incorrect for MODIS Aqua, 30% for MODIS Terra, 20% for Sentinel 3A, and 26% for VIIRS SNPP. In the course of the work, it was shown that the MODIS Aqua satellite provides the most high-quality and reliable information about the optical characteristics of the Black Sea.

In August 2020, during the 80th cruise of the R/V “Akademik Mstislav Keldysh”, the chlorophyll a concentration (Chl-a) and spectral coefficients of light absorption by phytoplankton pigments, non-algal particles (NAP) and colored dissolved organic matter (CDOM) were measured in the Norwegian Sea, the Barents Sea and the adjacent area of the Arctic Ocean. It was shown that the spatial distribution of the three light-absorbing components in the explored Arctic region was non-homogenous. It was revealed that CDOM contributed largely to the total non-water light absorption (a_tot(λ) = a_ph(λ) + a_NAP(λ) + a_CDOM(λ)) in the blue spectral range in the Arctic Ocean and the Barents Sea. The fraction of NAP in the total non-water absorption was low (less than 20%). The depth of the euphotic zone depended on a_tot(λ) in the surface water layer, which was described by a power equation. The Arctic Ocean, the Norwegian Sea and the Barents Sea did not differ in the Chl-a-specific light absorption coefficients of phytoplankton. In the blue maximum of phytoplankton absorption spectra, Chl-a-specific light absorption coefficients of phytoplankton in the upper mixed layer (UML) were higher than those below the UML. Relationships between phytoplankton absorption coefficients and Chl-a were derived by least squares fitting to power functions for the whole visible domain with a 1 nm interval. The OCI, OC3 and GIOP algorithms were validated using a database of co-located results (day-to-day) of in situ measurements (n = 63) and the ocean color scanner data: the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Terra (EOS AM) and Aqua (EOS PM) satellites, the Visible and Infrared Imager/Radiometer Suite (VIIRS) onboard the Suomi National Polar-orbiting Partnership (S-NPP) and JPSS-1 satellites (also known as NOAA-20), and the Ocean and the Land Color Imager (OLCI) onboard the Sentinel-3A and Sentinel-3B satellites. The comparison showed that despite the technological progress in optical scanners and the algorithms refinement, the considered standard products (chlor_a, chl_ocx, aph_443, adg_443) carried little information about inherent optical properties in Arctic waters. Based on the statistic metrics (Bias, MdAD, MAE and RMSE), it was concluded that refinement of the algorithm for retrieval of water bio-optical properties based on remote sensing data was required for the Arctic region.

В отличие от контактных методов, использование спутниковых данных для изучения динамики хлорофилла a даёт возможность проводить исследования на больших масштабах с высокой частотой измерений. Такие наблюдения выполнены в глубоководной части и прибрежном районе Чёрного моря в период с 1998 по 2015 г. Они позволили определить годовую динамику концентрации хлорофилла a в поверхностном слое и выявить периодичность, интенсивность и продолжительность массового развития водорослей. Годовая динамика концентрации хлорофилла в западном и восточном циклонических круговоротах, а также в зоне шельфа у крымских и кавказских берегов имела одинаковый характер, повторяющийся из года в год. С августа-сентября по апрель-май следующего года изменение хлорофилла в большинстве случаев соответствовало нормальному распределению и имело вид колоколообразной кривой с максимумом в декабре-январе. В течение осеннего периода значения концентрации хлорофилла постепенно повышались по мере охлаждения воды, ослабления сезонного пикноклина и увеличения скорости потока биогенных веществ из нижних слоёв к поверхности. Зимой глубина перемешиваемого слоя достигала значений, в несколько раз больших, чем осенью, что могло приводить к уменьшению концентрации хлорофилла в единице объёма воды. Весеннее интенсивное развитие фитопланктона наблюдалось в конце марта ― начале апреля при ослаблении конвективного перемешивания и повышении устойчивости водного столба.