Sergey V. Sheberstov

The aim of the present study is to develop a method for calculating the diffuse component of the bidirectional reflectance of a strongly absorbing and highly-forward scattering medium like seawater. The relevance of this issue is due to the widespread use of the bidirectional reflectance in the processing of ocean color measurements. Explicit expressions for the first two terms of the expansion are derived. The first term reproduces the well-known result of quasi-single-scattering approximation, and the second corresponds to the contribution of quasi-double scattering (the small-angle multiple scattering before, between, and after two events of large-angle scattering). To validate our analytical results, we carry out numerical integration of the radiative transfer equation for the Henyey-Greenstein phase function, as well as for the two-term Henyey-Greenstein function that models light scattering in seawater. It is shown that for optical parameters typical to seawater, the first two terms of the expansion (i. e. the sum of quasi-single- and quasi-double-scattering contributions) prove to be sufficient to describe the bidirectional reflectance with high accuracy over a wide range of illumination/viewing angles.

Using numerical methods for solving the radiation transfer equation, ocean albedo values were calculated for a set of bio-op tical characteristics corresponding to situations with different chlorophyll concentrations (1 μg/L and 10 μg/L) and the case of intense coccolithophore bloom (8–12 million cells/L). Calculations were carried out in the spectral range of 280–2800 nm for cases of cloudless sky at various wind speeds and atmospheric transmission. It has been shown that for Case 1 waters, a change in chlorophyll concentration from 1 to 10 μg/L does not lead to changes in albedo. In the case of intense coccolithophore blooms, the ocean albedo can increase more than threefold. Calculation of average monthly albedo values for selected points in the Barents and Black seas showed that the presence of intense coccolithophore blooms significantly increases average monthly albedo values. The calculation of the values of radiation absorbed in the seawater column depending on the time of day, carried out for these points, demonstrated that the presence of blooms significantly reduces the values of absorbed radiation. It is shown that the contribution to the albedo of radiation emerging from water used in the state-of-the-art NEMO numerical ocean model, amounting to 0.005 ± ± 0.0005, corresponds only to Case 1 waters. Intense coccolithophore blooms can increase this contribution by more than 14 times. A simple formula is proposed for correcting albedo values taking into account the influence of bio-optical characteristics.

The paper presents the results of testing the possibility of using empirical orthogonal functions to develop algorithms for estimating the concentration of chlorophyll a and suspended matter, the biomass of cyanobacteria in the eastern part of the Gulf of Finland. To develop the algorithms, we used an array of data from field measurements of the subsurface radiance reflectance in 2012–2014, carried out simultaneously with the determination of bio-optical characteristics. It turned out that in the case of the concentration of chlorophyll a and suspended matter, such algorithms can be created not only using the hyperspectral radiance reflectance, but also for the spectral channels of satellite color scanners MODIS and OLCI. An estimation of the cyanobacteria biomass with the empirical orthogonal functions method is not applicable in the case of using satellite channels. A study of the possibility of the most prone to atmospheric correction errors shortwave MODIS channels exclusion was also made. It turned out that the concentration of chlorophyll a is more sensitive to such exclusion than the concentration of suspended matter. Validation on a MODIS data showed that empirical orthogonal functions algorithms give results no worse than regression ones.

Interannual variability (2002–2018) of primary production, surface seawater temperature (T₀), photosynthetically active radiation (PAR) and ice cover area in the East Siberian Sea (ESS) was studied based on the data of the MODIS-Aqua ocean color scanner. The annual total primary production (PP_tot) decreased by 1.7 TgC over 17 years. The multi-year negative trend in PP_tot was 0.1 TgC y^–1 (1.38% y^–1). The decrease in PP_tot was accompanied by the decrease in the area-specific primary production (IPP). Over the investigated period the annual mean IPP decreased by 13 mgC m^–2 d^–1. The multi-year negative trend in IPP was 0.76 mgC m^–2 d^–1 y^–1 (0.84% y^–1). The decrease in PP_tot and IPP was accompanied by statistically insignificant increase in T₀ (R² = 0.09) and statistically significant decline in PAR (R² = 0.29). Over the 17 years, T₀ in ESS increased by 0.68°С (0.04°С y^–1 or 3.2% y^–1) and PAR declined by 3.4 Ein m^–2 d^–1 (0.2 Ein m^–2 d^–1 y^–1 or 1.9% y^–1). The ice-free area slightly diminished during the study period (R² = 0.10) by 64.94×10³ km². The statistically insignificant negative multi-year trend in the ESS ice-free area was 3.82 × 10³ km² y^–1 (0.87% y^–1). Long-term variability in PP_tot in ESS from 2002 to 2018 depended not only on the decrease in the ice cover but also on PAR level and IPP.

The extended in situ measurements of spectral radiometric characteristics were carried out in the South-Eastern Baltic jointly with satellite data of ocean color scanners MODIS and MERIS and in situ data on the chlorophyll concentration (Chl) and total suspended matter concentration (TSM). Due to the research new corrected bio-optical algorithms of satellite data have been developed providing better accuracy of Chl and TSM estimation in comparison with the standard algorithms.

The data of the color satellite ocean scanners SeaWiFS and MODIS-Aqua provide evidence that a sharp increase in the backscattering coefficient of the suspended particles occurred within a...