The horizontal and vertical chlorophyll-a concentrations (Chl) distributions along the ship route of 79^th cruise of R/V “Akademik Mstislav Keldysh” in the Atlantic Ocean in December 2019—January 2020 were obtained. Analysis of the observations was conducted for the biogeochemical provinces which were determined by means of the different hydrometeorological conditions for the phytoplankton community functioning.

Three methods of Chl measurements were applied: standard extraction method of the seawater samples, the fluorometer underway measurements in the flow-through water system at the depth of 5 m and in-situ profiling fluorometer measurements at stations. Also, measurements of the seawater temperature and salinity in the flow-through water system, the CTD and optical profiling, and ARGO floats were used.

The maximum Chl values in the near-surface ocean layer are observed at zones of divergence in the Canary upwelling systems (CNRY) (1.3 mg/m³) and in the Falkland (Malvinas) Current system (4.8 mg/m³)^, and also in the Drake Strait (1.2 mg/m³)^. The least Chl values (0.01–0.12 mg/m³) are observed in the tropical, equatorial areas and in the South Atlantic Subtropical Anticyclonic Gyre (SASTG) at the ship route.

Vertical Chl profiles with absence of the significant Chl maximum were found. In the CNRY region the Chl values were not changed much within the euphotic zone. While in the region of the Antarctic Circumpolar Current in the Drake Strait and in the offshore stream of the Falkland Current the quasi-homogeneous Chl profiles were observed in the upper layer which is almost 2 times deeper than the euphotic zone.

In the profiles with the significant local Chl maximum the most Chl value of 3.5 mg/m^{3 i}s observed at the depth about 12 m in the upwelling region on the Patagonian Shelf. In other regions the depth of Chl maximum localization (0.2–0.9 mg/m³) varies widely from 25 to 120 m. In the western part of the SASTG region the absolute Chl maxima were found at the depth of 120 m, that is deeper, then the euphotic zone which is equal to 100 m.

This study proposes a method for obtaining information on the salinity of the Sea of Azov, based on the use of contact and remote sensing data. The approach to the salinity fields recovery is based on obtaining generalized regression equations relating in situ archival data with regional biooptical products obtained from standard level-2 MODIS products. This analysis showed the possibility of using various approaches to obtain generalized empirical (regression) equations for the spring and summer seasons, the differences in which are ~10 %. The results of the recovered salinity values were verified using in situ data. It was found that the plots of the average values of the recovered salinity are in the region of 95 % of the confidence bands of the modern long-term average trends for 1986–2018 and 2000–2018. The possibility of using the results of the proposed method in the construction of spatial maps of the Azov Sea salinity, synchronized in time with satellite scenes, is shown.

In this paper we discuss the problem how to analyze the results of spectral volume scattering function measurements in natural waters and artificial ones. Analytical predictions and experimental proves of the remarkable changes in the properties of the light scattering at small angles after a filtration of water sample are presented. It was proposed that a certain correlations in the positions of scattering centers always exist in the water which were resulted in the volume scattering function of any natural water was strong forward peaked. Based on Mie theory the sums of scattering amplitudes and scattering radiances were calculated for a power-law particle size distribution while the maximal radius was varied. The square of the sum of the scattering amplitudes was compared with the value of the classical term. It is shown that in the “blue” a significant compensation of the effect of spatial correlation of particles occurs with an increasing of the maximum radius of particles, starting from the radius is about 0.2 μm. The contribution of the additional term decreases faster at general angles region. The spectral properties of light scattering by seawater during sequential filtration through nucleopore filters with a pore diameter of 2, 1, and 0.45 μm have been measured. The volume scattering function data at scattering angles from 6 to 90 degrees were analyzed. Water samples were taken from the surface of the Black Sea at the beginning of October. The Secchi depths are varied from 16 up to 19m. It was found that after filtration through a filter with pores diameter equaled to 1 μm, the typical deep at 30° of spectral slope of volume scattering function was appeared. Such deep becomes stronger after the next stage of filtration, when diameter of pores was equal to 0.45 µm. It is shown that the angular dependence of the exponent of the spectral slope obtained as a result of processing the experimental data is in consistent with theoretical predictions.

The paper presents an analysis of field measurements of the spectral reflectance of the Black Sea and the bio-optical and colorimetric characteristics calculated from it, as well as chlorophyll concentrations and the Secchi disk depth. For the regions of the Black Sea with different characteristics, the averaged values of the calculated characteristics are presented: the effective and dominant wavelength in the reflectance spectrum, the purity of the water color. Using the previously developed bio-optical algorithm, the concentration of chlorophyll, absorption by dissolved organic matter, backscattering by suspended matter, and the vertical light attenuation coefficient were calculated.
Inverse relationships are established between the Secchi disk depth and the maximum reflectance, as well as between the Secchi disk depth and the dominant wavelength. Linear relationships with high correlation coefficients are obtained between the surface chlorophyll concentration and the effective and dominant wavelength. A significant relationship was also established between the surface chlorophyll concentration and the maximum reflectance. Among the relationships between bio-optical and colorimetric parameters, a high correlation is observed between the vertical light attenuation coefficient and color purity.
Chlorophyll concentrations calculated from the data of the spectral reflectance coefficient using a semi-analytical algorithm are in good agreement with the data of field measurements. Comparison of calculated and measured chlorophyll concentrations with satellite products of optical ocean color scanners shows an overestimation of chlorophyll concentrations by satellite data processing algorithms. The obtained correlations can be used in the future to assess bio-optical parameters in the northern part of the Black Sea.

The article describes results of the suspended particulate matter during 80^th cruise of the RV Akademik Mstislav Keldysh in the Franz-Victoria Through, northern Barents Sea. Two perpendicular transections were carried out by means of small-angle laser diffractometer LISST-Deep operating in the red range with a wavelength of 670 nm. Observed data consist 7 thousand volume concentration and particle size distributions (PSD) measurements. Weather conditions during expedition allowed to extend the transection to southern part of Nansen basin. The volume concentration of suspended matter varied up to 7.58 μl/l in the euphotic zone and layer and from 0.2 to 0.5 μl/l in the nepheloid layer. Paper demonstrates construction possibilities of the particle size distributions profiles by depth. Transection comparison with measurement results by the conductometric method on a Coulter Counter of the bathometric samples were carried out.

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.

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.

The complex processing of data from the shipborne lidar survey of the Black Sea coastal areas was carried out. In this area, internal waves are often observed according to contact and satellite observations. The source of probing linearly polarized laser pulses in the lidar is a solid-state laser with diode pumping of the YAG: Nd active element with a pulse power of 20 mJ at a wavelength of 532 nm and a duration of 7 ns. The co- and cross-polarized components of the echo signal were recorded during the lidar survey. The cross-polarized component of the echo signal was mainly used due to its greater sensitivity to various kinds of inhomogeneities in the vertical distribution of hydrooptical characteristics. The purpose of processing is to identify quasi-periodic structures in the spatial distribution of the characteristics of lidar echo signals caused the propagation of short-period internal waves. Three processing methods were applied: the approximation method, the wavelet analysis method, and the Hilbert-Huang transform method. A large array of data obtained during 50 hours of lidar survey has been processed. Three 60-minute tracks containing quasi-periodic structures have been identified. The three processing methods results are in good agreement with each other and make it possible to obtain the most complete information about the parameters of the studied processes. The parameters of the recorded quasi-periodic processes are typical for internal waves observed in the coastal areas of the Black Sea. In the future, when processing a large amount of lidar sounding data, it is advisable to carry out fast processing by automated spectral methods at the first stage, and to carry out detailed processing by the approximation method only for those survey areas where quasi-periodic processes have been identified.

It is noted that the high instrumental accuracy of modern marine spectrophotometers for reflectance measurements is difficult to implement due to large methodological errors caused by the sea surface waves by weather conditions and illumination geometry. A two-beam spectrophotometer based on the use of a multi-channel photomultiplier eliminating the disadvantages is proposed. The design and operation principle of a spectrophotometer, developed on the basis of a compact high-aperture monochromator and a photometric unit, in which the channels of the upwelling radiation and the irradiance of the sea surface are located, are described. In the instrument, instead of absolute measurements, the light fluxes are compared in a differential photometer instead of absolute measurements in order to reduce methodological errors. The use of a differential photometer, which measures the spectral reflectance of water, can significantly increase the measurement accuracy, because does not require absolute calibration of optical channels. The use of a multichannel photomultiplier leads to an additional decrease in methodological errors by reducing the time of measurements of the seawater spectral reflectance and makes it possible to select a sufficient number of spectra in a large volume of data without interference from waves and lighting conditions. The photomultiplier signals are digitized using a compact USB ADC/DAC module with digital input/output functions. Amplified signals from photocathodes are received at the module input, the module output is connected to the USB input of a computer recording signals and controlling the spectrophotometer work.

The wavy surface effect on the resolution of optical imaging systems employing narrow modulated probing beams is studied. Relations are formulated connecting the complex amplitudes of the photon density waves (propagating through the wavy interface and the water layer) to the main parameters of the problem. These relations consider the finite waves height and the change in the photon trajectory length due to random refraction of rays upon entering the water. The optical transfer function of the wavy surface and the beam scattering function averaged over the ensemble of surface waving realizations are introduced. The dependences of these functions on the surface waves integral parameters, namely, the elevations and slopes dispersions, are examined. The contributions of surface waving and water layer to the generation of optical transfer functions and to the overall signal level from an underwater object when it is imaged using photon density waves are estimated. It is shown that in a certain range of depths, spatial frequencies, and illumination beam modulation frequencies, the systems employing photon density waves can demonstrate advantages over the systems with stationary illumination.  

The article is dedicated to the memory of Oleg Viktorovich Kopelevich, Doctor of Physical and Mathematical Sciences, who has been conducting research in the field of ocean optics for more than 50 years and has been the head of the Ocean Optics Laboratory of the Shirshov Institute of Oceanology for more than 30 years. The paper presents the main results obtained by Dr. Kopelevich in different sections of ocean optics. At the first stage of his scientific activity, he was engaged in the study of light scattering in seawater. He identified four types of seawater scattering phase functions based on the results obtained in ocean expeditions. Subsequently, Dr. Kopelevich developed a low-parameter model of the seawater inherent optical properties. Since the late 1970s, an important place in his work has been the development of satellite methods for studying the ocean, in particular, the creation and improvement of regional bio-optical algorithms. The use of these algorithms for the waters of the Russian seas became the basis for the creation of the Atlas of their bio-optical characteristics, published regularly since 2002. The results of the use of regional algorithms were summed up in a collective monograph published in 2018. In addition, O.V. Kopelevich contributed to the study of light fields and the factors affecting them — cloud cover and mass phytoplankton blooms.