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This study is aimed at developing and verifying a method that uses a digital holographic camera to measure the gas volumetric flux, which is relevant for the monitoring of gas emissions, in particular methane in the Arctic seas. The method is based on the analysis of histograms of cross-sectional areas of gas bubbles and their velocities obtained from holographic data. The result of the study is the determination of a constant calibration factor k = 2, taking into account the geometric factor of the camera and the deformation of the bubbles. The coefficient is determined in laboratory conditions, taking into account the area of the gas-generating site of a bubble generator simulating a gas flare. It is found that k remains stable in a wide range of a gas volumetric flux from 5 × 10−4 m3·m−2·s−1 to 15 × 10−4 m3·m−2·s−1 that limits the applicability of a working formula. Verification of the method in the field conditions of the Arctic expedition showed good agreement with the data obtained by the standard trap method: the discrepancy was only 5%. It was shown that the method is applicable for quantitative assessment of weak gas emissions, in particular methane, in the Arctic seas, where the measured volumetric fluxes are orders of magnitude lower than the established upper limit of the method.

Monitoring the parameters and behavior of plankton makes it possible to assess the state of the aquatic ecosystem and detect the beginning of an environmental disaster at an early stage. In this respect, the most informative method for the in situ plankton study is underwater digital holography. This method allows obtaining information on the size, shape, and location of plankton individuals, as well as performing their classification and biotesting according to their behavioral responses using a submersible holographic camera non-invasively, in real time, and in the automatic mode. The monitoring series of the ecosystem functions can be used to assess the state of the ecosystem. One of them is the time series of the concentration of individuals of various plankton taxa in a certain volume. There are characteristic rhythms in the ecosystem function caused by both plankton biorhythms and changes in habitat parameters, as well as their synchronization, whereas a change in this rhythm may serve as an alarm signal for the ecosystem deprivation. By constructing the analogies based on the spectroscopy of atoms and molecules the paper shows the bioindication capabilities of the Fourier spectra of the plankton ecosystem function, built during monitoring measurements using a submersible digital holographic camera. The spectroscopic study of plankton allows determining the pollution in the plankton habitat at early stages. The in situ experimental data suggest that the order and chaos of plankton biocenosis are reflected in the structure of the spectral lines of the ecosystem functions associated with plankton. Various self-oscillatory processes in the biocenosis that regulate the plankton number and rhythm form the basis for plankton spectroscopy, which may be used for bioindication monitoring.

This study considers the applicability and effectiveness of the Viola–Jones method to automatically distinguish zooplankton particles from the background in images reconstructed from digital holograms obtained in natural conditions. For the first time, this algorithm is applied to holographic images containing coherent noise and residual defocusing. The method was trained on 880 annotated (marked) holographic images of Daphnia magna along with 120 background frames. It was then tested on independent laboratory and field datasets, including morphologically related taxa. With optimized settings, the precision of the algorithm reached ~90% and F1~85% on noisy holographic images, and the algorithm also demonstrated the preliminary ability to recognize similar taxa without retraining. The algorithm is well suited for analyzing holographic data as a fast and resource-efficient pre-filter—it effectively separates particles from the background and thereby allows subsequent classification or its application in real-time aquatic environment monitoring systems. The article presents experimental results demonstrating the efficiency of this algorithm during plankton monitoring in situ.

The paper presents an underwater holographic sensor to study marine particles—a miniDHC digital holographic camera, which may be used as part of a hydrobiological probe for accompanying (background) measurements. The results of field measurements of plankton are given and interpreted, their verification is performed. Errors of measurements and classification of plankton particles are estimated. MiniDHC allows measurement of the following set of background data, which is confirmed by field tests: plankton concentration, average size and size dispersion of individuals, particle size distribution, including on major taxa, as well as water turbidity and suspension statistics. Version of constructing measuring systems based on modern carriers of operational oceanography for the purpose of ecological diagnostics of the world ocean using autochthonous plankton are discussed. The results of field measurements of plankton using miniDHC as part of a hydrobiological probe are presented and interpreted, and their verification is carried out. The results of comparing the data on the concentration of individual taxa obtained using miniDHC with the data obtained by the traditional method using plankton catching with a net showed a difference of no more than 23%. The article also contains recommendations for expanding the potential of miniDHC, its purpose indicators, and improving metrological characteristics.

The measurements of plankton concentration performed on various-scale registration systems are quite different from each other. This work shows that these differences are caused by the spatiotemporal structure of plankton biocenosis, as well as the features of data processing methods that take into account the scale of measurements of the corresponding methods and registration tools. Hence, this leads to the modality and periodicity of the measured plankton concentration time series, whose parameters are consistent with environmental factors. Such modality and periodicity are studied in this work by analyzing the data obtained through a natural digital holographic experiment. The paper suggests the metrics of modality and periodicity of digital holographic data characterizing their informational value. It also shows the role of the segmentation of monitoring time series at the bioindication station. The proposed approach makes it possible to link the time scales of observations with the spatial scales of the studied plankton groups. Thus, we are able to determine the spatiotemporal frameworks of bioindication estimates, reasonably plan the measurement experiments, and correctly compare the data obtained using different tools.

The paper presents a diagnostic complex for plankton studies using the miniDHC (digital holographic camera). Its capabilities to study the rhythmic processes in plankton ecosystems were demonstrated using the natural testing in Lake Baikal in summer. The results of in situ measurements of plankton to detect the synchronization of collective biological rhythms with medium parameters are presented and interpreted. The most significant rhythms in terms of the correlation of their parameters with medium factors are identified. The study shows that the correlation with water temperature at the mooring site has the greatest significance and reliability. The results are verified with biodiversity data obtained by the traditional mesh method. The experience and results of the study can be used for the construction of a stationary station to monitor the ecological state of the water area through the digitalization of plankton behavior.

The study presents a bioindication complex and a technology of the experiment based on a submersible digital holographic camera with advanced monitoring capabilities for the study of plankton and its behavioral characteristics in situ. Additional mechanical and software options expand the capabilities of the digital holographic camera, thus making it possible to adapt the depth of the holographing scene to the parameters of the plankton habitat, perform automatic registration of the “zero” frame and automatic calibration, and carry out natural experiments with plankton photostimulation. The paper considers the results of a long-term digital holographic experiment on the biotesting of the water area in Arctic latitudes. It shows additional possibilities arising during the spectral processing of long time series of plankton parameters obtained during monitoring measurements by a submersible digital holographic camera. In particular, information on the rhythmic components of the ecosystem and behavioral characteristics of plankton, which can be used as a marker of the ecosystem well-being disturbance, is thus obtained.

The paper studies the influence of coherent noises on the quality of images of particles reconstructed from digital holograms. Standard indicators (for example, signal-to-noise ratio) and such indicators as the boundary contrast and boundary intensity jump previously proposed by the authors are used to quantify the image quality. With the use of these parameters, for examples of some known methods of suppressing coherent noises in a holographic image (eliminating the mutual influence of virtual and real images in in-line holography, and time averaging), the features and ranges of applicability of such correction were determined. It was shown that the use of the complex field amplitude reconstruction method based on the Gerchberg–Saxton algorithm and the spatial-frequency method improves the quality of determining the particle image boundary (by boundary intensity jump) starting from the distance between a hologram and a particle, which is about twice the Rayleigh distance. In physical experiments with model particles, averaging methods were studied to suppress non-stationary coherent noises (speckles). It was also shown that averaging over three digital holograms or over three holographic images is sufficient to provide a quality of particle image boundary suitable for particle recognition. In the case of multiple scattering, when it is necessary to impose a limit on the working volume length (depth of scene) of the holographic camera, the paper provides estimates that allow selecting the optimal working volume length. The estimates were made using the example of a submersible digital holographic camera for plankton studies.

The paper proposes an equivalent optical scheme of an in-line digital holographic system for particle recording and a mathematical model that establishes a one-to-one correspondence between the dimensional and spatial parameters of a digital holographic image of a particle and the imaged particle itself. The values of the model coefficients used to determine the real size and longitudinal coordinate of a particle according to its holographic image are found by calibration. The model was tested in field and laboratory conditions to calibrate a submersible digital holographic camera designed to study plankton in its habitat. It was shown that four calibration measurements are sufficient enough to determine the model coefficients, and the developed design of the submersible digital holographic camera makes it possible to perform these measurements during the recording of each hologram. In addition, this neither requires data on the refractive index of the medium with particles nor on the parameters of the optical elements of the scheme. The paper presents the results of marine experiments in the Kara Sea and the Laptev Sea, as well as in fresh water in laboratory conditions and in Lake Baikal. The error in measuring the particle size in seawater without the use of the model is 53.8%, while the error in determining their longitudinal coordinates is 79.3%. In fresh water, the same errors were 59% and 54.5%, respectively. The error in determining the position of a particle with the use of the designed mathematical model does not exceed 1.5%, and the error in determining the size is 4.8%. The model is sensitive to changes in the optical properties of the medium, so it is necessary to perform calibration in each water area, and one calibration is quite sufficient within the same water area. At the same time, the developed design of the submersible holographic camera allows, if necessary, calibration at each holographing of the medium volume with particles.

This paper considers the features of a time series of plankton concentrations, which are further compared with such phenomena as the alteration of day and night and tidal processes. The analysis of experimental data recorded as a result of long-term monitoring measurements under field conditions showed that the diurnal variability in plankton concentrations can be described using a model harmonic function. At the same time, based on the parameters of the diurnal variability model, it is possible to build a bioindication system to detect the influence of abnormal environmental factors estimated as pollution. This study discusses the ideology of building such a system based on regular observations of the behavior of autochthonous plankton using a submersible digital holographic camera.