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Microplastics (MPs) are recognized as a global emerging threat to aquatic ecosystems and biodiversity worldwide. Though the number of publications and interest to the MP research have been increased rapidly, it is still hamper to compare the obtained data due to the usage of different methodologies in MP assay. Thus, there is an urgent need for a standardized approach to the procedures of MP quantification in order to produce comparative assessments. In this pilot study, the conventional NOAA protocol of MP extraction from seawater was combined with a simple and inexpensive method for analyzing shape and size spectrum of all MP particles making up the sample. A common flatbed scanner equipped with slide adapter was applied for image acquisition while MP dispersive properties (particle abundance, shape and size spectrum) were quantified using ImageJ software. Feret’s diameter and circularity (or roundness) appeared to be the most efficient shape descriptors for the particle analysis. The total silhouette area of MP particles was shown to produce a confident approximation of the MP overall mass. The first reliable estimates of MP concentrations in the Black Sea coastal waters (Sevastopol Bay) accounted for 0.6 to 7 items m-3 and 6 to750 µg m-3 in terms of abundance and mass, respectively. No steady-state gradients have been revealed in MP distribution along the transect from the mouth of the bay to its corner. Inflow of MP to the bay waters and their transport along the bay seemed to be controlled by a complex combination of factors including rainfalls, wind regimes, currents and the Black river discharge.

To assess microplastics abundance in the aquatic environment different filtering tools (nets, meshes and filters) are generally applied. As a result, their naturally occurring size-frequency distributions are altered because of removing of items smaller than the mesh size. This hampers comparison of the results between studies utilized filters and nets of different mesh size. To assess the process of MPs generation and filtration, a stochastic model of macroplastic destruction coupled with the model of MPs filtration on the net was proposed. The stochastic model of macroplastic destruction incorporates empirical parameterizations of fracture position and fracture probability as a function of particle shape. The model sensitivity was tested in respect to the input parameters: the initial number of particles, the initial size of the macroparticles, the number of fracture steps (number of generations, i.e. the final ‘age’ of the particles), and the number of independent ‘sources’ in the final particle set.The simulation results were compared with the available publications as well as with the data collected at Lake Onego. This allowed us to achieve qualitative agreement between the modelled and the observed distributions based on the similarity of the shape of size-frequency distribution curve in log-log scale.Large particles, which have all three dimensions larger than the mesh size, are retained by the net efficiently. For others the probability of particle retention by the net depends on the particle shape, smallest and largest dimensions, and particle orientation in space.To simulate the actual filtration process on the net, a mathematical model of filtration process was developed. The model passes a given set of three-dimensional particles (in quasi-elliptic approximation) through a two-dimensional net with a given cell size. Randomly given parameters determine the position of a particle in space, the size of its projection on the two-dimensional plane (net), and the position of the particle centre with respect to the corners of the cell (net). The results of plastic breaking simulation were coupled with a stochastic MP particle filtration model. This allowed us to show qualitatively how the shape of size-frequency distribution of MP particles is altered after the filtering through the net with a mesh size close to the lower boundary. This information can be used to compare the results obtained in studies utilized different neuston nets and filters, which is one of the most relevant tasks in the assessment of environmental contamination by microplastics on a global scale.The study was supported by the Russian Science Foundation grant number 19-17-00035.

Abundance, chemical composition and ecological risk of microplastics (MPs) in terrestrial and marine environments have merited substantial attention from the research communities. This is the first attempt to comprehend the ecological risk of MPs in sediments along the Indian coast using meta-data. Polymer hazard index (PHI), pollution load index (PLI) and potential ecological risk index (PERI) were used to evaluate the quality of sediments. Areas have high PHI values (1000) due to the presence of polymers with high hazard scores such as polyamide (PA) and polystyrene (PS). According to PLI values, sediments along the west coast of India (WCI) are moderately contaminated with MPs (PLI: 3.03 to 15.5), whereas sediments along the east coast of India (ECI) are less contaminated (PLI: 1 to 6.14). The PERI values of sediments along the Indian coast showed higher ecological risk for the metropolitan cities, river mouths, potential fishing zones and the remote islands.

Combination of altimetry-based method of eddy identification and historical hydrological measurements for 1992–2015 is used to analyze the thermohaline and dynamic structure of the Black Sea eddies and its relation with eddy intensity, eddy age and season of a year. Anticyclonic eddies (AEs) are characterized by negative salinity anomalies, which can reach −1.7 psu at the depth of the main halocline. The temperature anomalies are positive in their upper layers, and negative in the deeper layers, because of the vertical displacement of the waters of the Cold Intermediate Layer (CIL). Cyclonic eddies (CEs) have the opposite structure with increased salinity, colder upper layers and warmer deeper layers. Thermohaline anomalies in the eddies of both signs are maximal in summer, while in winter they are shallowest and minimal. The displacement of pycnocline in eddies causes the decrease/increase of stratification in the upper layer of AEs/CEs and opposite increase/decrease in their deeper layers. It also causes the deepening/uplift of the layer of maximum geostrophic vertical shear in AEs/CEs. The latter is the probable reason of the observed higher intensity and deeper penetration of orbital velocities in AEs than in CEs. The changes of isopycnals positions during the eddies’ lifetime are used to quantify the evolution of vertical velocity in AEs and CEs. In the beginning of AEs life during intensification phase, vertical velocity is directed downward, while during the decaying phase it change its sign and is directed upward. The opposite is observed in CEs. Vertical velocity is maximal at the pycnocline depth of 100–110 m with values changing from (−8 to 8) ∗ 10−6 m/s in AEs, and from (+5 to −25) ∗ 10−6 m/s in CEs. Eddies thermohaline structure and altimetry-derived orbital velocity is tightly related. This relation obtained in the study and altimetry-derived data on the distribution of eddy frequency, translational speed and orbital velocity is used to quantify eddies salt, heat content and transport in the basin. The transport velocity of water in the eddies core (2–4 cm/s) is significantly smaller than the average velocity of the large-scale currents (∼10–40 cm/s). Such slowing causes the “relative” transport of eddies against the mean flow direction. This effect leads to the accumulation of brackish and cold water in the deep layers of east Black Sea and maintain the observed east-west asymmetry of the basin thermohaline fields.

The parameters of barotropic and baroclinic oscillations in the subinertial range have been obtained by numerical simulations of the Black Sea circulation using a model developed at the Marchuk Institute of Numerical Mathematics, Russian Academy of Sciences. The structures and periods of barotropic and baroclinic seiches are in good agreement with the known theoretical predictions and results of numerical modeling of seiches obtained for the Black Sea subbasins.

Microlitter (0.5-5mm) concentrations in water column (depth range from 0 to 217.5m) of the main Baltic Proper basins are reported. In total, 95 water samples collected in 6 research cruises in 2015-2016 in the Bornholm, Gdansk, and Gotland basins were analysed. Water from 10- and 30-litre Niskin bathometers was filtered through the 174μm filters, and the filtrate was examined under optical microscope (40×). The bulk mean concentration was 0.40±0.58 items per litre, with fibres making 77% of them. Other types of particles are the paint flakes (19%) and fragments (4%); no microbeads or pellets. The highest concentrations are found in the near-bottom samples from the coastal zone (2.2-2.7 items per litre max) and from near-surface waters (0.5m) in the Bornholm basin (5 samples, 1.6-2.5 items per litre). Distribution of particles over depths, types, and geographical regions is presented.

Distribution of microplastics particles (MPs) in the water column is investigated on the base of 95 water samples collected from various depths in the Baltic Sea Proper in 2015–2016. Fibres are the prevalent type of MPs: 7% of the samples contained small films; about 40% had (presumably) paint flakes, while 63% contained coloured fibres in concentrations from 0.07 to 2.6 items per litre. Near-surface and near-bottom layers (defined as one tenth of the local depth) have 3–5 times larger fibre concentrations than intermediate layers. Laboratory tests demonstrated that sinking behaviour of a small and flexible fibre can be complicated, with 4-fold difference in sinking velocity for various random fibres' curvature during its free fall. Numerical tests on transport of fibres in the Baltic Sea Proper were performed using HIROMB reanalysis data (2007) for the horizontal velocity field and laboratory order-of-magnitude estimates for the sinking velocity of fibres. The model takes into account (i) motion of fibres together with currents, (ii) their very slow sinking, and (iii) their low re-suspension threshold. Sensitivity of the final distribution of fibres to variations of those parameters is examined. These experiments are the first step towards modelling of transport of fibres in marine environment and they seem to reproduce the main features of fibres distribution quite well.

The Black Sea shelf is a region of intense manifestation of various dynamical processes. Under the influence of different natural forces, eddy-wave phenomena develop here, which influence the general circulation of sea waters, biological productivity, and the condition of the engineering structures. Modern numerical models allow us to simulate and analyze the processes of the joint dynamics of marine circulation and large-scale waves. In this work, we study the spatiotemporal spectral characteristics of the sea level and velocity fluctuations formed due to atmospheric forcing and tidal potential. The hydrophysical fields are calculated using the Institute of Numerical Mathematics, Russian Academy of Sciences (INM RAS), σ model based on primitive equations. We use the CORE data as atmospheric forcing at the sea surface; the tidal potential is described by the semidiurnal lunar constituent M2. Analyzing the simulation results makes it possible to emphasize that accounting for the semidiurnal tidal potential not only improves the accuracy of the sea-level calculation at coastal stations, but also generates subinertial baroclinic oscillations previously found in the Black Sea from the data of in situ observations.

The transition from winter vertical mixing to the formation of the spring thermocline in the southeastern Baltic Sea is studied based on data from the hydrophysical measurements program (11 expeditions) in the Russian part of Gdansk Bay in March–June 2010, 2011, and 2013. CTD measurements were taken along the standard 18-km transect across the isobaths with a 500-m step abeam the city of Baltiysk. A set of frequently measured data was collected in a 1–2 week interval from the end of March to the beginning of May, which made it possible to analyze the transformation of the vertical thermal structure of water from inverse winter type to the summer stratification with the transition of temperature over the temperature of the density maximum. Series of repeated measurements at the deep and coastal stations as well as surface and subsurface towed measurements were carried out. The fact that lenses of freshened warmer water appear at the surface almost simultaneously with intensification of cold intrusions in intermediate (10–40 m) layers makes it possible not only to confirm the advective nature of the formation of the spring thermocline in the Baltic Sea, but also to hypothesize about the intensification of intrabasin exchange when winter-time vertical mixing ceases: the potential energy excess supported by vertical mixing in the 60-m upper quasi-homogeneous layer (UQL) of the Baltic Proper, in which the horizontal estuarine salinity gradient is significant, is converted to kinetic energy of exchange currents as the mixing process terminates. Such water dynamics makes it possible to explain the intensification of intrusions in the Baltic in spring and the formation of the cold intermediate layer due to the fast propagation of late-winter UQL water from the Bornholm Basin to the Baltic Proper. The results agree well with earlier published studies of other authors.

Purpose. The purpose consists in a combined analysis of satellite observations of surface manifestations of internal waves near the Crimean coast and the results of numerical simulation of influence of seasonal thermohaline conditions and the relief on their structure, dynamics and intensification. Methods and Results. Based on the analysis of remote sensing data using high-resolution sensors of the Landsat-8 and Sentinel-2 satellites and the theoretical estimates, the main spatial and temporal characteristics of the internal waves on the Black Sea shelf near the Heracles Peninsula were determined. According to the temperature and salinity data obtained from the satellite measurements and the research vessels measurements in 1951–2008 from the Oceanographic Data Bank of Marine Hydrophysical Institute, the structure of density stratification was investigated, and buoyancy frequency profiles in the shelf and slope area from Yevpatoria to Yalta were obtained. Vertical velocity profiles of internal waves of the first three modes on the shelf were constructed. It was revealed that phase velocity of the internal waves of the first mode in the deep-sea part varied within the range of 2.6–5 m/s, the waves of the second mode – within 1.1–2.3 m/s, and the waves of the third mode – within 0.7–1.4 m/s. The average length of the waves detected from the satellite data was 0.4 km; the longest waves, about 1.1 km, were observed most often between Yevpatoria and Sevastopol, propagating predominantly to the northeast. Within the same train, wave dispersion occurred resulting in the wavelength diminution to 0.1–0.3 km. Conclusions. The stated assumption on the cause of generation of intense internal waves conditioned by the interaction of the Rim Current jet with the shelf edge was confirmed by the results of numerical calculations. Spatial and temporal characteristics of the internal waves, the integrated data of remote sensing and the modeling results make it possible to estimate vertical exchange at the shelf and to determine the depth of the maximum Brunt–Väisälä frequency.

The paper considers the internal waves on the shelf of the Crimean coast, their structure and dynamics at the periphery of the Black Sea basin. Manifestations of internal waves were studied by numerical experiments and by processing of remote sensing data from high-resolution data. Maps of the vertical velocity of the first three modes of internal waves on the shelf of the Crimean coast were constructed. The phase velocity of the internal waves in the deep part of the sea for the first mode at the given profiles varies from 2.6 to 5 ms\(^{-1}\); for the waves of the second mode—1.1–2.3 ms\(^{-1}\); for the waves of the third mode—0.7–1.4 ms\(^{-1}\). Wavelengths detected from the satellite data had a mean value of 0.4 km. The longest waves, around 0.8 km, were observed between Sevastopol and Yevpatoria. During June, 2017 the waves of \(2^{nd}\) and \(3^{rd}\) modes were observed in the area of study the most frequently.

Thermohaline intrusions are a typical feature of the Baltic Sea water body. We report observations of vivid colder/saltier intrusion activity in intermediate layers and upper pycnocline of the Gdansk Bay (the south-eastern part of the Baltic Sea) in early spring (March–April, 2013). Extremely low water temperature (down to 1.4–2 °C) and specific water salinity (SA7.5–7.8) of the intrusions point at a possible area of the intrusion waters formation: within the upper mixed layer in the Arkona/Bornholm basins at the end of March. Data on water temperature and salinity available from the International Council for the Exploration of the Sea (ICES) data repository, data of the Arkona Becken automated station, meteorological information, and remote sensing data for March–April 2013 are used to confirm the conclusions. It is confirmed once again that the transfer from the two-layered winter water stratification to the three-layered summer one in the Baltic Sea is a consequence of the sea-scale exchange process. The latter is manifested as simultaneous advection in upper and intermediate layers of waters with lower and higher salinity, correspondingly. The reported extremely cold intrusions indicate the beginning of the formation of the Cold Intermediate Layer (CIL), which thus contains waters of the Arkona/Bornholm basins. The main environmental factors driving the observed vivid intrusions in April 2013 are easterly winds and negative buoyancy fluxes due to seasonal solar heating of waters with the temperature below that of the density maximum. Substantial prolongation of the period of seasonal vertical mixing might be the most important process related to the transition of water temperature over the temperature of the density maximum (Tmd) in the Baltic Sea. With the climate warming, the cooling of surface waters below the Tmd becomes rare, limiting deep ventilation of Baltic waters in spring.

Simplified physical models and geometrical considerations reveal general physical and dynamical properties of microplastic particles (0.5–5 mm) of different density, shape and size in marine environment. Windage of extremely light foamed particles, surface area and fouling rate of slightly positively buoyant microplastic spheres, films and fibres and settling velocities of negatively buoyant particles are analysed. For the Baltic Sea dimensions and under the considered idealised external conditions, (i) only one day is required for a foamed polystyrene particle to cross the sea (ca. 250 km); (ii) polyethylene fibres should spend about 6–8 months in the euphotic zone before sinking due to bio-fouling, whilst spherical particles can be retained on the surface up to 10–15 years; (iii) for heavy microplastic particles, the time of settling through the water column in the central Gotland basin (ca. 250 m) is less than 18 h. Proper physical setting of the problem of microplastics transport and developing of physically-based parameterisations are seen as applications.

Mechanical fragmentation of four commonly used plastics, from 2-cm squares or cubes to microplastics (MPs

Physical and dynamic properties of marine microplastic (MP) particles are summarized, with relevance to their behavior in the coastal zone. The density range, size classes, and typical particle shapes are presented, and their variation with time due to weathering, biofouling, and mechanical degradation in the swash zone is considered. The dependence of the time rate of increase of integral particle density on the particle shape due to its biofouling shows that floating fibers and threads (“one-dimensional” (1-D) particles) are the first to begin sinking, followed by 2-D films and flakes, and then 3-D fragments. Mechanical fragmentation in the swash zone of plastic samples made of low-density polyethylene (LDPE), polystyrene (PS), polypropylene (PP), and foamed polystyrene (foamed PS) shows qualitative features of the generated MP particles and indicates that increase in mass of MPs with time is exponential. Dynamic behavior of MPs is discussed on the base of knowledge from classical hydrodynamics, marine sedimentology, and physical oceanography. Shape-dependent manner of the particle sinking and the value of the terminal settling velocity are characterized in terms of the particle size, density, and angularity. Critical velocity of resuspension by a unidirectional flow is addressed in terms of the Shields diagram, summarizing available published data on laboratory experiments on the particles with properties similar to those of MPs. Consideration of motion of particles in an oscillatory flow under surface waves and in roll structures illuminates mechanisms of mixing of MPs and natural marine debris. Analysis of the observed sorting of sediment grains under the influence of surface waves raises the question of probable sorting of MPs in the water column of the coastal zone, leading to different transport for different kinds of MP particles, including the offshore export of smaller MPs, onshore transport of larger particles, and possible effect of arresting of finer MPs under coarser sediment grains. Typical velocity scales of natural convective flows and upward/downward motions within the Langmuir circulation cells are shown to be of the same order of magnitude as the settling velocity of common MPs—units of centimeters per second—which suggests the importance of these mixing mechanisms for the motion of MP particles. As a natural case study, the events of washouts of heavily intermixed patches of seaweed, amber stones, and MPs onto the beaches of the Baltic Sea after severe storms are examined. It is concluded that stormy episodes seem to bring repeatedly larger plastics to the surf and swash zone and export smaller pieces offshore.

An overview of modern approaches to the problem of parametrisation of sources of marine waters microplastics pollution from the coastline is conducted. The estimates of Europe’s plastic production along with mismanaged plastic waste percentage that might be the source of microplastics particles input to marine environment are presented. A semi-empirical for-mulation for the particles source intensity is suggested. It considers the main factors of local anthropogenic pressure for the coastal spot location for the given coordinates. Both ad-vantages and disadvantages of such an approach along with possible ways for improvement are discussed.

The Black Sea is an enclosed deep marine basin, where the structure of tidal movements is dominated by the direct influence of the tidal force on the proper water body. We investigated the spatial structure of its climatic circulation under the impact of tides. We developed a program module extending the numerical general circulation model of the Black Sea which was designed in the Institute of numerical mathematics, Moscow. It allows the lunar semidiurnal harmonics (M_2) influence to be taken into account explicitly via the discrete analogues of the differential equations of motion. Our work reflects the main results of the numerical experiment on the 4x4 km horizontal grid and 40 vertical σ-levels. It was a one-year model run using the CORE atmospheric climatology forcing. We compared the first and the last weeks of simulation and found out that the characteristics of a tidal mode M2 were established at a very short period of time (7 days), which is the estimate of the model’s energy redistribution time scale. The coastal areas where the tidal impact is substantial (~10 cm) were located mainly at the shallow-shelf inlets highly influenced by the climate change. Validation of the cotidal maps showed the reliability of our model at the climatological time scale. In future we will focus on the baroclinic tidal movements and validation with the Marine Hydrophysical Institute database in order to shed new light on physical and ecological processes at the frontal zone along the Rim Current.

External oceanographic conditions rather than anthropogenic influence are shown to cause the 3-dimensional distribution of anthropogenic microparticles (MP, 0.5–5 mm) within the body of sandy beaches of a non-tidal sea with strong wind/wave climate and seasonal sea level variations (the Baltic Sea). A patchy structure is confirmed in all three dimensions, with background concentrations of several tens of MP items per kg of dry sample weight versus peaking spots with several hundreds of items per kg dry weight. The background MP concentrations are of the same order of magnitude for the beach surface, beach body, and sands of underwater coastal slopes, highlighting that the contaminated by MPs sand cover of the entire sea coastal zone is one single entity, repeatedly re-distributed between its underwater and beach parts by every next storm. Peaking concentrations are related to stormy events and places with stronger water dynamics, and are associated with locations of coarser sands within the beach body and wracklines at the beach surface. This suggests that marine waters are the source of anthropogenic microparticles for the beach, and not vice versa. The prevalence of wave-driven over wind-driven beaching mechanism for MPs extracted from the beach samples is confirmed by the flotation tests. Size distribution of the extracted MPs is found to be similar to that obtained for plastics floating at the ocean surface. Such a coherency for different oceanic environments speaks in favor of independence of general fragmentation processes on the particular external conditions, shifting the attention to the fragmentation process and material properties of synthetic particles in marine environment. Capsule Stormy winds, surface waves, and sea level variations rather than anthropogenic load define 3-d variability of pollution by synthetic microparticles within the sand body of beaches.

A three-dimensional numerical model of the pollution of the water column and bottom sediments of the Black Sea by polychlorobiphenyls (PCBs) is developed. The model consists of a physically complete hydrodynamic block and also the transport and transformation modules for the detritus and PCB. The PCB transfer module calculates three functions: the concentration of dissolved PCB,the PCBon sinking detritus particlesand inthe upper layer of thebottom sediments. Wetake into account the processes of adsorption-desorption of PCB on sinking detritus particles, the alternating- sign flow of the substance on the water-sediment boundary, and destruction of the detritus. A model spin-up calculation is performed for the scenario of an instantaneous PCB emission from the Sfantu Gheorghe branch of the Danube. It is shown that the PCB transport on detritus is a natural buffer mechanism damping the spread of a steady organochlorine pollution. The method of mathematical modelling is an efficient approach to ecological stud- ies of the marine environment. The construction of numerical models and the study of biogeochemical cycles of the group of stable organochlorine pollutants damag- ing the marine ecosystem are urgent problems. Such compounds possess a series of specific features. These are their global prevalence, extreme resistance to physical, chemical, and biological transformations, their bioaccumulation caused by their low water solubility and high solubility in lipids, and their toxic impact on living organ- isms in extremely small doses. PCBs, which are some of the most widely-spread and toxic synthetic substances, belong to the class of aromatic compounds. When PCBs are included into bio- logical food chains, due to their selective biotransformation a progressive loss of low-chlorinated components occurs. Therefore, human and animal organisms accu- mulate the most dangerous high-chlorinated PCBs (21). Polychlorinated biphenyls were massively produced beginning with 1929. The synthesis and extensive use of PCBs is explained by their physical and chemical properties: exceptional ther- mal and electrical insulating characteristics, heat resistance, etc. Until the end of their industrial production in 1986, approximately 2 million tons of PCBs had been produced in the world. Polychlorobiphenyls get into the sea via river runoff, at- mospheric precipitation, and also with industrial and municipal drainage effluent.