The main objectives of this study were to investigate the distribution features of the 210Po in abiotic (water and bottom sediments) and biotic (zooplankton, mollusks, fish) components of the North Crimean Canal (NCC) aquatic ecosystem and adjacent irrigated soils as well as assessment of the doses received by water organisms from α-radiation of absorbed 210Po. The samples were processed using standard radiochemical methods accepted in international practice. The activity of 210Po in the samples was measured using the alpha-spectrometric OCTETE Plus complex (ORTEC-AMETEK, USA). The measurement error did not exceed 20

—Sedimentation transport of 137 Cs can lead to the significant accumulation of this radionuclide at depths that could not be reached through the vertical water exchange alone; therefore, a comparative assessment of its contents for different types of suspended matter (SM) and regions of the Black Sea is of particular interest. For this purpose, we have collected samples of SM and seawater at deep-water and coastal stations for the subsequent determination of the 137 Cs activity in the surface water layer. To calculate the fraction of lithogenic matter, the potassium content in the SM was additionally determined. The range of 137 Cs content on SM at different stations differed by more than an order of magnitude: from 7 to 111 Bq/kg for specific activity, and from 0.03 to 0.69% for its content on SM, in % of the total content in the surface water layer. Stations located farther away from the coast were characterized by the lowest percentage of 137 Cs in SM, while its content at the coastal stations was more variable. The comparison of the lithogenic and biogenic contribution to SM and data on 137 Cs for different stations suggests that the content of this radionuclide on SM is primarily determined by the dynamic variations of the lithogenic matter. Based on the 137 Cs migration on SM, the Black Sea is divided into at least two types of regions. One regions are water areas far removed from sources of lithogenic matter where SM is formed mainly due to the hydrobiont activity. The 137 Cs content in SM due to the predominance of biogenic matter and an insignificant concentration of lithogenic matter in this case accounts for hundredths of a percent of its total content in the surface water layer. Other regions are coastal and shelf water basins, which, on the one hand, are subjected to the significant coastal and river lithogenic runoff, and on the other, they are characterized by the elevated trophicity and biological productivity. In these water areas, 137 Cs content on SM owing to the variability of biotic and abiotic factors is more variable and can fluctuate from values typical for the open sea to an order of magnitude higher.

The latitudinal distribution of the anthropogenic radionuclide 137Cs in the Atlantic surface waters was studied according to the results of expeditionary research carried out during 79 cruise of the R/V «Akademik Mstislav Keldysh» (March-May 2020). Radiospectrometric analysis of the samples showed that the 137Cs activity concentration in the Atlantic Ocean water was in the range of 0.65 ± 0.10–1.44 ± 0.21 Bq/m3. It was found that the 137Cs activity peaks corresponded to the global fallout peaks in the middle latitudes of the Earth. Comparison of the data obtained in 2020 with the results of the 1988/1989, 1992/1993, 2002 expeditions revealed a tendency to smooth out the 137Cs activity peaks in the middle latitudes due to hydrodynamic processes in the World Ocean.

The comparative study of technogenic 90Sr behavior features in Crimea lakes'ecosystems with dif-ferent levels of salinity was carried out in 2016-2021.Two sources of 90Sr input were identified for all the studied lakes:the primary source concerned with atmospheric fallout immediately after the Chernobyl NPP accident and the secondary long-term input of this radionuclide by waterway.The half-life of the 90Sr concentration in the water of the hypersaline Lake Sasyk-Sivash was estimated to vary from 0.8 to 1.1 years after the closure of the North Crimean Canal(NCC).Biogeochemical processes in the lake under the absence of the secondary source of the radionuclide input were shown to decrease in the 90Sr residence time in the water column by 131 times.For brackish water bodies,a significant factor influencing the radionuclide concentration in ecosystems of lakes was the pH of their water,while for hypersaline lakes the level of water salinity was the main factor determining 90Sr behavior.The concentration of 90Sr in bottom sediments of studied lakes depended mainly on this radionuclide con-centration in a water environment.Calculated 90Sr distri-bution factors(Kd)for studied lakes'bottom sediments varied in a range of n·100 ÷ n·102 for hypersaline lakes and of n·101 ÷ n·102 for lakes with brackish waters.Due to the closure of the NCC,the 90Sr redistribution took place in lake ecosystems only under the geochemical processes within the water bodies themselves.The results obtained in this work are of particular importance as a starting point or a basis for further radioecological studies of the Crimean inland waters after the reopening of the NCC and the Dnieper waters re-entering the territory of Crimea in 2022 after the 8 years of their absence.

A long-term study of the activity concentration of the artificial radionuclide Cs-137 in the abiotic components of the ecosystems of the Crimean salt lakes during 2016-2020 is carried out. It was found that the contribution of the atmospheric inflow of the post-accident Cs-137 into the Crimean salt lakes accounted for 88.5 %, while the water pathway of the inflow of the dissolved radionuclide accounted for 11.5 %. Activity of the dissolved form of Cs-137 in lake water is within the range of 167.4 Bq.m(-3) from below detectable, whereas in the bottom sediments it is within the range of 15.3 Bq.kg(-1) from below detectable (data 2019). The Cs-137 concentration levels in the bottom sediments of the salt lakes are 3 or more times lower than that in the bottom sediments of the Black Sea, while the distribution coefficients (KdS) are within values 10(1)-10(1) (Bq. kg(-1)Bq. kg(-1)). It is found that the salinity of water bodies is the main geochemical factor affecting distribution of Cs-137 between abiotic components of the studied lakes. The Cs-137 inventory in water and bottom sediments of the studied lakes are 6.5 GBq and 45.4 GBq, respectively. It determines the bottom sediments of the Crimean salt lakes as a sink of the post-accident artificial radionuclide Cs-137.

Based on the results of field research in the cruise 68 of the RV Akademik Mstislav Keldysh, the content of anthropogenic 137Cs radionuclides in surface waters and bottom sediments of the Barents Sea was estimated. As a result of a comparative assessment of current levels of radiocesium in other seas, it is concluded that the radioecological situation in the Barents Sea is favorable, despite the presence of a large number of potential sources of radiation pollution.

Based on the results of expeditionary research carried out during the 82nd cruise of the RV “Akademik M. A. Lavrentyev” (01.06.2018–20.07.2018), the assessment of current levels of concentration activity of technogenic radionuclide 137Cs in surface waters of Far Eastern seas is given. The studies were carried out in the northwestern part of the Sea of Japan, the southern part of the Sea of Okhotsk, the coastal waters of the Pacific Ocean near the Kamchatka Peninsula, and the western part of the Bering Sea. Activity of 137Cs in seawater samples was determined by sorption method using two series-connected adsorbers with subsequent measurement of 137Cs content via its gamma-emitting daughter radionuclide 137mBa. Sorption efficiency was assessed by the difference in activity on the first and second adsorbers. A comparative analysis of contamination levels of water areas studied was made. It was revealed that 137Cs volumetric activity in surface water of the Sea of Japan varied from (2.9 ± 0.1) to (5.1 ± 0.3) Bq·m−3, in the Sea of Okhotsk – from (1.8 ± 0.1) to (2.3 ± 0.1) Bq·m−3, and in the Bering Sea – from (1.7 ± 0.1) to (3.1 ± 0.1) Bq·m−3. The maximum 137Cs concentrations were registered in the Sea of Japan, which might be due to its isolation from other water areas and presence of secondary sources of radionuclide intake. In general, contamination of adjacent water areas is insignificant, and fluctuations in concentrations occur within technogenic isotopes global background in the marginal seas of the Pacific Ocean.

Knowledge of trace elements content and their behavior in aquatic ecosystems is important for their sustainable use. There is a lack of such data for saline and, especially, hypersaline lakes and lagoons. Concentrations of more than 20 elements were evaluated in bottom sediments of 15 saline/hypersaline lakes and Lagoon Sivash in Crimea. An average salinity varied from 4 to 335 g/L in studied water bodies. The concentration of the trace elements varied from lake to lake. The highest variability was recorded for Cd, from 4.13 mg/kg to below the detectable level (CV = 1.463), and for Se, from 5.52 to 0.05 mg/kg (CV = 1.053). The lowest variability demonstrated by Cr, from 368 to 17 mg/kg (CV = 0.463), and by V, from 67.8 to 1.7 mg/kg (CV = 0.481). According to the found content of studied elements, all lakes were separated into three groups, and Lagoon Sivash was not included in these clusters. Salinity affected the concentration of some elements in bottom sediments, and this effect was not linear or unidirectional. In some cases, the action of other factors, often unknown, masked the effect of salinity. The geochemical background affects the structure and functioning of aquatic ecosystems, but the state of these ecosystems can significantly modify this background. An understanding of the differences in the elemental composition of bottom sediments in different lakes is possible only based on an integrated consideration of the interaction of all landscape, intra-ecosystem, and anthropogenic processes and factors that can influence this.

In the framework of radioecological monitoring, the features of the distribution of the post-Chernobyl nuclear power plant (NPP) accident artificial radionuclides of 137Cs and 90Sr in the Crimean saline lakes were studied. Samples were collected from 12 Crimean lakes with a salinity range from 2 to 400 g/L in 2014–2017. Concentration of 90Sr varied from 5.9 to 313.6 Bq/m3, and 137Cs from 0.8 to 106.7 Bq/m3. Closing of the North Crimean Canal resulted in a decrease in radionuclide concentration in the lakes. About 61% of the total variability of 90Sr and 33% of 137Cs in lake water can be explained by salinity changes. The salinity affects the behavior of radionuclides in water, mainly influencing their solubility, on isotope exchange between the solution and bottom sediments, and also, probably, through its influence on accumulation of isotopes by aquatic organisms. Salinity is not the alone factor determining the behavior of radionuclides in the lakes, it only acts by interacting with other factors.

Исследовано содержание техногенного радионуклида 137Cs в разных группах соленых озер Крымского п-ова. Установлено, что одним из основных источников поступления в них 137Cs является Северо-Крымский канал, который до 2014 г. приносил в Крым днепровскую воду с высоким содержанием радионуклидов чернобыльского происхождения. Другой источник поступления 137Cs – воды Черного моря за счет их дренажной и прямой связи с прибрежными солеными озерами Крыма, в которых наблюдалась положительная корреляция между концентрацией 137Cs и соленостью воды.

The content of the man-made radioactive nuclide 137Cs was investigated in different groups of the salt lakes of Crimea peninsula. One of the main sources of 137Cs was determined to be the North-Crimea channel, which supplied the Dnepr water to Crimea until 2014 with a high content of radioactive nuclides of Chernobyl origin. The other source of 137Cs is the Black Sea water owing to its drainage and direct connection with the coastal salt lakes of Crimea, which demonstrates a positive correlation between 137Cs concentration and water salinity.

According to results of the field work conducted in 42nd cruise of the research vessel Akademik Boris Petrov (January-March, 2017) and 68th cruise of RV Akademik Mstislav Keldysh (June-August, 2017), the data on concentration of the man-made radionuclide caesium-137 in surface waters of the East China Sea, the South China Sea, Equatorial part of the Indian Ocean, the Red Sea and Mediterranean, Biscay Gulf, the English Channel, the Baltic Sea, North Atlantic, Norwegian Sea and the Barents Sea have been obtained. It was found that concentration of caesium-137 in the surface water of studied Eurasian seas is at a low level. The exception is the Baltic Sea, where the level of caesium-137 is much higher than the values observed before the Chernobyl accident. A higher concentration of caesium-137 in the surface waters of the East China Sea, compared with previous estimates, was found, which may be due to the significant discharge of a high-level liquid radioactive waste after the Fukushima-1 accident in 2011.

The concentration of the technogenic radionuclide 137Cs was studied in different groups of salt lakes of Crimean Peninsula. It was established that one of the major sources of 137Cs input in them is the North Crimean Canal, which, up to 2014, had been delivering the Dnieper water with a high concentration of radionuclides of Chernobyl origin. Another source of 137Cs inflow is Black Sea water through its drainage and direct interaction with the coastal salt lakes of Crimea, in which a positive correlation was observed between 137Cs concentration and water salinity.

По результатам экспедиционных исследований, проведенных в 42-м рейсе НИС “Академик Борис Петров” (январь-март 2017 г.) и 68-м рейсе НИС «Академик Мстислав Келдыш» (июнь-август 2017 г.), получены новые данные по содержанию техногенного радионуклида цезия-137 в поверхностных водах Восточно-Китайского и Южно-Китайского морей, экваториальной части Индийского океана, Красного и Средиземного морей, Бискайского залива, пролива Ла-Манш, Балтийского моря, Северной Атлантики, Норвежского и Баренцева морей. Установлено, что концентрация цезия-137 в поверхностной воде большинства исследованных морей Евразии находится на достаточно низком уровне. Исключение составляет Балтийское море, где уровень содержания цезия-137 значительно превышает величины, наблюдавшиеся в дочернобыльский период. Обнаружена более высокая, по сравнению с предыдущими оценками, концентрация цезия-137 в поверхностных водах Восточно-Китайского моря, что может быть связано со значительным сбросом высокоактивных жидких радиоактивных отходов в результате аварии на АЭС Фукусима-1 в 2011 году.

New data were obtained during Cruise 42 of the R/V Akademik Boris Petrov (January–March 2017) and Cruise 68 of the R/V Akademik Mstislav Keldysh (June–August 2017) on the concentration of the anthropogenic radionuclide cesium-137 in the surface waters of the East China Sea, South China Sea, equatorial Indian Ocean, Red Sea, Mediterranean Sea, Bay of Biscay, English Channel, Baltic Sea, North Atlantic, Norway Sea, and Barents Sea. Low cesium-137 concentrations were observed in the surface waters of the Eurasian seas. An exception is the Baltic Sea, where the cesium-137 content is much higher than the values obtained before the Chernobyl accident. Compared with previous estimates, higher cesium-137 concentration was observed in the surface water of the East China Sea, which could be related to the extensive discharge of high-level liquid radioactive waste after the Fukushima Daiichi nuclear accident of 2011.