During cruise 71 of the R/V Akademik Oparinm the radioecological state of the Pacific Ocean east of Japan and the Kuril Islands, as well as the seas of Okhotsk and Japan, was assessed in connection with polluted water discharge from the Fukushima-1 NPP. The results of the first samples of surface analysis showed a tritium content of 0.36–0.78 tritium units (0.04–0.09 Bq/L). The highest concentrations were found in the jet of the Kuroshio Current and in the area of the Southern Kuril Islands. Meanwhile, it is ten thousands time lower than the radio security norm. On the cruise, samples of water, bottom sediments, and marine biota were prepared to analyze radio isotopes of cesium, strontium, lead, radium, etc. A large volume of hydrographic, chemical, biological, gas-geochemical, and mercury measurements and samplings was obtained. Mooring systems were recovered and redeployed. New data on the water state and dynamics of the study region were obtained, as well as biogeochemical parameters and their interannual variability in association with climate change and increasing anthropogenic load.

Investigation of the interannual and short-term variability of the water parameters in the seas of Japan and Okhotsk and their ecological state were continued on cruises 77 of the R/V Professor Gagarinskiy and 97 of the R/V Akademik M.A. Lavrentyev in fall–winter 2021. CTD profiling and water sampling for chemical analyses, including methane and radioisotopes, were carried out. Autonomous bottom stations were moored for 2.5 months. Long-term trends of bottom water warming, eutrophication, and acidification of the Sea of Japan have been confirmed. The specific features of fall–winter restructuring of the field of currents in the northern Sea of Japan, multifrontality of the Sakhalin upwelling structure, and bimodality of the Primorye Current are shown.

The results of research carried out on cruise 62 of the R/V Akademik Oparin from December 14 to 29, 2020 continued monitoring of the interannual variability of the Japan Sea water parameters, confirmed the long-term warming trend of bottom waters, revealed its slowdown during the last decade, demonstrated the formation of a cyclonic gyre in the northern part of the sea and development of upwelling, contributing to more intensive cooling of the sea surface layer. A detailed survey of a mesoscale eddy was carried out in the area of the Primorye Current. The hydrochemical parameters, zooplankton, and methane and tritium in various water structures were assayed. The mercury content in the atmospheric water layer was measured.

The paper provides information about a complex of geological studies in Chaun Bay in the East Siberian Sea during cruise 60 of the R/V Akademik Oparin in October 2020. Preliminary results of the scientific expedition are discussed.

Multiyear monitoring of the marine environment of the Japan/East Sea was continued by the joint Korean–Russian expedition of the R/V Akademik Oparin (cruise 58) in October–November 2019, including changes in the sea circulation and ventilation and biogeochemical processes as a result of current climate changes and growing anthropogenic impacts. Studies of mesoscale eddies associated with the branches of the Tsushima Warm Current have been continued. Methane flare has been discovered for the first time at the eastern Primorye slope.

In February 2020, geochemical tracers along with hydrochemical and hydrological characteristics were used to reveal a discharge site of salt groundwaters in the head of the Razdol’naya R. estuary at a distance of 22 km from its mouth bar. The elevated activity of 224Ra isotopes from 11.1 ± 0.1 to 2.2 ± 0.05 Bq/m3 in the bottom water layer over a distance of 15 km indicates to the propagation of slightly saline waters from a pool 8 m in depth toward the receiving basin. The distribution of stable water isotopes (δ18O and δD) and ions of basic salt composition showed that water of the top aquifer at the site of discharge area can suffer the effect of seawater. The major cause of the observed phenomenon is supposed to be the penetration of seawater into the top aquifer within the estuary during winter low-water season and the seepage of this water in the region with largest depth. The contrast of the temperature of ground and surface water was accompanied by a positive temperature anomaly in the zone of their interaction and a decrease in ice thickness along the mixing zone.

Приведены сведения о комплексе геологических исследований в Чаунской губе Восточно-Сибирского моря в 60-м рейсе НИС “Академик Опарин” в октябре 2020 г. Обсуждаются предварительные результаты научной экспедиции.