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.

During cruise 65 of the R/V Akademik Oparin from November 26 to December 29, 2022, research was continued in the area of Avachinskiy Bay, Kamchatka, to study the possible factors that caused the mass mortality of marine organisms in fall 2020, and perform integrated hydrographic and hydrochemical surveys on the northeastern shelf of Sakhalin Island to monitor the environmental situation in areas of oil and gas production and in the southern Sea of Okhotsk. According to data from two autonomous moorings deployed in the coastal zone of Avachinskiy Bay with an exposure of 169 days, relatively calm water dynamics in the bay were noted throughout the entire summer–fall season of 2022, which contributed to enrichment of waters in nutrients necessary for plankton development. No noticeable decrease in oxygen content was recorded. A survey of the southern part of the East Kamchatka Current was carried out, the evolution and the structure of the anticyclonic eddy of Avachinskiy Bay at the time of its splitting was traced. Studies have been carried out on the hydrochemical characteristics of coastal bays and rivers during the freeze-up period. Samples of plankton and bottom sediments were taken for special analyses in the laboratories of the Pacific Institute of Bioorganic Chemistry and National Center for Marine Biology, Far Eastern Branch, Russian Academy of Sciences.

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.

On cruise 80 of the R/V Professor Gagarinskiy from June 15 to July 18, 2022, integrated hydrographic and hydrochemical studies were carried out in Avachinskiy Bay of the Kamchatka Peninsula, on the northeastern shelf of Sakhalin Island, and in Peter the Great Bay to assess current environmental risks in the coastal zone of key areas of Russia’s Far Eastern seas and, in particular, to study possible abiotic factors that caused the mass mortality of marine organisms in Kamchatka in fall 2020. Comprehensive observations were performed, including CTD, hydrochemical water sampling, moorings, and plankton and bottom sediment sampling. An increased nutrient content was found, both in subsurface and intermediate Pacific waters, in comparison with 1990s–2000s data. Intensive snowmelt on adjacent mountain slopes during the survey period showed the significant impact of continental runoff on the hydrochemical characteristics of Avachinskiy Bay waters, causing decreased salinity and significantly increased concentrations of nutrients and chlorophyll a in the coastal zone. The dynamics of mesoscale eddies ensures transport of coastal waters to the ocean over a distance of more than 100 nautical miles. The results suggest that the catastrophic phenomenon off Kamchatka, associated with anomalous harmful algae blooms, is due to conjugate biogeochemical and hydrographic factors: river runoff and intrusion of deep waters on the shelf caused by coastal wind and dynamic upwelling during the period of maximum warming and eutrophication of subarctic Pacific waters, as well as the orographic and topographic features of Avachinskiy Bay.

The hydrological and hydrochemical data of surface and bottom waters of Academy Bay were obtained on two POI FEB RAS cruises carried out from July 11 to 14, 2016 and from September 15 to 19, 2020. Observations in 2016 were carried out during the summer flood and after thawing sea ice in the bay which were confirmed by the hydrological characteristics of the waters. Supersaturation of bottom and surface waters with atmospheric oxygen and undersaturation with atmospheric carbon dioxide indicate that production processes dominate in the bay. However, low contents of nutrients and chlorophyll a in the bay and in the estuaries of the Syran and Ulban rivers characterize the bay as rather an oligotrophic basin. This is supported by the common procedure of eutrophication assessment. In September 2020, the majority of bottom waters were undersaturated with oxygen and supersaturated with carbon dioxide, meaning organic matter degradation processes dominated. In the fall 2020, the apex of Ulban Bay was characterized by very high concentrations of dissolved inorganic phosphorus, reaching 10–14 μmol/L, and by chlorophyl a concentrations of surface waters higher than 10 mg/m 3 . Values of obtained parameters suggest that waters of Academy Bay have high eutrophic status. The difference between observations implemented in 2016 and 2020 cannot be explained by the interannual variability of the hydrochemical parameters of the waters of Academy Bay, rather it can be explained by an additional episodic source of nutrients that occurs in the late summer–autumn season. Salmon that died after spawning can serve as such an source. These dead salmon provide eutrophication of Academy Bay waters. High eutrophic waters probably provide the feeding base for polar whales.

Fluxes of nutrients (ammonium, total nitrogen, dissolved inorganic phosphorus, total phosphorus, silica, and dissolved organic carbon) across the interface between bottom water and sediments were studied in three shallow bights (Voevoda, Novgorodskaya, and Uglovoj) of the Peter the Great Bay in the course of survey in September–October, 2019. At two stations in each of the bights, nutrient concentrations were measured in seawater sampled at 10 cm above the seafloor and in the pore waters of the sediment cores; and organic carbon, fulvic acids, humic acids, and chlorophyll a were analyzed in the solid phase of the sediments. Bioturbation coefficients were estimated from variations in chlorophyll a concentration with depth in the sediment cores. The fluxes of nutrients (ammonium, total nitrogen, phosphorus, total phosphorus, and silicon) and dissolved organic carbon (DOC) were evaluated by model calculations. The estimated nutrients fluxes were, with regard to the bioturbation, about one order of magnitude higher than those estimated by the Fick law. The highest concentrations of organic carbon, 6.5 and 5.5% per dry weight of sediments, were found in the top layer of sediments retrieved at sites covered by Zostera marina L. in the Voevoda and Novgorodskaya bights, respectively. However, the highest fluxes across the water–sediment interface were obtained for a site devoid of seagrass in the Novgorodskaya bight, where the top sediment layer contained 2.8% organic carbon. The annual specific fluxes of nutrients estimated for the water–sediment interface were comparable to or even higher than the annual specific fluxes of nutrients brought per surface area unit of the Amur bay by the Razdolnaya River and domestic waste waters from the city of Vladivostok.

In the period of summer flood 2016, a number of chemical characteristics associated with carbon cycle were studied in the estuaries of the Syran and Ul’ban rivers in the area of the Shantarskii Archipelago with the total water discharge of 194.7 m3/s. The zone of mixing at the salinity of <20‰ is the source of CO2 for the atmosphere with the estimated CO2 flux up to 112.7 mmol m−2 day−1. At the salinity >20‰, the thickness of the photic layer increases abruptly and photosynthesis starts to dominate, resulting in the formation of CO2 flow from the atmosphere into water with a rate of up to 30 mmol m−2 day−1. A model CO2 flow at the water/atmosphere interface is presented at a wind speed from 2 to 15 m/s for the entire mixing zone, the water in which generally absorb atmospheric CO2. A specific feature of the basin compared with the estuaries of the Uda and Usalgin rivers in the zone of the Shantarskii Archipelago is the combination of the relatively low volume of water and solid runoff with a relatively high phosphorus flux in mineral and organic forms.

The article presents the data of an comprehensive expedition of the Pacific Oceanological Institute, Far East Branch, Russian Academy of Sciences, performed in the mixing zones of the Uda and Usalgin rivers in July 2016. During the flood period, they occupy most of the catchment basins: Udskaya Bay and Nikolaya Bay. The estimated discharge of the Uda and Usalgin rivers was 4390 and 173 m3/s, respectively, and estuarine waters with salinity to 8‰ spread from their mouths to a distance of 25 km in Udskaya Bay and 2.5 km in Nikolaya Bay. The boundary of the estuarine seashore distinguished by the isohaline 30‰ was at a distance of 85 km from the river mouths in the both mixing zones. River runoff—the main supplier of suspended particulate matter and silicates—limited the thickness of the photic layer and photosynthesis at the early stage of mixing zones to the salinity of ~24‰, where water was a source of CO2 for the atmosphere. At the later stage of the mixing zones, under the conditions of a greater thickness of the photic layer, production dominated, and the water area was a sink for atmospheric CO2. Mineralization of autochthonous organic matter on the bottom of the receiving basins was the main source of inorganic forms of nutrients. The lack of silicates (DISi/DIN < 1 and the relatively low DIN/DIP ratio in the range of 1.8–8) was a limiting factor of photosynthesis at the late stage of the mixing zone.

Hydrological and chemical surveys were conducted in the Uglovoy Bight in October, 2019, February, May and June, 2020 (in total 120 stations) and chemical analyses of water from 13 small rivers running into the bight were done on October 21-22, 2020. Extremely high concentration of nutrients was detected in the Peschanka, Saperka and Gryaznukha Rivers that was obviously caused by waste waters discharge. These rivers were the main source of the bight eutrophication. Within the bight, the highest anomalies of chemical parameters, as low oxygen content, low pH, high concentrations of nutrients (N, P, Si), high turbidity, and high CO2 partial pressure were observed close to these rivers mouths, in particular under the ice in winter, when wind mixing was absent. The hypoxia disappeared in the warm period of year because of wind mixing. High concentrations of total nitrogen (10.0-40.0 μmol/L), total phosphorus (1.5-2.0 gmol/L), dissolved organic carbon (3-5 mgC/L), and chlorophyll a (0.5-2.0 μg/L) in all seasons were the results of active production-destruction processes, obviously with prevalence of organic matter destruction, since the water in the bight was undersaturated with oxygen and supersaturated with carbon dioxide — the bight accumulates and mineralizes organic matter from terrestrial and riverine discharge. Underwater photographs did not detect Zostera meadows at the bottom, which were observed in the northwestern Uglovoy bight in the past. Comparison of historical data on episodic studies in the bight with results of the surveys indicates degradation of its ecosystem, with such signs as disappearance of seagrass, hypoxia in winter, and CO2 flux into the atmosphere. Reduce in water exchange between the bight and the Amur Bay caused by construction of the underwater pipeline in 1982 and the bridge in 2012 is suggested as a reason of the degradation.

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.

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.

According to the Plan of Complex Scientific Research of the World Ocean for 2017–2022, the V.I. Il’ichev Pacific Oceanological Institute, Far Eastern Branch, Russian Academy of Sciences, continues to study the northern part of the Sea of Japan and the Tatar Strait, carrying out a three-year plan of expeditionary research of this region. The expedition aboard the R/V Akademik Oparin (cruise no. 55) spanned from October 2 to 19, 2018. Geophysical and gas-geological study of the water area in the central and southern parts of the Tatar Strait was continued. On the rookery of Tyuleny Island, in the Sea of Okhotsk, information on sea lions and larghas were recorded, three individuals of northern fur seal were tagged with satellite trackers to study their migration routes. CTD observations of the upper layer (down to 600 m) were made on transects across mesoscale eddies in the northern part of the Sea of Japan. Expeditionary research was supported by the Council of the Earth’s Hydorsphere of the Ministry of Science and Higher Education of the Russian Federation.

In February and July 2014, multidisciplinary geochemical studies of the sediments were carried out at two stations. One of them was located in Voevoda Bight, which has a bottom depth of 4 m, covered by eelgrass (Zostera marina L.); the other one served as a background station, because it is devoid of seagrass and is located in the northern part of Amur Bay (near Rechnoj Island). This station has a bottom depth of 6 m. The chemical composition of sediment pore water was studied along the depth of the core (80 cm) for the following: concentrations of dissolved organic carbon, carbohydrates, humic substance, nutrients (inorganic phosphorus, silicon, ammonium), and the parameters of the carbonate system (pH, total alkalinity-TA, dissolved inorganic carbon-DIC, CO(2)partial pressure-pCO(2)). Contents of organic carbon (OC), fulvic acids, humic acids, and water were measured in the sediment solid phase. It was established that that the OC concentration was more than 5 and 2% in Voevoda Bight and at the background station, respectively. The sum of the fulvic and humic acids was about 40% of the OC content. It was found that the OC content decreased by 50% within 40 cm of the top layer of the sediments in Voevoda Bight. Within the same layer, a sharply increased nutrient concentration, TA, and DIC and decreased pH and sulfate ion concentrations were observed, which were the opposite of the background station. Seasonal variability of the observed parameters was weak, except for the significant decrease in pH and increase in pCO(2)observed in summer due to temperature effect on the constants of the carbonate system. The chemical composition of pore water suggests that sulfate reduction was an important process. However, it cannot explain the sharp decrease in OC; therefore, it was suggested that OC is transformed into infauna biomass by the food chain.

Content of accumulated carbon (Cdep, which is referred to as blue carbon, BC) was calculated for the uppermost 1-m thicknesses of sediments in bights of the Peter the Great Bay of the Sea of Japan in 2014–2020. In the absence of Zostera Marina (ZM) meadows in Voevoda, Novgorodskaya and Uglovoy bights, BC contents were 140, 99, and 55 tC/ha, respectively. The presence of sea grass significantly increases these values, which were 180 and 126 tC/ha for Voevoda and Novgorodskaya bights, respectively. Using the measured radioactivity of the 137Cs isotope, the rate of BC accumulation in the bights without meadows ZM was estimated at 17, 69, and 98 gC/(m2 year) in Uglovoj, Novgorodskaya, and Voevoda bights, respectively, and at 101 and 144 gC/(m2 year) at sites covered by ZM meadows in Novgorodskaya and Voevoda bights, respectively. Using data of chlorophyll a measurements in sediments of the basins in 2018 through 2020, the bioturbation coefficients (62.7–5.3 cm2/day) and the constants of apparent organic carbon degradation in sediments (0.003–0.068 day–1) were estimated. The maximum BC degradation rates were found in the upper horizons: 550, 39, 6, and 4 gC/(m2 year) at sites with ZM in Voevoda, and without ZM in Voevoda, Novgorodskaya, and Uglovoy bights, respectively. It was found out that ZM meadows cause the maximum concentration of refractory organic carbon ( $${{{\text{C}}}_{{{\text{org}}}}} = {\text{1}}{\text{.9}{\% }}$$ in sediments of Voevoda bight).