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.

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.

Data on dissolved organic concentration (DOC) and concentration of nutrients (phosphorus, silicon, and nitrogen of ammonium, nitrite and nitrate) in the Razdolnaya/Suifen River water are presented. The samples were collected fortnightly, as a rule, during more than a year (2013–2014). The nutrients concentration decreased and DOC and humic substances concentration increased with the river run-off increasing. In conditions of monsoon climate, the nutrients discharge from the Razdolnaya/Suifen into the Amur Bay had great pulsations that promoted sometimes producing of «excessive» phytoplankton biomass in the bay and provided a background for hypoxia at the bottom. Natural terrestrial fluxes of nutrients and DOC into the bay are much higher than these substances supply with waste waters of Vladivostok City. Interannual variability of the nutrients and dissolved organics fluxes into the Amur Bay is traced. Tendency to their increasing is supposed since 2003 because of the Razdolnaya/Suifen River annual discharge increasing observed by Hydrometeorological Agency in 2003–2017.

Обсуждаются продукционные и гидрохимические параметры эстуарных экосистем рек Уда и Усалгин с расходом воды 4390 и 173 м3/с соответственно. Их эстуарии являются макроприливными с высотой приливов до 9,7 м. Приливы разрушают вертикальную плотностную стратификацию воды. Это, в совокупности с относительно низкими концентрациями неорганических форм азота в речных водах, снижает величину автотрофной продукции в эстуариях. Однако высокие концентрации растворенного органического углерода в речных водах обеспечивают благоприятные условия для образования гетеротрофной продукции не только в реках, но и в зонах смешения. Неорганические формы биогенных веществ, поставляемые реками, являются основой автотрофной продукции, что в большей степени проявляется при солености воды более 24 . В целом эстуарии рек Уда и Усалгин можно характеризовать как гетеротрофные бассейны. Parameters showing the direction of the processes of production / destruction of organic matter of the estuarine ecosystems of the Uda and Usalgin rivers with a flow rate of 4,390 and 173 m3/s, respectively, during the summer flood of 2016 are discussed. The estuaries of the Uda and Usalgin rivers are macro-tidal with tidal heights of up to 9.7 m. Tides break vertical density stratification of water. The concentrations of inorganic forms of nitrogen in river waters are low. These two factors reduce the intensity of autotrophic production in estuaries. However, high concentrations of dissolved organic carbon in river waters provide favorable conditions for heterotrophic production not only in rivers, but also in mixing zones. The source of inorganic forms of nutrients is the result of distraction of organic matter supplied by rivers, but this source, which is the basis of autotrophic products, is to a greater extent at the late stage of mixing zones with water salinity of more than 24 . In general, estuaries of these rivers can be characterized as heterotrophic basins.

The contents of C org , humiс acids (HA), and fulvic acids (FA) in soil and riverine and marine bottom sediments have been investigated along a transect between the Razdolnaya River and Amur Bay. The revealed higher C org content of soil compared to that of riverine and marine sediments is associated with peatlands. The increased HA content of sediments compared to the FA content, corresponding to the initial period of water mixing, is explained by flocculation. The identified riverine sediments with increased FA, Fe, and Mn contents indicate active processes of flocculation and the formation of Fe and Mn oxyhydroxides. Soil shows significantly larger amounts of humified organic matter than marine sediments. Marine sediments contain mostly biochemically unrecycled organic matter.

Исследовано содержание Сорг, гуминовых (ГК) и фульвокислот (ФК) в почве, речных и морских донных осадках на разрезе р. Раздольная — Амурский залив. Выявлено повышенное содержание Сорг в почве по сравнению с осадками реки и морскими отложениями, что связанно с торфяниками. Показано увеличение содержания ГК по сравнению с ФК в осадках, соответствующих начальному периоду смешения вод, что связано с флокуляцией. Выявлены речные отложения, содержащие повышенное количество ФК, Fe и Mn, что свидетельствует об активном протекании флокуляции и формирования оксигидроксидов Fe и Mn. Показано, что почва содержит значительно больше гумифицированного органического вещества по сравнению с морскими отложениями. Морские осадки в большей мере содержат биохимически не переработанное органическое вещество.

The hydrological and hydrochemical parameters of the Tumen River estuary were collected at 13 stations in May and October 2015. Vertical temperature, conductivity, dissolved oxygen, chlorophyll fluorescence, and turbidity profiles were obtained. Water was sampled from the surface and bottom layer. The water samples were analyzed for major ions, pH, salinity, concentrations of dissolved oxygen, major nutrients, dissolved organic carbon, humic matter, and δ18О and δD isotopes. This estuary is attributed to microtidal type with a flushing time of about 10 h. A phytoplakton bloom occurred in the top layer of the estuary. For surface horizons, the hydrochemical parameters show a linear correlation with salinity. In the bottom horizons, all these parameters, except for major ions and δ18О and δD isotopes, reveal substantial nonconservative behavior. The nonconservative behavior of the hydrochemical parameters in the bottom waters was mainly caused by degradation of the phytoplankton biomass at the water/sediment interface. Hypoxic conditions were established in the bottom waters of the estuary in May.

During Cruise 62nd of the R/V “ Professor Gagarinsky ” in September, 2014, the carbonate system of sediments and contents of nutrients and organic carbon in pore water were studied in two geochemical stations located in hypoxia areas in the Peter the Great Bay. It was established that the concentrations of silica, phosphorus, and ammonium increase by 5, 10, and 20 times, respectively, with sediment depth to 70–80 cm. The alkalinity, dissolved inorganic carbon, and the partial pressure of carbon dioxide significantly increase with depth, while рН value and organic matter (ОM) decrease. Changes in the chemical composition of pore water with sediment depth (0–80 cm) are caused by anaerobic microbial degradation of OM, concentration of which in the top sediment layer is 2–3%. The degradation products of OM in the bottom waters of bay and pore waters of bottom sediments indicate that its main sources are diatoms. During hypoxia, the oxygen demand rate by sediment surface near Furugelm Island is estimated to be 5 mmol/(m 2 day). A combination of such factors as downwelling circulation, the absence of photosynthetically active radiation, and the high oxygen demand rate at the water/sediment interface provides hypoxia formation in the depressions of the Peter the Great Bay bottom topography.

In August 2013, anoxia of the bottom waters was established in the southern region of the Far East Marine Biosphere Reserve, Far East Branch, Russian Academy of Science, in the depression between Furugelm Island and coastal waters. Death of the benthic community was registered using a remotely operated underwater vehicle. The hydrochemical studies revealed that the area of the absence and/or presence of low oxygen contents corresponds to an area of anomalously high contents of ammonium, phosphates, and silicates, a high partial pressure of carbon dioxide and normalized alkalinity, and the presence of hydrogen sulfide. The microbiological decomposition of diatoms precipitated on the seafloor in the absence of oxygen regeneration was the reason for anoxia. Its formation in summer of 2013 was caused by anomalously abundant precipitates in the Far East.

Изучено содержание органического углерода во взвеси воды и в верхнем слое осадков эстуария р. Раздольной в период ледостава (02.2010 г.). Показано, что 30% от среднего содержания общего органического углерода во взвеси составляли гуминовые вещества и 14% – фитопланктон. Впервые спектрофотометрически определена концентрация гуминовых веществ во взвеси. По содержанию хлорофилла-а рассчитана концентрация углерода, производимого фитопланктоном. В верхнем слое осадка измерены содержания органического углерода, гуминовых веществ и хлорофилла-а. Показано, что с увеличением солености воды до 23 соотношение этих трех компонентов заметно не изменялось. Степень гумификации в осадках составляла около 52%. Концентрация органического углерода в осадке была ниже, чем во взвеси, почти на порядок, а доля легко усвояемого (не гумифицированного) углерода – ниже вдвое.

В летний сезон 2011 г. исследованы продукционные и гидрохимические характеристики эстуариев рек Артемовка и Шкотовка, впадающих в кутовую часть Уссурийского зал. (Японское море). Из-за высокой интенсивности деструкционных процессов парциальное давление в речных водах Шкотовки, Артемовки и Кневичанки (основной приток Артемовки) было 800, 1800 и 3700 мкатм соответственно. В отличие от эстуария р. Шкотовки, высокая степень трофности р. Артемовки и благоприятное сочетание гидрологических и гидрохимических условий на момент исследования обеспечили высокую интенсивность продукционных процессов (концентрация хлорофилла а достигала 200 мкг/л, пересыщение по кислороду было более чем в два раза, парциальное давление углекислого газа уменьшилось до 25 мкатм). Также была обнаружена консервативная и неконсервативная зависимость щелочности от солености в воде эстуариев рек Шкотовка и Артемовка соответственно.

Production and hydrochemical characteristics of the estuaries of the rivers of Artemovka and Shkotovka, flowing into the apex of the Ussuri Bay (the Sea of Japan), were studied in the summer of 2011. Because of the high rate of destruction processes, the partial pressure in river waters of the Shkotovka, Artemovka, and Knevichanka (the Artemovka’s major tributary) was 800, 1800, and 3700 μatm, respectively. Unlike the Shkotovka R. estuary, the high trophic level of the Artemovka R. and a favorable combination of hydrological and hydrochemical conditions in the study period determined the high rate of production processes (chlorophyll a concentration reached 200 μg/L, oxygen oversaturation was more than twofold, the partial pressure of carbon dioxide dropped to 25 μatm). A conservative and nonconservative dependence of alkalinity on salinity was also found to exist in the water of the Shkotovka and Artemovka rivers, respectively.

The paper presents data on organic carbon concentrations in riverine suspended matter and surficial bottom sediments measured in the estuary of the Razdol’naya River during the period of ice cover (February 2010). In the suspended matter, humic substances and phytoplankton accounted for 30% and 14% of total organic carbon, respectively. The concentration of humic substances in riverine suspended matter was determined for the first time by spectrophotometry. The concentration of phytoplankton carbon was calculated from the chlorophyll-a content. Concentrations of organic carbon, humic substances, and chlorophyll-a were measured in the surface sediment layer. The proportions of these three components show no marked variability with an increase in salinity to 23‰. The degree of humification in bottom sediments was about 52%. Organic carbon concentrations were almost an order of magnitude lower and the proportion of bioavailable (nonhumified) carbon was two times lower in the bottom sediments than in the riverine suspended matter.

The complex study of the river water and pore solutions from the bottom sediments in the lower reaches of the Razdol’naya River was conducted in February 2010. The major ion composition of the waters indicates the submarine origin of the near-bottom and pore waters in the lower reaches of the Razdol’naya River in the winter. The river estuary extends upstream for more than 20 km. It was established that the studied sediments are reduced oozes containing pyrite, hydrotroilite, and iron monosulfide, which is direct evidence for sulfate-reduction in the sediments. The diagenesis of organic matter is the main reason for the considerable decrease in the amount of sulfates and the increase in the alkalinity of the sediment pore water. The sedimentary pore water sampled from the deep river pits is characterized by excess alkalinity that cannot be explained by sulfate-reduction and methane genesis. It was suggested that the chemical weathering of silicate minerals and the bacterial mineralization of salts of organic acids could result in the excess alkalinity of the sediment pore water.