Hypoxia in Peter the Great Bay

Studies on hypoxia in Peter the Great Bay (Japan Sea) are reviewed. Seasonal hypoxia is observed in warm season at the bottom of three areas: Amur Bay, Ussuri Bay, and the southwestern part of Peter the Great Bay occupied by the Far-Eastern Marine Biosphere Reserve (FEMBR). Processes of the hypoxia forming are similar in all these areas. The main reason is the dissolved oxygen consumption by microbial degradation of organic matter within topographic depressions in conditions of limited ventilation because of strong summer stratification. The bottom depressions prevent horizontal water exchange and provide accumulation of organic and inorganic suspension, that is another factor important for development of hypoxia. The Amur Bay is the most subjected to hypoxia, being a semiclosed estuarine basin eutrophed by nutrients input from the Razdolnaya River and waste waters of Vladivostok city. The Ussuri Bay has better water exchange and less eutrophication, therefore there are scarce data about hypoxia in this area. FEMBR area has good water exchange and is only episodically influenced by nutrients discharge from the Tumen River, so hypoxia is observed there occasionally. Another consequence of microbial degradation of organic matter in these areas is acidification: pH decreased in 0.5 unit in the bottom water of the Amur Bay in eight decades from 1932 to 2013. Synchronism between regional and global processes of eutrophication, deoxygenation, and acidification of bottom waters is discussed.

Hydrochemical state of the Uglovoy Bight (Amur Bay) in different seasons

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.

Peculiarities of temporal variability of dissolved oxygen content in eelgrass Zostera marina Linnaeus, 1753 meadows in the Voevoda Bay (the Amur Bay, the Sea of Japan)

Currently, the shallow basins with Zostera marina L. meadows are considered as absorbers of atmospheric carbon dioxide, capable of restraining an increase in its concentration. Due to its high primary productivity, eelgrass releases a large amount of oxygen into the environment. To establish the peculiarities of production activity in shallow-water basins, covered with Z. marina meadows, we conducted monitoring of hydrological and production indicators with different measurement intervals on the example of the Voevoda Bay (the Amur Bay, the Sea of Japan). Observations were carried out for eight and a half months (22.09.2012–07.06.2013). Measurements of temperature, salinity, chlorophyll fluorescence, and turbidity were carried out in Z. marina meadows at a depth of 4 m every three hours by a Water Quality Monitor hydrological station. Dissolved oxygen content was determined every hour by an optical oxygen sensor ARO-USB. Two types of oxygen concentration variability were established: 1) seasonal variability, mostly resulting from seasonal variations in the environment; 2) daily variability during the freeze-up period, mostly determined by the intensity of photosynthetically active radiation penetration into sub-ice water. In the autumn season, low oxygen concentrations, up to hypoxic level, were recorded. In the winter and spring seasons, the oxygen content was, as a rule, at 100–130 % of saturation. High daily variability was observed during the freeze-up period, with no snow coverage. In February, the range of daily fluctuations of oxygen concentration reached 730 μmol·kg−1, with 3-fold supersaturation regarding atmospheric O2. As established, the maximum rate of oxygen production, relative to 1 g of Z. marina wet weight, is 6.5 mg O2·h−1·g−1. High daily dynamics of oxygen in seawater is analyzed in relation to eelgrass physiological peculiarities (air lacunae play an important role in oxygen dynamics in the environment), as well as to short-period tides. .

State of aggregations and biological characteristics for mysid Neomysis awatschensis in the northern Amur Bay (Peter the Great Bay, Japan Sea)

Size and sex structure of estuarine-freshwater mysid Neomysis awatschensis in the northern Amur Bay is considered, time of their aggregations formation is determined, and their biomass is estimated on the data of surveys conducted in 2014–2019. The samples were collected at the depth of 0.5–2.0 m in the daytime in May-July. The catches were recalculated per 1 m3 of water. All collected mysids (2212 ind.) were measured under binocular microscope with accuracy of 0.1 mm, weighted by torsion scales with accuracy of 1 mg, and separated to males, females and juveniles. The females were differentiated by 5 stages of their development: 1) immature females with oostegetic rudiments; 2) oviparous females; 3) females with embryos in the marsupia bags without stalk-eyes with black pigment; 4) females with embryos in marsupia bags with black eyes; 5) spawned females with empty marsupii. Time and duration of N. awatschensis aggregating changed by years in dependence on hydrological and weather conditions; the aggregations were either monospecific or mixed with another mysid species — N. mirabilis. The maximum mysid biomass could exceed 500 g/m3 that was appropriate for commercial fishery. The maximum body length of mysid females was 13.7 mm, of males — 12.0 mm. The average length varied from 6.7 to 10.3 mm, being shorter in summer months than in May. Portion of males in the aggregations was always significant (16.3–50.7 %), portion of females varied from 8.7 to 52.5 %, portion of juveniles was the largest in June (29.9–75.0 %). The females to males ratio was 1 : 1 in May and July of 2014–2017, but males prevailed in June 2015 (1.0 : 1.9), whereas females prevailed in June 2016 (1.8 : 1.0). Spawning of the mysids was the most intensive in May-June, though females had embryos at different stages of development in all samples that indicated an extended period of reproduction.

Influence of inter-annual variability of river runoff on the bottom sediments of the Razdolnaya River estuary, Primorye, Russia

Изучены вариации гранулометрического и химического состава верхнего двухсантиметрового слоя донных отложений эстуария р. Раздольная, отобранных трижды в течение 2015–2019 гг., когда водный и твердый стоки реки выросли в 2–3 и 5–6 раз соответственно по сравнению с 2014 г. Показано, что значительный вынос осадочного материала сопровождается увеличением на 10–20 % доли пелитовых (<0,01мм) фракций в отложениях внешней части эстуария в северной части Амурского залива. Вариации гранулометрического состава являются главным контролирующим фактором и определяют 67 % общей изменчивости химического состава осадков эстуария. Еще 20 % изменчивости обусловлены примесью загрязненных отложений залива, содержащих повышенную концентрацию Сорг., Hg, Zn, Pb, Cd, Ag, Cu, Mo, V, а также восстановительным диагенезом, приводящим к обеднению осадков марганцем и кобальтом на разрезе река–море. Установить влияние коагуляционных процессов в зоне смешения на валовый состав донных отложений эстуария р. Раздольная не удалось. Grain size and chemical composition have been studied of the upper 2 cm layer of bottom sediments of the Razdolnaya River estuary sampled thrice in 2015–2019. It was a period when the river runoff increased by 2–3 and 5–6 times for the water and suspended solids, respectively, compared to 2014. It has been shown that significant subtraction of sedimentary material is accompanied by 10–20 % increase of pellite fractions (grain size <0.01 mm) in the deposits of the outer part of anestuary in the North part of the Amur Bay. Grain size composition is the main controlling factor determining 67 % of the total variability of the chemical composition of estuarine bottom sediments. Another 20 % of variability are due to the admix of polluted sediments of the Amur Bay, containing an increased concentration of Corg., Hg, Zn, Pb, Cd, Ag, Cu, Mo, V, as well as diagenesis, leading to depletion of sediments by the Mn and Co on a river-sea section. It is not possible to establish the influence of coagulation processes in the mixing zone on total concentration of trace elements in the bottom sediments of Razdolnaya River estuary.

Peculiarities of the eutrophication of the Amur Bay (Japan Sea) by Razdolnaya River

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.