Seasonal variations in biogenic substances (nitrates, nitrites, ammonium, phosphates, and silicates), dissolved oxygen, dissolved organic matter, chlorophyll, humus, and suspended matter at seven stations on the Razdol’naya River are examined. Based on seasonal variations, the local sources of river water pollution by biogenic substances were identified. The annual and daily fluxes of biogenic substances through the Razdol’naya River into Amur Bay are calculated. A diffuse pollution source—agricultural fields in river valley—are shown to be the main source of biogenic substance fluxes. The vast hypoxia in the bottom water of Amur Bay, detected by the authors in 2007 and 2008, is supposed to be mostly caused by floods on the Razdol’naya R., which create favorable conditions of water blooming in Amur Bay in summer. The biomass of dead phytoplankton, experiencing microbiological decomposition and oxidation, causes hypoxia of bottom waters in the bay.

In the course of the expedition of the Pacific Oceanological Institute in August 2007, extensive hypoxia was found in the near-bottom layer of the Amur Bay water mass. The hypoxia’s formation was immediately reflected in the values and distribution of the carbonate parameters in the near-bottom waters of the bay. The maximum values of the carbon dioxide partial pressure, the dissolved inorganic carbon, and the total alkalinity were associated with the areas of the minimum oxygen content. The microbial destruction of the dead phytoplankton greatly increased the partial pressure of carbon dioxide, which was over 2000 μatm in the hypoxia centers at a depth of about 20 m. At the same time, the carbon dioxide partial pressure in the surface waters of Amur Bay was considerably lower than that in the atmosphere. Hence, the bay’s aquatic area was a sink for the atmospheric CO₂ despite its high content in the near-bottom waters. It was shown that the excess alkalinity associated with the hypoxia sites in the near-bottom layer of water was caused by the sulfate reduction proceeding in the upper layer of the sediment.

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...

Based on detailed hydrological and hydrochemical surveys carried out in each of the four seasons of 2008, Amursky Bay in the north west quadrant of the Japan Sea was found to experience seasonal hypoxia. The primary process of hypoxia formation is a microbiological degradation of the ¡§excess¡¨ amount of diatoms under rather low photosynthetic active radiation in bottom layer and weak water dynamics. The microbiological decay of dead diatoms under light deficient conditions intensively consumes dissolved oxygen and produces phosphates, ammonium, silicates, and dissolved inorganic carbon. Existence of a phytoplankton ¡§excess¡¨ is caused by phytoplankton bloom resulting from nutrient pulses into Amursky Bay. There are two main sources of these nutrients: the waste waters of Vladivostok city and discharge from Razdolnaya River. The river delivers more than two times the amount of nutrients than the waste waters of Vladivostok. It is suggested that the phytoplankton ¡§excess¡¨ might be caused by an enhanced supply of nutrients delivered into the surface layer resulting from the increased discharge of the river on a short time scale. Our data suggest that hypoxia is seasonal, with a peak at the end of summer. The upwelling of the Japan Sea water in the beginning of the fall season and its advection across the shelf is the primary process by which the hypoxia is destroyed. During the winter, strong vertical mixing due to termohaline convection makes the water column uniform and brings more oxygen into the water along with high primary production under the ice. Thus, during the winter season, the ecosystem of Amursky Bay recovers completely