The Hydrological, Hydrochemical, and Microbiological Characteristics of Amur Bay during the Summer Monsoon

An integrated hydrological and microbiological survey of Amur Bay in July 2022 showed that the levels and distribution of values of the thermohaline and hydrochemical parameters indicate the effect of the summer monsoon and river runoff. Under southerly winds, there was a setdown of the water surface along the western coast, which resulted in a decrease in temperature by 0.3–1°C and an increase in salinity by 2.6 PSU relative to those near the eastern coast. At the same time, a setup effect with blocking of freshened and warm, nutrient-enriched surface waters was observed in the northeastern part of the bay. These favorable conditions caused phytoplankton blooms accompanied by high concentrations of chlorophyll a (up to 6–9 μg/L) and oxygen saturation of water (up to 120%). In the near-bottom layers of this sector, hypoxia with oxygen concentrations lower than 5% was observed as a consequence of developing eutrophication. According to the results of microbiological testing, the waters of Amur Bay in the summer of 2022 were classified as mesosaprobic, enriched in organic compounds, with accumulation of organic matter prevailing over its degradation. The biological pollution of water was indicated by the high abundance of bacteria of the sanitary indicator group, which exceeded the maximum permissible concentration; the activity of plant communities was indicated by a high abundance of phenol-resistant microorganisms. Oil pollution was detected only in vicinities of sources of petroleum hydrocarbons: an oil terminal and a junction railway station. The low abundance of metal-resistant microorganisms indicated the insignificance of specialized (technogenic) pressure on the waters of the bay at the sampling time.