Hydrochemical Study of Academy Bay (Sea of Okhotsk)

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.