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 CO<sub>2</sub> 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 mmol/L), total phosphorus (1.5-2.0 μmol/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 CO<sub>2</sub> 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.