Effect of River Runoff on the Hydrochemical Characteristics of Water in Udskaya Bay and Nikolaya Bay (the Sea of Okhotsk) in Summer

The article presents the data of an comprehensive expedition of the Pacific Oceanological Institute, Far East Branch, Russian Academy of Sciences, performed in the mixing zones of the Uda and Usalgin rivers in July 2016. During the flood period, they occupy most of the catchment basins: Udskaya Bay and Nikolaya Bay. The estimated discharge of the Uda and Usalgin rivers was 4390 and 173 m 3 /s, respectively, and estuarine waters with salinity to 8‰ spread from their mouths to a distance of 25 km in Udskaya Bay and 2.5 km in Nikolaya Bay. The boundary of the estuarine seashore distinguished by the isohaline 30‰ was at a distance of 85 km from the river mouths in the both mixing zones. River runoff—the main supplier of suspended particulate matter and silicates—limited the thickness of the photic layer and photosynthesis at the early stage of mixing zones to the salinity of ~24‰, where water was a source of CO 2 for the atmosphere. At the later stage of the mixing zones, under the conditions of a greater thickness of the photic layer, production dominated, and the water area was a sink for atmospheric CO 2 . Mineralization of autochthonous organic matter on the bottom of the receiving basins was the main source of inorganic forms of nutrients. The lack of silicates (DISi/DIN < 1 and the relatively low DIN/DIP ratio in the range of 1.8–8) was a limiting factor of photosynthesis at the late stage of the mixing zone.