175 ISSN 0001-4370, Oceanology, 2018, Vol. 58, No. 2, pp. 175–186. © Pleiades Publishing, Inc., 2018. Original Russian Text © P.Ya. Tishchenko, P.Yu. Semkin, G.Yu. Pavlova, P.P. Tishchenko, V.B. Lobanov, A.A. Marjash, T.A. Mikhailik, S.G. Sagalaev, A.F. Sergeev, E.Yu. Tibenko, N.D. Khodorenko, R.V. Chichkin, M.G. Shvetsova, E.M. Shkirnikova, 2018, published in Okeanologiya, 2018, Vol. 58, No. 2, pp. 192–204. Hydrochemistry of the Tumen River Estuary, Sea of Japan P. Ya. Tishchenko*, P. Yu. Semkin, G. Yu. Pavlova, P. P. Tishchenko, V. B. Lobanov, A. A. Marjash, T. A. Mikhailik, S. G. Sagalaev, A. F. Sergeev, E. Yu. Tibenko, N. D. Khodorenko, R. V. Chichkin, M. G. Shvetsova, and E. M. Shkirnikova Il’ichev Pacific Oceanological Institute, Far East Branch, Russian Academy of Sciences, Vladivostok, 690041 Russia *e-mail: tpavel@poi.dvo.ru Received June 8, 2016 Abstract—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 fluo- rescence, and turbidity profiles were obtained. Water was sampled from the surface and bottom layer. The water samples were analyzed for major ions, pH, salinity, concentrations of dissolved oxygen, major nutri- ents, dissolved organic carbon, humic matter, and δ 18 О and δD isotopes. This estuary is attributed to microt- idal type with a flushing time of about 10 h. A phytoplakton bloom occurred in the top layer of the estuary. For surface horizons, the hydrochemical parameters show a linear correlation with salinity. In the bottom horizons, all these parameters, except for major ions and δ 18 О and δD isotopes, reveal substantial noncon- servative behavior. The nonconservative behavior of the hydrochemical parameters in the bottom waters was mainly caused by degradation of the phytoplankton biomass at the water/sediment interface. Hypoxic condi- tions were established in the bottom waters of the estuary in May. DOI: 10.1134/S0001437018010149 INTRODUCTION The Tumen River estuary aroused great social and scientific interest at the threshold of the 21st century in relation to an international project aimed at trans- forming the area into a free economic zone (around 100 km 2 ) developed under the aegis of the UNO (Tumen River Economic Zone—TREZ) [1, 16]. The next version of the project was the Tumen River Area Development Program (TRADP). Later, this project was again transformed, first into the Tumen River Economic Development Area (TREDA), and then into Tumangan (Korean name of the Tumen River), which proposed construction of the port megalopolis with the participation of North Korea, China, and Russia [1, 16]. Preliminary ecological studies are required to implement the project. In Russia, these studies were carried out in the mouth of the Tumen River and in the adjacent Russian water basin by the Institute of the Sea Biology (ISB) and Pacific Institute of Geography (PIG) of the Far East Branch of the Rus- sian Academy of Sciences with the assistance of the Far East Regional Hydrometeorological Research Institute (FERHRI) [16–18]. This work was comprehensive, i.e., with hydrological, hydrochemical, and hydrobio- logical studies, but it did not provide parameters that could characterize the intensity of production– destruction processes: dissolved oxygen, nutrients, and dissolved species of organic and inorganic carbon. The study of precisely these parameters made it possible in 2013 to establish anoxia of bottom waters in the southwestern part of the Far East Marine Bio- sphere Reserve, 23 km northeast of the mouth of the Tuman River [11, 33]. Further studies revealed sea- sonal variations of oxygen deficit in the basin between Furugelm Island and the continental slope [13]. The reasons for hypoxia formation in the marine reserve have not yet been completely understood. The com- paratively small distance from the mouth of the Tyu- men River to the hypoxic area (23 km) attracted our attention to the role of the river in biogeochemical processes in southwestern Peter the Great Bay. There- fore, detailed hydrological–hydrochemical studies were carried out in May and October 2015 in south- western Peter the Great Bay, which included a study of the Tumen River estuary. This work reports the results of a study of the Tumen River estuary carried out in the spring and autumn of 2015. MATERIALS AND METHODS The Tumen River (Tumynjahn in Chinese, Tuman- gan in Korean, Tumangang in the old Russian geo- graphical literature, Tyumen Ula up to 1974, and the Tumen or Dumen River in English literature) originates at the Changbai highland, in the eastern slope of extinct Pektusan volcano (41°58.167′ N, 128°10.933′ E). The total drainage area of the river is 33000 km 2 . Most part of the basin of the Tumen River is located in the mountainous area, which caused the clearly expressed mountainous character of the river [16]. This is the MARINE CHEMISTRY