Impact of the transboundary Razdolnaya and Tumannaya Rivers on deoxygenation of the Peter the Great Bay (Sea of Japan)

Abstract

It has been established that the waters of the transboundary Razdolnaya (Suifen) and Tumannaya (Tumen) Rivers are highly polluted by nutrients. Loads of nutrients supplied by the Razdolnaya and Tumannaya Rivers cause eutrophication and induce seasonal hypoxia in areas off the river mouths in the Amursky Bay (northwestern part of Peter the Great bay (PGB)) and the Far Eastern Marine Biosphere Reserve (FEMBR) area (southwestern part of PGB). In both cases, hypoxia developed in the bottom waters in topographic depressions in the basins after the peak nutrient fluxes that occur in summer due to the monsoon climate. In 2013, anoxia occurred in the FEMBR area as a result of unusually heavy atmospheric precipitation at the end of July. The hypoxic areas expanded over approximately 60 km2 and 17 km2 in Amursky Bay and the FEMBR area, respectively. The hypoxia durations were approximately three months in 2011 in the semi-enclosed Amursky Bay and three weeks in 2014 in the more open FEMBR area. The observed hydrochemical anomalies (oxygen, phosphorus, silica, pH, DIC, ammonium) suggest that the main cause of hypoxia is degradation of “excess” diatoms on the water/sediment interface under dynamic water limitation. Mooring observations by a water quality monitoring station registered very high chlorophyll concentrations (approximately 50 mg/m3) in the bottom waters of the FEMBR area. These high chlorophyll concentrations may be attributed to the topographic depression, which works as a sediment trap and accumulates organic matter. Sharply increasing of transparency of water column in the FEMBR area caused by replacement of coastal waters on the waters of open sea can to develop periphyton on seafloor. There is synchronization in deoxygenation of regional water basins (in our case it is the shelf and open part of the Sea of Japan) and global basins (oceans) which may be caused by variations in the chemical composition of atmosphere.