Fluxes of chemical oxygen demand and nutrients in coastal rivers and their influence on water quality evolution in the Bohai Sea

Abstract

Chemical oxygen demand (COD) and nutrient level in water are key parameters for monitoring and assessing water quality. This study systematically investigated the fluxes of COD (CODCr, acidic CODMn, and alkaline CODMn) and nutrients (NH4+-N, NO3−-N, NO2−-N, and PO43−-P) in 36 major coastal rivers and their influence on the evolution of water quality in the Bohai Sea (BS). The results showed that most of the rivers around the BS are already heavily polluted considering COD, whereas they were less polluted by nutrients. The proportions of COD, NO2−-N, and PO43−-P during the rainy season, normal water season, and withered water period decreased, but the proportions of NH4+-N and NO3−-N increased. The fluxes of COD and PO43−-P were significantly affected by runoff, whereas the flux of dissolved inorganic nitrogen (DIN) was significantly affected by the concentration of DIN The molar N/P ratios in coastal rivers were higher than those in the BS. The molar N/P ratios obtained from the State Oceanic Administration of China (SOA) data and those calculated according to river fluxes had a significant linear correlation. The variation in the alkaline CODMn concentration announced by SOA was consistent with that calculated according to river fluxes, but the variation in DIN and PO43−-P concentrations was not consistent. The fluxes of CODCr and nutrients have decreased in recent years due to the implementation of pollution management schemes. This study provides a scientific reference for restoring ecological environment and protecting water quality in the BS.