Effects of dissolved oxygen and nutrients from the Kuroshio on hypoxia off the Changjiang River estuary


The intrusion of the Kuroshio into the East China Sea (ECS) affects the development of hypoxia off the Changjiang (Yangtze) River estuary; however, quantitative analysis of its impacts is lacking. In this study, the Regional Ocean Modeling Systems (ROMS) model coupled with the Carbon, Silicate and Nitrogen Ecosystem (CoSiNE) model was used to investigate the relative importance of dissolved oxygen (DO) and different nutrients (silicate, nitrate, and phosphate) in the Kuroshio on hypoxia in the ECS. Results show that changes in DO concentrations in the Kuroshio modify the distribution and intensity of hypoxia through direct onshore transport by hydrodynamic processes. An increase in Kuroshio DO concentration by 25% or 50% would result in a decrease of the maximum hypoxia extent (MHE) in the ECS by 76% or 86%, respectively, while a 25% decrease in Kuroshio DO would increase the MHE by up to 219%. The contribution of DO in the Taiwan Strait is almost negligible. In contrast to Kuroshio DO, nutrients affect hypoxia in the ECS through onshore transport by hydrodynamic and biochemical processes. Changes in phosphate and nitrate concentrations by 25% in the Kuroshio would change the MHE by up to 30% and 18%, respectively, accompanied by apparent changes in surface chlorophyll-a concentrations. The effect of silicate on hypoxia is negligible because a 25% change in silicate concentrations in the Kuroshio would result in less than 1% change in the MHE. Our results reveal a hierarchical rank of importance for environmental variables in the Kuroshio (i.e., DO > phosphate > nitrate > silicate) in modifying the development of hypoxia in the ECS.
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