Response of bottom hypoxia off the Changjiang River Estuary to multiple factors: A numerical study

The formation and maintenance of low oxygen offshore of the Changjiang River Estuary have occurred with high frequency in certain regions, which are defined as bottom hypoxia hotspots. The two major hotspots are the southern shallow bank (Yangtze Bank) and the submarine canyon. For this study, a high resolution ecosystem model was used to simulate dissolved oxygen dynamics over the continental shelf and to investigate the sensitivity of bottom hypoxia extent to multiple factors. Scenario simulations that varied the river discharge (both freshwater amount and nutrient loads), riverine nutrient concentration, shelf wind speed, and strength of the Kuroshio subsurface water intrusion and intrusion-introduced oceanic nutrient concentration were examined and the extent to which these processes regulated the bottom hypoxia extent was analyzed. Model results indicated that the spatial extent of bottom hypoxia is markedly sensitive to riverine inputs, which have larger impacts on hypoxia over the shallow bank than over the submarine canyon. The simulated bottom hypoxia is more responsive to changes in river discharge than riverine nutrient concentration, because discharge has a dual control on the shelf vertical stratification as well as primary production. The increased wind speed enhanced vertical mixing and thus reduced the spatial extent as well as the total volume of bottom hypoxic water. Weakened Kuroshio intrusion resulted in prolonged near-bottom water residence time, which can cause increase in bottom hypoxia extent. Increasing Kuroshio-introduced nutrient concentration by 30% resulted in enhanced primary production, which caused larger increase in hypoxia extent over the submarine canyon and offshore regions, compared to the shallow bank.