Tidal modulation of the hypoxia adjacent to the Yangtze Estuary in summer

The influence of tide on the distribution of hypoxia adjacent to the Yangtze Estuary was investigated using a three-dimensional hydrodynamic eutrophication model. In the absence of tide, the hypoxic areal extent increased from 5226.75 km2 in the base model run to 19,514.25 km2, and it mainly occurred in the inner shelf region near the mouth of the Yangtze Estuary and in the inner part of Hangzhou Bay. During neap tide, the hypoxic areal extent further increased to cover 20,704.50 km2. Hypoxia first appeared to the southeast of Hangzhou Bay. During the neap to spring tide transition, the hypoxic areal extent decreased to 2430.00 km2 due to reduced stratification (2.65 × 10− 3 S-2) and the direction of the current. The hypoxic areal extent further decreased to 2319.75 km2 during spring tide along with increased current and reduced stratification. Also, hypoxia mainly appeared in the submarine valley.