Subsurface low dissolved oxygen occurred at fresh- and saline-water intersection of the Pearl River estuary during the summer period

Estuarine oxygen depletion is one of the worldwide problems, which is caused by the freshwater-input-derived severe stratification and high nutrients loading. In this study we presented the horizontal and vertical distributions of dissolved oxygen (DO) in the Pearl River estuary, together with temperature, salinity, chlorophyll a concentration and heterotrophic bacteria abundance obtained from two cruises during the summer (wet) and winter (dry) periods of 2015. In surface water, the DO level in the summer period was lower and varied greater, as compared to the winter period. The DO remained unsaturated in the summer period if salinity is < 12 and saturated if salinity is > 12; while in the winter period it remained saturated throughout the estuary. In subsurface (> 5 m) water, the DO level varied from 0.71 to 6.65 mg L− 1 and from 6.58 to 8.20 mg L− 1 in the summer and winter periods, respectively. Particularly, we observed an area of ~ 1500 km2 low DO zone in the subsurface water with a threshold of 4 mg DO L− 1 during this summer period, that located at the fresh- and saline-water intersection where is characterized with severe stratification and high heterotrophic bacteria abundance. In addition, our results indicate that spatial DO variability in surface water was contributed differently by biological and physio-chemical variables in the summer and winter periods, respectively.