Spatiotemporal patterns and driving factors of dissolved oxygen dynamics in a highly turbid subtropical macrotidal...

The increasing prevalence of hypoxia in highly turbid, macrotidal estuaries has raised growing concern, however, understanding of dissolved oxygen (DO) dynamics in these unique systems, particularly in subtropical regions remains limited. This study investigates the spatiotemporal dynamics of DO and its controlling factors in the Ao River Estuary, a subtropical macrotidal system characterized by persistent turbidity maximum zones (TMZs) located in the East China Sea. Long-term monitoring revealed significant DO improvements in the river-estuarine continuum, primarily due to reductions in oxygen-consuming pollutants. The primary drivers of DO variability exhibited distinct spatial patterns along the river-estuarine continuum: temperature was the dominant factor upstream, both temperature and oxygen-consuming pollutants played key roles in the midstream, while the downstream TMZs were governed by a complex interplay of tidal forcing, pollutant loads, temperature, and river discharge. Hypoxia (DO <2.0 mg/L) predominantly occurred in TMZs during summer coinciding with elevated river discharge and pollution levels. High-frequency observations demonstrated that short-term DO fluctuations in TMZs were primarily governed by temperature variability, as well as by changes in turbidity and pollution levels-mediated by tidal dynamics and freshwater inputs. In 2022, extreme warming events led to the co-occurrence of high turbidity, elevated temperatures, and pronounced DO depletion in TMZs, underscoring the system's vulnerability to extreme warming stress. Our findings revealed that the drivers of DO variability varied across spatial and temporal scales, emphasizing the necessity for integrated management strategies that incorporate both pollution control and climate adaptation measures to mitigate hypoxia in high-turbidity, macrotidal estuaries.