Long-term spatiotemporal variations in and expansion of low-oxygen conditions in the Pearl River estuary: a study synthesizing observations during 1976–2017

The Pearl River estuary (PRE) frequently experiences low-oxygen conditions in summer, with large-extent low-oxygen events and a long-term deoxygenation trend being reported recently. In this study, we provide a synthesis of the spatiotemporal patterns and incidence of different low-oxygen levels in the PRE based on the in situ observations collected from 1976 to 2017 and aim to elucidate the underlying mechanisms of low-oxygen conditions and their changes over the past 4 decades. The long-term observations show that the dissolved oxygen (DO) content in the PRE has had significant temporal variability and spatial heterogeneity. Low-oxygen conditions (DO < 4 mg L−1) have occurred mostly in the bottom waters of 5–30 m during summer and early autumn, with locations and severity varying substantially between years. Coastal waters from the southwest of Lantau Island to the northeast of the Wanshan Archipelago were identified as a hotspot area prone to subsurface low-oxygen conditions due to the combined effects of comparatively deep topography, a certain residence time and stability of the water column, and enhanced oxygen depletion related to high phytoplankton biomass. In addition, the low-oxygen waters, either directly imported from the upstream reaches or generated locally and further transported with the estuarine circulation, also had considerable impacts on the oxygen levels in the estuary. As for early autumn, marked low-oxygen conditions were present in both the surface and the bottom waters. A large area affected by low oxygen (∼ 4450 km2) was found in September 2006, where the low-oxygen conditions were comparable to the most severe ones observed in summer. The area was formed by the inflows of low-oxygen waters from the upstream reaches and enhanced oxygen depletion driven by an intricate coupling of physical and biogeochemical processes. Our analysis also reveals there has been an apparent expansion of the summertime low-oxygen conditions at the bottom of the PRE since the years around 2000, coincident with major environment changes in the Pearl River region. Overall, the PRE seems to be undergoing a transition from a system characterized by episodic, small-scale hypoxic events (DO < 2 mg L−1) to a system with seasonal, estuary-wide hypoxic conditions in summer. Although exacerbated eutrophication associated with anthropogenic nutrient inputs has generally been considered the primary cause for the deterioration of low-oxygen conditions in the PRE, the sharp decline in sediment load may play an important role as well via increasing water transparency and thereby supporting higher and broader phytoplankton biomass in the estuary.