Low dissolved oxygen in the Pearl River estuary in summer: Long-term spatio-temporal patterns, trends, and regulating factors.

Several large semi-enclosed seas and coastal bodies have formed seasonal hypoxic water masses over large areas. The dominant cause for such formations is believed to be the increasing inflow of nutrients from watersheds and urban areas into estuaries and coastal waters. Several studies have reported hypoxic events in the Pearl River estuary (PRE). However, hypoxia events appear to be episodic, which are neither seasonal nor estuary-wide. The reasons for such occurrences are not understood fully. The objectives of this study are to use 24-year time series data obtained during 1988-2011 to analyze the long-term variability in dissolved oxygen (DO) in summer over the entire estuary from urban areas to estuarine coastal waters and examine regulating factors for the occurrences of hypoxia. The results showed that various regulating factors were responsible for the variability of DO in different regions of the PRE. DO decreased in the urban reach, upstream and main estuarine zones of the PRE from about 2000. It was hypoxic (DO <2.0 mg L) in the urban reach zone and periodic low DO (DO <3.5 mg L) and episodic hypoxic in the upstream zone after 2000, mainly influenced by the increasing sewage discharge. The main estuarine zone experienced episodic low DO and sporadic hypoxia after 2000. The ecosystem buffering capacity was characterized by turbidity, well vertical mixing and short residence time which diluted the pressure for the formation of hypoxia. The eastern shelf appeared to be more vulnerable to hypoxia because of its deeper topography, plume front position, and stratification of the water column compared with the western shelf. Its bottom waters experienced seasonal low DO but seldom hypoxia. In comparison, low DO rarely occurred in the western shelf because of its shallow topography and short residence time. In brief, 24-year series data captured from 1988 to 2011 showed a lack of seasonal and estuary-wide hypoxia in the PRE.