Hypoxia in the upper reaches of the Pearl River Estuary and its maintenance mechanisms: A synthesis based on multiple year observations during 2000-2008

Based on our multiple year observations during 2000-2008 in the Pearl River Estuary, this study sought to synthesize the long-term pattern of hypoxia and its relationship to organic carbon and nutrient loading in this important world major estuary under significant human impacts. We confirmed previously observed year-round low dissolved oxygen (DO) of < 63 μmol kg− 1 reaching the threshold of hypoxia in the upper reaches of the Pearl River Estuary, extended from the Guangzhou Channel to downstream of the Humen Outlet, in the surface water, covering a water body of ~ 75 km length. The surface DO concentration had a significantly negative correlation with dissolved and particulate organic carbon, and NH4+ concentrations. Both aerobic respiration and nitrification highly varied spatially in the water column. The highest rates of respiration and nitrification were observed in the Guangzhou Channel, which decreased downstream along with organic carbon and NH4+ concentrations. Seasonally, the highest rates of total oxygen consumption upon normalization to the substrate (TOC, total organic carbon; and NH4+) were observed in summer, suggesting that both the substrate availability and water temperature were major factors controlling the oxygen consumption rates. Oxygen mass balance calculations showed that in summer, the oxygen consumption rate in the water column by aerobic respiration (21.3 × 106 mol O2 d− 1) and nitrification (14.5 × 106 mol O2 d− 1) was almost balanced by the reaeration (32.6 × 106 mol O2 d− 1) and net advective complement (2.3 × 106 mol O2 d− 1). The contributions of other processes (e.g., sediment oxygen consumption and photosynthesis) appeared to be minor. We estimated that the on-site biogenically produced organic matter, or autochthonous organic material, contributed only 13% ± 10% of the TOC being respired in the hypoxic area, suggesting that the allochthonous organic material, primarily derived from sewage discharge, dominated aerobic respiration and the associated oxygen consumption. Meanwhile, NH4+ which was clearly reflective of sewage loadings (if not all) dominated the nitrification process and the associated oxygen consumption. Taken together, the hypoxia in the studied area was profoundly anthropogenic and this conclusion should have many implications towards regional environmental management.