Hypoxia off the Changjiang (Yangtze River) Estuary: Oxygen depletion and organic matter decomposition

In an especially dry year (2006) in the Changjiang Estuary, three cruises were conducted between June and October, to study the process of oxygen depletion. Data for the hypoxic zone pooled for 1959 through 2006 suggest that a dramatic increase in the area of hypoxia has occurred in recent years, and that the center of hypoxia moved northwards in 2006. In August, the hypoxic area (dissolved oxygen, or DO, <62.5μM) in the northern region was 15,400km², which is comparable to that in the Gulf of Mexico. A large area of low DO (62.5μM<DO<94μM) also was found in the southern region. In near-bottom waters, particulate organic carbon (POC), dissolved inorganic phosphorus (DIP), dissolved inorganic nitrogen (DIN) and apparent oxygen utilization (AOU) showed coupled variation. For example, relationships can be found between AOU and POC/nutrients (POC/DIP: r=-0.47, POC/DIN: r=-0.50; p<0.001, n=86), and between AOU and Δσ of the water column (r=0.66, p<0.001, n=86; Δσ=density_{near-bottom waters}-density_{surface waters}). It is interesting that oxygen depletion in the northern and southern regions developed separately, and they showed distinct differences. Oxygen depletion in the southern region is milder and relatively long lived, whereas in the northern region it is more pronounced and short lived. The different relationships between AOU and inorganic nutrients, indicates different mechanisms for the occurrence of oxygen depletion between the southern and northern regions, respectively. This can be due to 1) the influence of dissolved organic nutrients as another decomposition product besides inorganic forms, 2) and/or different chemical composition of organic matter that decomposed in the near-bottom waters.