Oxygen depletion in the upper reach of the Pearl River estuary during a winter drought

We examined dissolved oxygen (DO), the carbonate system and nutrients in the upper reach of the Pearl River estuary in a very dry season in February of 2004. We observed very low DO, down to < 12–30 μmol O2 kg− 1 in the surface water, upstream of the vicinity of the Humen outlet, one of the major water channels into the estuary. The oxygen depleted water body encompasses a surface area of > 20 km2 within a salinity range between 1 and 5. Accompanied with the low DO were extremely high pCO2 (up to > 7000 μatm) and nutrients (ammonia > 600 μmol kg− 1 and nitrate > 200 μmol kg− 1). In addition to the aerobic respiration, processes such as nitrification substantially contributed to the consumption of DO, and may have a significant impact on the distribution pattern of the carbonate species including pH. Oxygen depletion was also observed in this area during our prior cruises in other seasons, signifying an alarming environmental condition in the region.