Summer hypoxia adjacent to the Changjiang Estuary

Changjiang, the third largest runoff in the world, empties into the East China Sea from Shanghai, the fastest developing area of China. With the increasing nutrient load from the river, a severe hypoxia zone was found to about 2  10⁴ km². The mechanism of hypoxia formation adjacent to the Changjiang Estuary receives more and more attention from both scientists and managers. This paper discusses the relationship between hypoxia and the water masses, primary production, particulate material transport and the density stratification in these areas according to data obtained from a cruise in September, 2003. Hypoxia is formed by organic detritus decay. The particulate organisms do not mainly come from the Changjiang river, or from the dead algal deposed locally, but from the local benthic algae or particles advected from the south. Maintenance of hypoxia is due to the large density stratification caused by the significant salinity difference between the fresh plume and salty water from Taiwan Strait. This applies also to other estuaries with large runoff and rapid economic growth drainage, such as the Pearl River. It is suggested that the hypoxic zone here is much more sensitive than that outside Mississippi River. More cruises over different weather and tide conditions are needed to prove this hypothesis. Interdisciplinary research should be further developed in the future.