Long-term changes of dissolved oxygen, hypoxia, and the responses of the ecosystems in the East China Sea from 1975 to 1995

Observation data obtained in the 32 degrees N transect (transect E) in 1975-1995 were used to analyze the long-term changes in dissolved oxygen (DO) concentration and near-bottom hypoxic water in the East China Sea (ECS). A declining trend in annual average DO concentration and the degree of DO saturation was observed. Consequently, the apparent oxygen utilization in the western waters of transect E was on the rise. There was a seasonal hypoxic phenomenon in near-bottom water in the western water of transect E. The width of hypoxic water formed in summer gradually extended eastward along the continental shelf (transect E) at the rate of 3.12 km year(-1). Three potential reasons might have caused the formation and maintenance of near-bottom hypoxic water. First, the special hydrological topography and hypoxic deep water of the Taiwan Warm Current provided a backdrop for the hypoxic zone. Second, in summer, the strength of water column stratification restricts water exchange. Third is the occurrence and decay of the phytoplankton bloom. In surface water, nutrient concentrations increased gradually, and chlorophyll (Chl a), primary production, and phytoplankton biomass in summer increased. On the other hand, the community structure of phytoplankton, zooplankton, and zoobenthos became simple. Blooming phytoplankton consumed plenty of nutrients in the surface, but the upwelling of nutritious bottom water was suppressed by the strong thermocline. As a result, sinking of phytoplankton was enhanced because of nutrient deficiency. In recent years, a serious lack of zoobenthos in the study area corresponded to a higher degree of hypoxia. This phenomenon would have a major effect on the evolution of ecological dynamic systems in the ECS.