Hydromorphological mechanisms leading to hypoxia off the Changjiang estuary.

Based on the seasonal surveying data and long-term data, the annual changes in the geographical locations, occurrence frequency, affected areas and the minimum oxygen level as well as the formation mechanism of the summer hypoxia off the Changjiang estuary are summarized and discussed in this paper. The historical data indicates that there were episodes of hypoxia in the past 50 years but not every year, and the event of summer hypoxia could be traced back to as early as late 1950s off the Changjiang estuary. The minimum oxygen levels in the hypoxia zone did not show any decline in the past 50 years, but all the events with large size of affected area (>5000km(2)) were observed after the late 1990s, suggesting an enlarging trend. The author argues that the development of summer hypoxia off the Changjiang estuary was related not only to stratification and input of suspended particulate matter, but also to the inflow of Taiwan warm current water as well as the bottom topography.