Study On Main Influencing Factors Of Formation And Deterioration Of Summer Hypoxia Off The Yangtze River Estuary

We studied the distribution characteristics and historical changes of summer hypoxia off the Yangtze River Estuary, and discussed the formation and aggravating reason of the hypoxia according to the historical and recent survey data (from Sep.1958 to Sep. 1959, Sep.2003, Jul.2005 and Aug.2009), and some other historical data collected from other papers and water monitoring systems (the DO value and location of the historical hypoxia, the historical variation of temperature in the Yangtze River estuary, the historical variation of the discharge of Yangtze River diluted water and sediments, the annual variation of plum rain strength in the middle and lower reaches of Yangtze River). The results showed that:1. The center of hypoxia zone was situated in the sea area of 30.75°N-32°N, 122.5°E-123.25°E, which was in the trough off the Yangtze River estuary. Throughout the year, the movement of monthly minimum DO zone had certain seasonal regularity. In spring, the low value area was located in the southernmost end of research area, and then moved towards north gradually with declining DO value, and then reached Yangtze River estuary in August, formed into the hypoxia. After that, it moved towards south gradually, along with which, the hypoxia disappeared gradually.2. Since the late 1950s, the hypoxia off the Yangtze River estuary presented a deterioration trend, which can be manifested from that the hypoxia event with the minimum DO value lower than 1.5mg/L mainly occurred after 1990s, and besides, the hypoxia area in 2009 is much larger than that in 1958, which shows the enlargement of hypoxia zone.3. Because there are complicated underwater topography and condition of hydrodynamics off the Yangtze River estuary and its adjacent sea area, the forming mechanism of summer hypoxia off the Yangtze River estuary must be very complex. Hypoxia was formed mainly under the interaction of physical and biochemical processes, and the physical process plays an important role. The formative process was affected by many external forcing factors, including some environmental factors like water temperature and salinity, and some coastal and ocean current systems like Yangtze River diluted water, Yellow Sea Coastal Current, upwelling, Taiwan Warm Current and Kuroshio Current. On one hand, these influential factors directly affect the forming process of hypoxia; on the other hand, through mutual effect and influence, they form cold pool and thermal barrier, and then eventually promote and determine the formation of hypoxia through cold pool and thermal barrier effect.1) The "cold pool" is a water mass situated below the 15-m surface layer off the Yangtze River Estuary. It has the characteristics of low temperature, isolation, close, and stability. It emerges in summer and is influenced by the Taiwan Warm Current, upwelling, and vortex circulation. It is the area where summer hypoxia forms.2) The "thermal barrier" is an area with higher temperature that forms around the periphery of the "cold pool". It is the barrier that prevents the "cold pool" from exchanging water with the peripheral areas, and it is affected by the Taiwan Warm Current, runoff from the Yangtze River, surface water of the Kuroshio, and stratification. It reinforces the stability.of the "cold pool".4. Through analyzing the deterioration reason of hypoxia under the influences of both climate change and human activities, it was regarded that sea water temperature rising and the variation of Yangtze River hydrological processes have certain effect on hypoxia zone. The rising of sea water temperature will cause the decreasing of dissolved oxygen solubility and increasing of water stratification, and then strengthen the hypoxia. The increase of plum rain strength (precipitation in summer) strengthened the stratification intensity and blocked the exchange of water in vertical direction, and accordingly, the hypoxia became more serious. The decrease of Yangtze River diluted water discharge and sediment delivery will theoretically make the hypoxia zone move shoreward but it is not in reality, which shows that the nutrients and organisms transported by the Yangtze River are not the main influential factors in the formation of hypoxia.