Seasonal Hypoxia And Its Affecting Factors In The Yangtze River Estuary

Hypoxia phenomena arise in the Yangtze River estuary and its adjacent area in summer. Due to the area of marine environment is closely related to coastal economic development, the existence and aggravation of hypoxia have a serious threat to the ecological environment and fishery resources. Therefore, much attention has been paid to the hypoxia issue in the Yangtze River estuary and its adjacent area. In this study, by comparing the historical data of hypoxic zones of the Yangtze River estuary and survey data of 12 cruises in recent years (1985.8、2004.8、2007.8、2007.11、 2009.5、2011.5、2012.2、2012.5、2012.8、2012.11、2014.2、2014.5、2014.8、 2014.11), seasonal changes of dissolved oxygen off the Yangtze River estuary, summer hypoxic zones distribution characteristics (size, location, lowest value of dissolved oxygen), historical changes, and the influencing factors were analyzed systematically. The major results are presented as follows:1. The seasonal distribution of dissolved oxygen in the Yangtze estuary indicated that the dissolved oxygen of the surface and bottom of the water presented the trend of higher near the coast, lower outer coast and mixing up and down uniformly in winter. While in spring, the stratification and algal blooms began to appear (chla=5.23μg/L), hypoxic waters began to develop. In summer, there is a surface water area of high oxygen, and the oxygen saturation is up to 182% which is consistent with the scope of the underlying hypoxic zone. In autumn, the low oxygen zone almost disappeared, but it will has residues of summer low oxygen core in a few years.2. Growing trends of hypoxic zone in Yangtze River estuary in 1959 manifested that hypoxia initially formed in May, the lowest value of dissolved oxygen was located in south of the Yangtze River estuary, and then gradually moved northward, dissolved oxygen value decreased. The lowest value of dissolved oxygen was near the Hangzhou Bay in June. In July, hypoxic zone formated, the lowest value of dissolved oxygen moved to north of the Yangtze River estuary. The lowest value of dissolved oxygen was located in northeast of the Yangtze River estuary and the degree of hypoxia reach annual peak in August. Low oxygen position began to move southward and the levels of low oxygen began to weaken.3. Yangtze River estuary belongs to seasonal hypoxia which existed in July, August, September in this survey and keeped stable in August every year. Besides, it also existed in May or November in some years. The low oxygen value began to appear in 10 meters. The lowest dissolved oxygen value off the Yangtze River estuary has a volatility decrease trend. The low oxygen core moved to north about 0.55 degrees while east-west direction basically unchanged. The areas of hypoxic zone in 2004,2007,2012 and 2007 are not less than that in 1985.4. The regeneration process of phosphate and nitrate in the bottom, especially phosphate, has great contribution to the consumption of dissolved oxygen in the bottom water.5. The main source of POC is terrigenous input and the consumption oxygen ability of terrigenous organic material is limited. Thus the contribution of terrigenous organic matter to hypoxic zone is limited. A large number of POC, conducting chemical and biological oxidation in the underlying, consuming a large amount of the underlying dissolved oxygen, which is from the surface phytoplankton growth, is an important reason for the formation of hypoxic zone.6. The position changes of edge water changes which was produced from Taiwan warm current consistented with the northward moving trend of hypoxic zone, illustrating that the increase of Taiwan warm current contributed to the low oxygen core northward moving and increased the hypoxia zone area. Mainly manifested in the following three points:(1) The enhance of the Taiwan warm current make part of the algae phytoplankton, organic detritus and the bottom low oxygen water move to north. So the hypoxia in the Yangtze River estuary is also made move to north. (2) The strength of upwelling is enhanced and the area is increased along with the enhancement of the Taiwan warm current. The upwelling provides rich nutrients and good light conditions for surface phytoplankton. So it indirectly aggravates the underlying oxygen consumption. (3)The area of the water stratification is increased along with the enhancement of the Taiwan warm current. So this increase the area of the bottom low oxygen water and provide external dynamic conditions for expanding the hypoxia area in Yangtze River estuary.7. The seasonal hypoxia characteristic in the Yangtze River estuary is affected by the seasonal change of water stratification intensity in a certain extent. The high value area of water stratification intensity is similar to the low value area of dissolved oxygen. Besides, the high value center of water stratification intensity and the low value center of dissolved oxygen is basically the same. The density stratification characteristics of hypoxic zone are very strong. The water stratification intensity, which is the necessary condition for forming hypoxic zone, has a very significant negative correlation relationship with the bottom dissolved oxygen value.8. Changjiang diluted water provides nutrient support for hypoxic zone. The decrease of the Yangtze River sediment discharge improved the waters light conditions which is influenced by the Changjiang diluted water. It makes phytoplankton in larger waters have light conditions that they need to grow. Besides, it deepens the euphotic depth and improves the marine primary productivity. So it has an indirect impact on the hypoxia zone area and degree of hypoxia.