Study On The Variation Of Water Body Environment In The Yangtze River Estuary After The Tropical Cyclone

The Yangtze River Estuary and adjacent sea area is frequently influenced by the tropical cyclone every year. The tropical cyclone has become one of the important factors to impact the water body environment of the Yangtze River Estuary. The water physical、chemical environment and biological production changes were largely due to the tropical cyclone which brought heavy rainfall to the Yangtze River Estuary and caused horizontal transport and vertical mixing in the water body. But there are few related research on this aspect in the Yangtze River Estuary and adjacent sea area because of the difficulty of sampling during the tropical cyclone. Based on the field observation in the Yangtze River Estuary before and after the passage of the tropical cyclone1007"Kompasu" and1109"Muifa", this paper studied the influence of the tropical cyclone to the water body environment in the Yangtze River Estuary and had important theoretical significance to improve the deficiencies of the research.First, the tropical cyclone passed through the Yangtze River Estuary strengthened the water mixing action and changed the water temperature and salinity structure. The surface water was entrained to the bottom which increased the temperature and reduced the salinity of the bottom water. By contrast, the surface temperature dropped and its salinity increased. This study also found that the bottom temperature increased and its salinity dropped after the tropical cyclone "Kompasu" transited the Yangtze River Estuary. But the surface temperature ascended due to the high temperature and the surface salinity dropped. The depth of thermocline and halocline descended after the tropical cyclone-and their stratifications got weaker than before. The Yangtze River Estuary water temperature was still relatively high after the transit of the "Muifa", but the temperature was close to the level before the transit of the tropical cyclone in several water layers. The surface salinity decreased significantly because of the influence of the Yangtze River diluted water, but the bottom salinity did not recover to the pre-level and the thermohaline stratification layers of the water was less than before.Second, the distribution and content of the chemical elements had changed after the tropical cyclone passed through the Yangtze River Estuary. By the combined impacts of the water temperature and phytoplanktop respiration, the DO concentration decreased after the tropical cyclone "Kompasu" transit the Yangtze River Estuary. But the DO content has gradually increased after few days of the tropical cyclone "Muifa" transit. The concentrations of DIN and SiO3-Si increased significantly after the tropical cyclone "Kompasu", PO4-P concentration decreased in the nearshore waters because of the adsorption, but it increased in the offshore waters. The fluxes of DIN、PO4-P and SiO3-Si increased by3.87×109mol、0.13×109mol and4.70×109mol respectively in the Yangtze River Estuary after "Kompasu", and the N/P raised with these changes. A majority of the concentrations of DIN and SiO3-Si reduced after "Muifa", and the PO4-P concentration increased in the lateral surface water but dropped in the bottom water. Especially in the rich-phytoplankton layers, the nutrient concentration was lower and formed a low-nutrient zone.Third, the biological production had changed by the influence of the alteration of the water physical and chemical environment. The survey data showed that the value of Fm and Fo raised after the tropical cyclone "Kompasu" transit the Yangtze River Estuary. These phenomenon indicated that the content of chlorophyll and phytoplankton biomass increased after the tropical cyclone. The position of subsurface chlorophyll maximum became much deeper and its layer thickness increased after the tropical cyclone.