The Seasonal Variation Characteristics Of Freshwater Lens In Yongxing Island And Its Impact Factors

Freshwater lens is an important freshwater resource on coral reefs.It is of great importance to study its seasonal and spatial changes for the development and utilization of groundwater in Yongxing Island.Based on the data of anchoring depth observation,horizontal observation and vertical continuous observation in Yongxing Island,the seasonal variation,horizontal and vertical variation characteristics of freshwater lens are analyzed in this paper.The main conclusions are as follows:(1)The temperature and salinity of the freshwater lens have obvious seasonal variations.The continuous anchoring observation results show that the freshwater lens features low temperature and high salinity in summer,high temperature and low salinity in winter,while water level changes little throughout the year.The characteristics of the freshwater lens has different change trend in surface layer and in subsurface layer.Correlation analysis show that the meteorological factors such as temperature,humidity and air pressure have little influence on the characteristics of the freshwater lens,while the average sea level height is highly correlated with the variation of the freshwater lens,indicating that the average sea level height is of great significance to the seasonal variation of the freshwater lens.As an important source of the freshwater lens in Yongxing Island,precipitation would instantaneously affect the salinity of the freshwater lens.(2)Based on the horizontal observation data,we first outlined the horizontal structure of the freshwater lens in Yongxing Island: the center is thick,the edge is thin,and the upper surface is about 15 m higher than the base level.Its main coverage area is the original ellipse-shape basin area of the island.The annual mean thickness of the freshwater lens is about 12.6 m.Though the thickness in the center of the freshwater lens is thick in summer and autumn,thin in spring and winter,but its coverage shrinks in summer and autumn.This characteristic is also found in vertical continuous observation.Horizontally,there is a sharp front between freshwater and saltwater of freshwater lens,and the transition band is narrow.(3)The effect of tides on the freshwater lens mainly results in daily periodicity of the freshwater lens.During the neap tide,the envelope of the freshwater lens would not deform by the tide which the main effect is supporting,while during the high tide,the interface between freshwater and saltwater will rise,forming a structure similar to an up-cone at the high tide,thus destroying the envelope shape of the lens.At this time,besides the supporting effect,the tide will cause the mixing of freshwater and saltwater.The observation results in different seasons show that the effect of diurnal tide on freshwater lens is greater than that of semidiurnal tide,and the larger the amplitude of water level,the more obvious the fluctuation of the lower interface of the freshwater lens.(4)Besides physical parameters,ecological parameters also have obvious seasonal and spatial variations.