High-Resolution Remote Sensing Monitoring And Dynamics Of Coral Islands And Reefs In The Xisha Islands,South China Sea

The dynamic changes of coral islands and reefs can not only reflect their development and evolution process,but also present the changes of ecological environment characteristics of coral islands and reefs and their game relationship with the socio-economic development and policy system of coral island and reef area.High-accuracy monitoring and long-term dynamics analysis of coral islands and reefs are the basis of development and sustainable management of coral islands and reefs resources.In the coral islands and reefs of the South China Sea,the coral reef ecosystem of the Xisha Islands is the most representative in China,and it is the key for developing the resources of the South China Sea and protecting the environment of the South China Sea.Therefore,the Xisha Islands has important political,military and ecological values.In this paper,the coral islands and reefs of the Xisha Islands are taken as a systematic whole.Using high-resolution optical remote sensing images combined with field survey data,high-accuracy water depth retrieval model around coral islands and reefs and high-resolution coral shoals detection model are developed to find out the water depth around coral islands and reefs and the distribution of coral shoals,the changes of coral reef sandbanks and coral islands under the influence of natural conditions and human activities are clarified,and finally a large-scale spatio-temporal dynamic information of coral islands and reefs in the Xisha Islands is obtained efficiently and accurately.The main research contents and conclusions are as follows:1)High-accuracy remote sensing retrieval of water depth around coral islands and reefs in the Xisha Islands:The water depth around coral islands and reefs is very irregular,and accurate water depth data is very important to ensure the safety of navigation.A Long-Short Term Memory(LSTM)neural network in deep learning of new generation AI technology is introduced for multispectral remote sensing detection of water depth,and an LSTM neural network model suitable for remote sensing water depth retrieval for coral islands and reefs was developed.The water depth retrieval results based on the LSTM model are highly satisfactory for water depths of 5–25 m with MREs averaging 7%–9%(minimum:4.02%,maximum:14.62%).They can not only clearly reflect the geomorphic units of the entire reef such as the reef flat,front slope(seaward slope)and patch reef,but also reveal subtle geomorphologic features of the reef flat surface and the valley system.Meanwhile,LSTM water depth retrieval model has good universality and scalability;Even when the number of control points is small,it also shows strong water depth retrieval ability.2)High-resolution optical remote sensing detection for coral shoals in the Xisha Islands:The spectral characteristics of coral shoals and the sea are very similar,and the light signal in coral shoals area is very weak due to its exponential attenuation in seawater,so it is difficult to identify coral shoals with high accuracy from optical remote sensing images.In this paper,a high-accuracy optical remote sensing detection method for coral shoals is proposed by combining deep belief network(DBN)classification model with the edge detection algorithm gradient vector flow(GVF)Snake to obtain the accurate location,range and other information of coral shoals.The detection results by DBN combined with GVF-Snake are more precise in the description of shoal contour,and the appropriate manual correction is carried out based on the detection results can replace the tedious process of manually drawing the contour of coral shoals.Depending on the detection results of multi-phase remote sensing images comprehensively,Yinli Shoal is distributed in northwest-southeast direction and its area is about 20 km~2.Binmei Shoal is distributed in northeast-southwest direction,covering an area of about 130 km~2.Beibianlang composes of three oval underwater reefs with a total area of about 5.1 km~2.Zhanhan Shoal is roughly oval in shape,with an area of about 6.4 km~2.Shanhudong Shoal is distributed in northeast-southwest oval shape,with a small area of only 0.05 km~2.Yongnan Shoal is basically distributed in a semicircle in east-west direction,covering an area of about 2.4 km~2.3)Stability of coral reef sandbanks in the Xisha Islands:Most of the coral reef sandbanks have no vegetation growth and are easily affected by climatic and hydrodynamic conditions,with unstable position and changeable shape.Through the assessment,Bei Sandbank,Zhong Sandbank,Panshiyu and Kuangzaibei Island are relatively stable,with the total displacement of the center of mass less than 30 m,while Xixin Sandbank,Sanzhizai,Quanfu Island and Yinyuzai are yet in an unstable stage,with the total displacement of the center of mass more than 100 m.The stability of the coral reef sandbanks come from the integrated effect of sediment supply,marine dynamic environmental conditions and coral reef topography.Under the same monsoon wave and typhoon wave,each sandbank is affected by different hydrodynamic forces due to the different position of each sandbank in the reef flat and the different beach sediment types,resulting in individual evolution process and stable state of eight sandbanks.4)Spatial and temporal dynamics of typical coral islands in the Xisha Islands:After the establishment of the Sansha City in 2012,the community construction and tourism and port industry of the Xisha Islands are developing vigorously,and the development intensity of main coral islands is constantly increasing.This paper analyzed the spatial and temporal changes of the islands where the Xisha Islands'communities,territorial sea base points and natural reserves are located.Community resident islands have changed significantly,especially Yongxing Island,Zhaoshu Island,Kuangzai Sandbank and Yinyu.These changes were mainly caused by the island-filling activities and the increasing development and utilization activities on the islands.The islands where territorial sea base points and natural reserves are located are relatively stable,and the topography,landform and vegetation of the islands are basically kept as the original natural state.Most of the changes were distributed at the end of each island.The beach rocks play a great role in protecting the islands.The islands with well-developed beach rocks basically maintain a stable state,while islands without beach rocks or with damaged beach rocks change greatly.The research results can provide supporting data and technical methods for the reasonable development and utilization of coral islands and reefs resources and the protection of coral reef ecosystem in the South China Sea,and then provide decision support services for the ocean management and marine rights protection of the South China Sea and the Sansha City.