| Coral reef ecosystem is one of the ecosystems with the highest primary productivity in the world.It plays an important role in maintaining marine biodiversity,preventing waves and consolidating beaches,and it is great resources for mankind.Coral reef resources in China are mainly distributed in the South China Sea.However,due to global change and continuous human activities,coral reefs in the South China Sea are generally degraded,which is mainly manifested by a significant decline in the live coral cover,which will have a long-term significant ecological negative impact on the ecological status of coral reefs,and once the health status of coral reefs in the South China Sea is coerced,it will seriously endanger China's marine territorial rights and interests and ecological security.It is urgent to carry out comprehensive,large-scale and spatially continuous fine monitoring of coral reefs in China,so as to effectively serve the management,protection,restoration and scientific utilization of coral reefs in the South China Sea.The live coral cover and its changing pattern are one of the important indicators to describe the health of coral reef ecosystems.However,at present,the live coral cover is still mainly obtained by scattered site or cross-sectional surveys,which are subjective and unable to achieve large-scale continuous mapping.High-resolution remote sensing has been proved to be the most effective means for large-scale and high-precision monitoring and mapping of coral reefs.Due to the particularity of the investigation area,it is difficult to carry out large-scale field investigation,and because of the limited width of high-scoring remote sensing images and the influence of clouds in tropical waters,it is more difficult to synchronize the acquisition time of field investigation data and remote sensing images.Therefore,this article explores a method of extracting coral reef geomorphic units and establishing the corresponding relationship between different geomorphic units and live coral cover.Taking Xisha Islands as the research area,the estimated live coral cover of 28 islands and reefs in Xisha Islands in 2005-2008,2010-2012 and 2015-2018(hereinafter referred to as 2005,2012 and 2015)were obtained.Firstly,a remote sensing inversion method of live coral cover based on water column correction is developed by using the field survey data of live coral cover in Xisha Islands which can be quasi-synchronized with remote sensing images in 2010.At the same time,a classification method of coral reef geomorphic units for live coral cover assessment is studied and proposed,and the corresponding relationship between sediment types and live coral cover is established by analyzing the spatial characteristics of live coral cover inversion results and geomorphic units classification results.On this basis,based on the classification results of sediment types in Xisha Islands in 2005,2010 and 2015,the spatial distribution results of live coral cover in the corresponding years were obtained.The changes of the estimated spatial distribution results of live coral cover in Xisha Islands were analyzed,and the coral reef health status of 28 coral reefs in Xisha Islands was evaluated by the change of live coral coverage.(1)Based on high-resolution satellite remote sensing data and live coral cover field data,the inversion method of live coral cover is studied by using stochastic gradient boosting algorithm.The results show that: The accuracy of the proposed inversion method is high,and the average relative error is less than 11%;Water column correction can improve the inversion accuracy,but the average relative error of the inversion results can still be less than 15% when the water column correction can not be carried out;From the inversion model of live coral cover,DVI index has a high contribution rate in each inversion model,which is of great significance to the inversion of live coral cover.(2)According to the spatial characteristics of live coral cover,the geomorphic characteristics of coral reefs and the indicators reflecting the influence of marine processes and dynamic factors in the formation of coral reefs,a classification system of coral reef geomorphic units for the evaluation of live coral cover was established.There are 11 types,including deep forereef slope,shallow forereef slope,forereef terrace,reef crest,coral area dominated,coral and sediment area,sandbank,sand flat,reef pond,patch reef and lagoon slope;Based on the constructed classification system of coral reef geomorphology units and high-resolution remote sensing images,coral reef geomorphology units are extracted from coral reefs in the study area.The classification result obtained by Random Patch Network(RPNet)method is the most ideal,and the overall accuracy is 93.70%,the Kappa coefficient is 0.93;Finally,the relationship between the live coral cover and coral reef geomorphic units was analyzed.It is found that different coral reef geomorphic units correspond to different levels of live coral coverage,and the corresponding relationship remains basically unchanged.And there is also a relationship between water depth and live coral cover,that is,the greater the water depth,the smaller the live coral cover.(3)Based on the RPNet classification method and human-computer interactive interpretation,the distribution information of geomorphological units in the study area is obtained,and according to the corresponding relationship between coral reef geomorphological units and live coral cover,the live coral cover distribution in the study area is obtained.According to the change of live coral cover of coral reefs,it is known that most of the coral reefs in the study area are in poor health,and the degradation rate is greater than the recovery rate.The natural factors causing the change of live coral cover include rising sea temperature,tropical cyclones and long-spiny starfish,while man-made factors include the development and utilization of islands and reefs,fishing and so on. |
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