Estimation Of The Areas Of The Coral Reefs In The Xisha Islands

The area is the most basic property of the coral reefs,which serves as the foundation for assessing resources and environmental effects of the coral reefs.Due to the lack of a reliable method to estimate the area of the coral reefs,there is little clear agreement on the areas of the coral reefs in the South China Sea until now.To address this problem,this study uses the Lingyang Reef of Xisha Islands as a case study to explore a method for estimating the area of the coral reefs by using multi-temporal and multi-spectral remote sensing images and evaluate its accuracy by comparing the results with in-situ data.Then this method is applied to other coral reefs in Xisha Island(including Bei Reef,Jinyin Island,Lingyang Reef,Ganquan Island,Shanhu Island,Shanhudong Bank,Quanfu Island,Yagong Island,Xiansheyu,Yinyu,Shiyu-Jinqing Island,Guangjin-Chenhang Island,Yongnan Bank,Huaguang Reef,Panshiyu,Zhongjian Island,Yuzhuo Reef,Langhua Reef,Qilianyu,Yinli Shoal,Dong Island,Zhanhan Shoal,Binmei Shoal,Beibianlang,and Gaojian Rocks).The total area of the shallow coral reefs with a depth of 30 m in the Xisha Islands(except Xidu Shoal and Zhanhan Shoal)and the area of their geomorphic zonations are estimated based on calculating the area of each coral reef one by one.Finally,the trend and main reasons for the change of geomorphic zonations of coral reefs in Xisha Island from 1993 to 2021 are analyzed after constructing the biennial area variation sequence of the coral reef geomorphic zonnations of Xisha Islands by using this method.The main conclusions are as follows:(1)Based on Sentinel-2 multi-temporal remote sensing images(MSI),a low-cost semi-automatic method of estimation of the areas of the coral reefs in the Xisha Islands was proposed by using an image edge detection algorithm.Automatic geomorphic boundary refinements were done using the Gradient Vector Flow-Snake(GVF-Snake)model after determining the approximate location of the boundary between geomorphic zonations of coral reefs.Thereafter,the extracted multi-temporal geomorphic zone boundaries were converted to geomorphic zone areas and then fused to establish a reliable and accurate geomorphic zone.The total area of shallow coral reef with a depth of 30 m and areas of each geomorphic zonation were obtained through the above steps.The accuracy of the area estimation results was evaluated by Landsat 8 multi-temporal remote sensing image of operational Land Imager(OLI),in-field survey data,and2 m high-resolution World View-2 image.The results show that the differences between the outline of the coral reef and the 30 m isobath line is also within 1pixel(5.7～9.5 m).Furthermore,the complete,the correction,and the quality of the boundaries are improved from 60%,64%,and 54%for the single-image method to 84%,83%,and 72%for our multi-temporal method,respectively.Meanwhile,the standard deviations of the estimated coral reef area of atolls for Sentinel-2 MSI and Landsat 8 OLI are less than 0.01 km~2 and 0.05 km~2,respectively,when more than 6 scenes of satellite images are used,and the difference between the area extracted from multi-temporal images by using the proposed method and the area determined by using a high-resolution World View-2 image are less than 0.2%and 0.5%,respectively.The standard deviations of the estimated coral reef area of platform reefs for Sentinel-2 MSI and Landsat 8 OLI are less than 0.03 km~2 and 0.02 km~2 respectively,when more than 6 scenes of satellite images are used,and the difference between the area extracted from multi-temporal images by using the proposed method and the area determined by using a high-resolution World View-2 image is less than 0.4%and 0.9%,respectively.The standard deviations of the estimated coral reef area of banks for Sentinel-2 MSI and Landsat 8 OLI are less than 0.03 km~2 and 0.01 km~2respectively,when more than 6 scenes of satellite images are used,and the difference between the area extracted from multi-temporal images by using the proposed method and the area determined by using a high-resolution World View-2 image is less than 2.2%and 3.4%,respectively.(2)Based on sentinel-2 MSI multi-temporal remote sensing images and the proposed method,area information of coral reefs and associated geomorphic zonations in the Xisha Islands were calculated.The results show that the total area of shallow coral reefs at 30 m in the Xisha Islands is 950.14 km~2(excluding compound lagoon,Xidu Shoal,and Songtao Shoal)or 1,833.93 km~2(excluding compound lagoon,Xidu Shoal and Songtao Shoal).The areas of reef front slope,reef-flat-lagoon-slope,lagoon,and sandbank/island,respectively,are 477.87 km~2,286.09 km~2,175.55 km~2,and 10.65 km~2 of the total coral reef area of 950.14 km~2.(3)Based on 482 sentinel-2 and Landsat remote sensing images,a time series of geomorphic zone area change of the coral reefs in the Xisha Islands from 1993to 2021 was constructed with an interval of 2 years.According to the results,the area of the reef-flat-lagoon-slope has decreased by 1,183,202.92 m~2 in the past 30years,whereas the lagoon area has increased by 877,673.98 m~2.From 1993 to2007,the reef-flat-lagoon-slope grew by 379,759.11 m~2/a,while the lagoon fell by 126,799.4 m~2/a.From 2007 to 2021,the area of reef-flat-lagoon-slope decreased by 311,193.42 m~2/a 2007 to 2021,while the area of lagoon increased by 196,931.73 m~2/a.(4)Combined with datas of sea level anomaly,live coral cover,and sea surface temperature,the primary factors for the geomorphic zone area change of the coral reefs in the Xisha Islands from 1993 to 2021 were analyzed.The results demonstrated that sea level change and coral growth conditions are the principal drivers of coral reef geomorphic alteration in the Xisha Islands.The rate of sea level rise in the South China Sea increased from 1993 to 2021(3.47 mm/a before2007,4.25 mm/a after 2007).Ecological events such as the continual rise in sea surface temperature and the outbreak of Acanthaster planci wreaked havoc on the coral reef ecosystem during the same period.For example,the live coral cover in the Xisha Islands was>50%before 2007,and it dropped to<25%after 2007.As a consequence of this situation,the speed of coral reef growth was slower than the rate of sea level rise,leading to shrinking in the reef-flat-lagoon-slope of the coral reefs in the Xisha Islands,while the lagoon expanded.