Spatial Observation And Health Assessment Of Coral Bottom In Hainan Island Based On Multisource Remote Sensing Data

The coral reef ecosystem is the most productive and diverse biological ecosystem in the world,which is of great significance to the global Marine ecological environment system.Since the 1950 s,the coastal coral reef community in Hainan Island has been facing unprecedented pressure due to climate change and human disturbance.In order to provide reference information for the management and protection of coral reefs in Hainan Island,it is urgent to conduct continuous spatial monitoring of benthic materials of coastal coral reefs in Hainan Island.At present,satellite remote sensing technology has become an important technical means to carry out large-scale coral reef observation.However,low spatial resolution image and pixel-based extraction methods are difficult to monitor the complex coral reef ecosystem due to their limitations.To solve the above problems,this paper takes the coastal coral reef areas of Sanya,Wenchang and Danzhou of Hainan Island as the study area,and uses Landsat-8 OLI and Sentinel-2 remote sensing images to combine threshold classification and objectoriented classification.The benthic material of coral reef in the study area was divided into living coral,bleach coral,sand,alage and other substances,and sample points were verified according to Google Earth images and field survey data.Overall accuracy(OA)and Kappa coefficient were used to evaluate the classification results of coral reef sediment.The classification area of coral reef benthic substances from2013 to 2020 was calculated,and the spatial and temporal distribution pattern and dynamic change law of regional coral reefs were analyzed,and the correlation between sea surface temperature,sea surface salinity,chlorophyll a concentration and bleaching rate was discussed.The health changes of coral reef ecosystem in Sanya coral reef Protection area were analyzed from the aspects of coral reef water environment,coral reef resources and coral reef health index.The following conclusions were reached:(1)The classification accuracy of coral reef benthic materials in the study area was more than 80%,and the Kappa coefficient was more than 0.7,and the classification results of Danzhou in 2020 showing a high degree of consistency.The object-oriented classification method can make full use of spectral information,texture information and location information of remote sensing image to construct interpretation rules and effectively extract the benthic material information of coral reef.(2)From 2013 to 2020,coral degradation occurred on both the left and right sides of the West Island of Sanya City.The degradation on the left side of the East Island was more serious.Degradation occurred in the far offshore area of Luhuitou,and the situation of coral near the coast was improved.The coral improvement area in Wenchang City extends from Coconut Bay south to Changpi Port,while the change in Changpi Port south is not obvious.The overall improvement area of Danzhou was larger than the degraded area.The living coral rate was negatively correlated with sea surface temperature and positively correlated with sea surface salinity and chlorophyll concentration.(3)The water environment of Sanya Coral Reef Protection area is in a healthy state.In 2021,Luhuithui-Yulin Jiao area has the best water environment,followed by Yalong Bay area,and East Island and West Island area.The coverage rate of reefforming coral showed a downward trend in 2014 and 2019.2014 was the minimum in the study period.In 2021,the coverage rate of reef-forming coral on the left side of East and West Bill Island was higher than that on the right side,Luhuitou Sea area was higher than that in large and small Donghai Sea area,and the overall coverage rate of Yalong Bay was at an average level.The addition of reef-building coral showed a downward trend in 2014 and 2019,and an obvious upward trend in 2015 and 2021.In2021,the addition of reef-building coral in Dadong Hal and Xiao Dong Hal and Yalong Bay were higher.The coral reef ecosystem health index was the lowest in 2015.After2015,the health status of Sanya Coral Reef Protection Area increased steadily,and in2021,it was in a very good state.