Modeling Analysis Of Past Process And Future Trends Of Living Coral Cover In Weizhou Island Based On Remote Sensing Observation Data

Coral reefs are called the "tropical rainforests of the sea" as coral reef is one of the most biodiversity-rich ecosystems in the ocean.They are of great importance as they can help to maintain the ecological balance,biological resources,and species diversity on Earth.However,coral reefs were globally threatened by factors such as human activities and climate change'with the live coral cover(LCC)being declining.Therefore,coral reefs are necessary to be managed and protected when they are exploited and utilized.Such management and protection need to be based on comprehending the knowledge of the impacts of environments on the coral reef ecological status.With the'technological development,human research on corals has no longer been limited to field investigations.As remote sensing can be used to investigate the health status of coral reef ecosystems and monitor their changes.It is one of the important methods to assist coral reef research.Weizhou Island is suitable for coral growth and reproduction and then was chosen as the study area.The coastal waters have good light condition,appropriate transparency,excellent water exchange conditions,and high quality of shallow sea bottom environment for coral growth.Corresponding live coral cover(LCC)data recorded in the historic documents and three environmental parameters obtained by MODIS(2003-2015).The environmental parameters contain chlorophyll-a concentration(Chl-a),sea surface temperature(SST),and photosynthetically active radiation(PAR),were used to explore the relationship between the LCC and the environmental parameters in this paper.Firstly,several environmental parameter statistics that have the greatest impact on LCC were calculated and chosen,and then principal component analysis(PCA)was used to remove the redundant information.Thereafter,to overcome the difficult caused by the fact that there is a lack of LCC data in some years for the study sites,a nonlinear constrained optimization method was used to construct the relationship between the LCC and the environmental parameter statistics.This is an innovative application in remote sensing research in coral reefs which is rarely considered in previous studies.The research results show that:(1)According to the regression equation,the environmental parameters(Chl-a,SST and PAR)all have the same effect on the change of LCC at different research sites in Weizhou Island,i.e.the different environmental parameters are consistent with the regression equation coefficients of LCC.(2)According to the environmental parameters and the LCC modeling results of LCC,the Ch1-a has the most negative impact on LCC,followed by SST,while PAR has the least positive impact on LCC.It can be inferred that SST may not be the main reason for the decrease of LCC in Weizhou Island,which is located at high latitude.However,the eutrophication of sea water(indicated by Chl-a)caused by human activities may be main reason.In addition,the reason for the smaller absolute value of PAR coefficient may be that PAR promotes the growth of corals and algae,which is its competitors,at the same time,positive and negative effects are offset.(3)To validate the model,this study used Leave-one-out(LOO)for verification,obtained mean absolute error,mean relative error,root mean square error and the maximum error are 0.093,12.9%,0.151 and 0.237 respectively.Based on the established model of LCC change in Weizhou Island,the future coral reef coverage were simulated and predicted.The results are shown as follows:the LCC around Weizhou Island will be stable at around 13%if it is under the condition of maintaining the existing environment,indicating that the pressure of existing environmental parameters for coral growth are insufficient to reduce the LCC to zero.However,if the SST continues to rise(140 years of fluctuations increase by about 1.5?,and regardless the change of Chl-a,PAR and other extreme environmental conditions),the corals around Weizhou Island may be reduced to a low level around 2140.In other words,the analysis suggests that even under the conditions of continuous global warming,resulting in a continuous increase in SST,the corals are still likely to be greatly reduced even if Weizhou Island is considered to be the "Refuge Hypothesis".However,comparing to the time of other areas that are not "Refuge Hypothesis",this time can be delayed by at least 40?110 years.