Assessing Coastal Land-water-biodiversity Nexus For Resilience-based Management

Human activities directly or indirectly interfere with coastal land-water-biodiversity nexus(LWB nexus),leading to losses of crucial ecosystem services and further contributing to ecosystem degradation.This study identified typical indicators in Xiamen,China,related to coastal LWB nexus as key elements for measuring coastal ecosystems and established a framework for coastal LWB nexus.Three genres of indicators were identified,including land-related elements(e.g.,built-up land,arable area,coastal reclaimed area,aquaculture area,food provision,carbon storage),water-related factors(e.g.,drinking water quality meets required standards,seawater quality meets required standards,total discharge of water pollutants into ocean,number of algal blooms,water purification,erosion prevention),and biodiversity-related elements(e.g.,normalized difference vegetation index,coastal wetland area,abundance of iconic rare species,fishery resources,habitat quality).The spatial and temporal LWB nexus were calculated based on catastrophic models,InVEST models(Carbon Storage and Sequestration Model,Nutrient Delivery Ratio Model,Habitat Quality Model)and ArcGIS-based models(Revised Universal Soil Loss Equation),and then the key indicators that influence the regime shifts of LWB nexus were identified.We also identified the thresholds of biodiversity-related ecosystem services(ES)response to land and water-related ES using piecewise linear regression by R software,and the nexus mechanism between subsystems was analyzed.This study aimed to provide suggestions for resilience-based management.The results are shown as follows:Firstly,the temporal changes of LWB nexus indicated that LWB nexus gradually decreased between 1996 and 2015.Although the nexus of water subsystem remained at a high level,its dramatic changes showed that the system was faced with a higher vulnerability.It was obvious that seawater quality standards as well as the total amount of pollutants were the key drivers of water system degradation.Therefore,the main masks for improving water quality would be controlling land-based pollution and monitoring pollution sources along the coast and the estuary.The results of the biodiversity and land subsystems showed significant degradation from 1996 to 2015,indicating that these two subsystems were key drivers of LWB nexus degradation.Among them,four biodiversity indicators affected the transition of the steady state,including normalized difference vegetation index,coastal wetland area,abundance of iconic rare species and species richness.The built-up land and the coastal reclaimed area were the key drivers of land subsystems.Therefore,this study identified the key drivers of catastrophe in nexus system,which could provide supports for resilience-based management.Secondly,this study selected coastal land,water,and biodiversity-related ES,including land-related ES:carbon storage and food provision;water-related ES:water purification and erosion prevention;and biodiversity-related ES:habitat quality.These services were regarded as key spatial indicators of coastal LWB nexus.The In VEST models and the ArcGIS-based models were used to measure the influence of land use change and sea use change on the spatiotemporal changes of ES in Xiamen between 2005 and 2015.The results showed that Siming and Huli districts on Xiamen island and its surrounding sea areas(e.g.,Eastern sea,Southern sea,Western sea)had relatively moderate regulating services(water purification and erosion prevention)and low food provision and habitat quality services.Developing districts and its surrounding sea areas(Haicang,Jimei,Tong'an,and Xiang'an,Jiulong river estuary,Tong'an bay,Maluan bay,and Dadeng sea)presented relatively high regulating services and habitat quality services,and medium food provision services.Furthermore,from 2005 to 2015,with the artificial land expanded dramatically,all of terrestrial ES showed a trend of decrease.The majority of marine ES(e.g.,food provision,water purification,and habitat quality services)also witnessed downward trends,except for carbon storage and erosion prevention services due to the government's adjustment of the sea area use structure.Based on the above results,we could draw a conclusion that city planners and policy makers should lay emphasis on improving carbon storage in southern region and coastline,food provision in western region,water purification in southern region and estuaries,erosion prevention in northern region and coastline,and habitat quality in coastline.In addition,this study used R software to analyze the nexus of land,water,and biodiversity-related ES,and the nexus results indicated:land-biodiversity>land-water>water-biodiversity.We also fitted piecewise linear regression of land-water,land-biodiversity,and water-biodiversity to analyze the thresholds of the changes of ES.The results showed that when the standardized value of land-related ES exceeded 0.885 in 2005/0.879 in 2015 or water-related ES exceeded 0.032 in 2005/0.050 in 2015,biodiversity-related ES would reduce gradually.The thresholds identification provided data support for coastal resilience-based management.According to the results,this study proposed an LWB nexus approaches based on resilience-based management.