Remote Sensing Monitoring Of Macroalgae Farming Area And Environmental Benefits Assessment

Macroaglae is the primary producer in marine ecosystem and exhibits many ecological functions such as nitrogen(N),phosphorous(P)and iodine(I)absorption,carbon(C)sequestration,and oxygen(O2)releasing.In addition,the high economic values of it have contributed to the booming farming area in recent years.Therefore,it is of great significance to delineating the large-scale farming information(farming area and distributions),and to achieving the environmental and economic benefits at the same time.This study explored the application of remote sensing(RS)technology in monitoring macroalgae farming area.Firstly,we compared the performances of pixel-and object-based approaches in identifying the Porphyra farms using Gaofen satellite no.2 remote sensing image.Then,three Gaofen satellite no.l remote sensing images with different dates were selected to establish the traditional classification tree(CT).Meanwhile,modified CT was also developed from the traditional CT by adjusting the thresholds based on the statistical relationship between spectral characteristics of two images.After that,modified CTs were applied in the long-time series classification to obtain the spatiotemporal distribution of macroalgae farms.Besides,we analyzed the driving factors of the changes of macroalgae farms and provided the data support,scientific basis and decision reference for further managing and planning macroalgae aquaculture.Finally,the environmental and ecological benefits of different macroalgae species in coastal area of China were quantitatively evaluated.The spatial distribution of these benefits was mapped,and possible suggestions were given based on the various coastal environments.The final results of this study are as follows:(1)Object-based classification method achieved higher overall accuracy and kappa coefficient than the pixel-based classification method in identifying macroalgae farms with high spatial resolution images,especially for macroalgae identification.This result suggests that the object-based methods can be employed to extract macroalgae farming information with high spatial resolution images and monitor the spatiotemporal distribution of macroalgae farms.(2)The classification results of modified CT and traditional CT for the same image were similar.Therefore,modified CT can be applied to identify the macroalgae farms in different years without ground-truth data.(3)Overall,from 1985 to 2016,the area of Porphyra farms in Dayu Bay kept booming,except for the fluctuation in the initial years and the obvious decline in 2013.The total area of Porphyra farms in Dayu Bay are 34.8 ha(1985),31.7 ha(1991),149.7 ha(1998),716.3 ha(2002),1,150.1 ha(2008),1,678.4 ha(2010),1,019.9 ha(2013),1,990.4 ha(2014)and 2,120.3 ha(2016),respectively.Besides,this study found that the possible driving forces of these changes were:economic factors such as gross product of primary industry and per capita income,typhoons and other natural disasters,cultivation techniques and government policies.Temperature has little effect on the farming area in this study area,though it possibly influences the farming models and growing status of Porphyra.Therefore,it would be conducive to the healthy development of macroalgae aquaculture to develope the aquaculture technology,strengthen the government intervention,and take effective responses.(4)A quantitative assessment of the environmental benefits of macroalgae aquaculture was conducted and the distribution of these benefits was mapped.As a result,we estimated that in 2015,the cultivated macroalgae in China has brought in plenty of environmental benefits,including about 70,000 tons N,9,000 tons P,and 5,000 tons I absorption,around 530,000 tons C sequestration,and more than 1,000,000 tons O2 release.What's more,it nearly saved 30,000 tons fertilizer,2,000 tons pesticide and 60,000 ha land resources compared with terrestrial vegetables.Chinese aquaculture macroalgae production and farming area keep booming recently.Thus,it is a unique opportunity for us to strengthen the macroalgae farming industry and to reach a win-win situation that brings great environmental benefits while obtains substantial economic return.This study explored the application of RS technology in monitoring and environmental benefits assessment of macroalgae farms,which provided technical support and reasonable suggestions for the healthy development of macroalgae farming.In addition,more research is expected in the future:(1)Large-scale and detailed monitoring of macroalgae farms using RS technology,such as obtaining the distribution,area and yield information of macroalgae farms in China;(2)combining more ecological and socio-economic data to fully assess the driving factors related to the changes of macroalgae farming area and environmental benefits.