Ulva Prolifera Detection Using Time-series GOCI Images And Analysis On The Key Environmental Factors To Its Growth

Since 2007 the first time Ulva prolifera(U.prolifer a)had occurred in the South Yellow Sea(SYS),and in 2008 the large-scale blooms of U.prolifer a that intruded into Qingdao Olympic Sailing Competition Arena had reached widespread attention,the U.prolifer a has routinely visited SYS for 11 years until today,showing a normalized outbreak trend.It has caused an incalculable loss on regional marine ecological environment and economic development.In June 2010,Korea's Space Agency launched the world's first geostationary ocean color satellite which is designed specialize for ocean color studies.GOCI has a revisit time of around 1 hour and a spatial resolution of 500 meters that can observe east coastal zones of China up to 8 times a day in diurnal cycle continuously.Now it has been widely used by many scholars in different fields.In this paper,time series GOCI images was applied to monitoring and extracting the U.prolifer a in SYS,which can effectively make up for the lack of high timeliness data,realize timely monitoring of the floating green macroalgae,and provide valid reference for predicting the drift trajectory and outbreak trends of U.prolifer a.The study collected GOCI images from 2011 to 2017 during the outbreak time of U.prolifera and NDVI was used to extract the algae.The long-term bloom patterns of U.prolifera were derived through the datasets which is infrequent to see before this.In order to determine the environmental factors to the U.prolifera's growth,the paper also collected and processed several environmental satellite data like precipitation,sea surface temperature(SST),photosynthetically active radiation(PAR)and sea surface wind,besides,historical data like water quality in SYS and seawater cultivation in Jiangsu coastal are also collected from 2011 to 2016.Based on this abundant accumulation of data,this paper analyzed and discussed the general characteristics of the growth process of U.prolifer a at annual scale.We also took advantages of GOCI data to analyze the short-term changes and characteristics of U.prolifera at daily scale.Then combined with hydrology and meteorology data,we furtherly analyzed the key environment factors and important mechanisms that related to U.prolifera's outbreak intensity,drift path,and distribution condition during the whole process.The main conclusions of the article are as follows:(1)Applying NDVI to GOCI images can achieve high-age,short-term,and continuous monitoring and extraction of U.prolifera in SYS,but heterogeneity of mixed pixels and saturation of NDVI caused deviation of the extraction results in near-shore and thin cloud-covered sea areas.Different thresholds for different regions can effectively improve this problem and promote the accuracy of the extraction of U.prolifera.(2)In terms of inter-annual outbreak intensity and process of U.prolifera,the outbreak scale has shown an upward trend since 2011.The scale of outbreaks of U.prolifera reached the highest level in 2015 which up to15374.5km2.The spatial variation during the period of outbreak in every year presents an overall trend of"distribution,aggregation,and distribution";the drift trajectory is directly affected by the sea wind field.The rapid growth of U.prolifera occurs at the very beginning of its life cycle,and the rapid decline occurs at the end of the life cycle.When ascending growth of U.prolifera reached a certain level,there will be a relatively stable period of time.(3)Light,precipitation,and sea surface temperature play different roles in different stages of the development of U.prolifera.In terms of illumination,suitable light conditions in May will contribute to the early growth rate of U.prolifer a.In terms of precipitation,precipitation on the one hand will indirectly provide rich amount of nutrients for the growth of U.prolifer a,on the other hand,it can make up for the water and ease its consumption process,prevents U.prolifer a from prematurely deteriorating due to excessive light exposure.In terms of sea surface temperature,the SST in the SYS from May to July is basically suitable for the growth of U.prolifera,also,with higher SST in May and good illumination conditions,the time of maximum area will appear earlier,the relatively low SST in July is one of the reasons for delaying the decay process of U.prolifera.(4)Since 2011,the scale of Porphyra aquaculture in coastal area of Jiangsu Province has been very large.The aquaculture water provides abundant nutrition for the growth of U.prolifera,and the Porphyra aquaculture rafts provide the attachment base for the initial growth of U.prolifera,which is the main cause of the outbreaks in SYS in recent years.