| Red tide is a sudden marine ecological disaster.The eutrophication of sea water is the main causes of red tides.In recent years,overfishing of fishery resources in the East China Sea,development of large-scale ocean projects,discharge of pollutants from the Yangtze River and other coastal areas have caused frequent occurrence of red tides in the East China Sea,which has a great harm to the marine ecological environment,fishery,social economy and public health.Therefore,it is of great significance to carry out monitoring research on red tides in the East China Sea.The traditional monitoring method of red tide is mainly based on manual survey,and it will consume huge manpower and material resources and timeliness is also not high.Satellite remote sensing technology has the advantages of real-time synchronization and wide observation range.It can be used to extract the informations of red tide and can make up for the defects of traditional methods.Compared with ocean color satellites such as MODIS and SeaWIFS,GOCI has a high temporal resolution of 1 hour and eight scenes a day.It is of great application prospect to monitor sudden events such as red tides in the East China Sea.Based on the ocean color remote sensing data obtained by the GOCI satellite,in this paper,firstly,an image preprocessing was performed such as atmospheric correction on the L1B data.Secondly,the three chlorophyll inversion algorithms of OC2,OC3G,and YOC were used to invert the chlorophyll-a concentration in the East China Sea,and then the measured data was used to verify and analyze the results of the three inversion algorithms.The inversion accuracy of the three algorithms was compared by the average absolute error,average relative error and correlation coefficient between the inversion value and the measured value,and the optimal algorithm for inversion of chlorophyll-a concentration in the East China Sea was determined.The chlorophyll-a concentration in the East China Sea from 2011 to 2017 was inverted using the optimal algorithm,and the characteristics and causes of diurnal variation were analyzed.Then the red tide information of the East China Sea from 2011 to 2016 was extracted using the chlorophyll-a concentration threshold method and the red tide index threshold method respectively,and the characteristics and causes of the red tide change were analyzed.And the accuracy was compared between chlorophyll-a concentration threshold method and the red tide index threshold method.Finally,the chlorophyll-a concentration in the middle and late period of the typhoon Canhong was inverted using the YOC algorithm,and the effect of Canhong on the chlorophyll-a concentration in the East China Sea was analyzed.The results of this study are summarized as follows:?1?The average absolute error of YOC algorithm is 0.39 mg·m-3,the average relative error is 42.47%,and the correlation coefficient is 0.88,which is the smallest average absolute error and average relative error and the highest correlation coefficient in the three algorithms?OC2,OC3G,and YOC?.Therefore,the YOC algorithm is the best algorithm for inverting the chlorophyll-a concentration in the East China Sea.During red tides frequently occurred,the characteristics of chlorophyll-a concentrations changes and distrubutions near the ocean area of Wenzhou on May 29,2011,near the waters of Hangzhou Bay on June 5,2012,the waters of Taizhou on May25,2013,the waters of the Yangtze River estuary on May 23,2014,the waters of Wenzhou on June 15,2015,the water of Zhoushan on May 22,2016,and the water of Xiangshan on May 22,2017 are as follows:chlorophyll-a concentration increases first and then decrease,and it reaches the peak around 10:30.?2?Based on the statistical analysis of multi-phase chlorophyll-a concentration,the threshold value of chlorophyll-a concentration for extracting red tide information from the East China Sea was determined to be 12 mg·m-3.This threshold was used to extract information on red tide from the East China Sea from 2011 to 2016.The diurnal variation of red tide is that it moves from the nearshore area to the offshore area and ted tides area increases first and then decreases,reaching a maximum around 10:30,which is mainly caused by the vertical migration of algae and the consumption of nutrients in seawater.The red tide extracted on May 29,2011 is mainly concentrated in the waters near Wenzhou,Zhejiang Province,which is basically the same as that recorded in the“China Ocean Disaster Bulletin”.From 2011 to 2016,the average relative error of the extracted red tide area is 0.83,which is about 83%higher than the measured red tide area.This is mainly due to the fact that the range of sea areas observed by remote sensing satellites is wider than that observed by artificial walking.?3?The red tide information in the East China Sea is extracted using the red tide index threshold value of 3.56.The bloom area of red tide is basically the same as that based on the chlorophyll-a concentration threshold method,which was basically the same as that recorded in the“China Marine Disaster Bulletin”.The area of red tide also increases first and then decreases,reaching a maximum around 10:30.Therefore,the relevant departments can choose to begin monitoring and controlling the red tide before10:00.The average relative error that extracts maximum area of red tide is 0.73,which is0.10 less than the average relative error of the chlorophyll-a threshold method.This shows that the improved red tide index threshold method is better than the chlorophyll-a concentration for extracting the red tide information in the East China Sea.?4?Using strong typhoon Canhong as a case to study the changes of chlorophyll-a concentrations before and after the typhoon landfall in the East China Sea?July 4-18,2015?,and discussed the different responses of Chlorophyll-a Concentration in the near-shore and off-shore areas under Typhoon Influence.It was found that typhoons could increase the chlorophyll-a concentration significantly in both the near-shore and the offshore areas.There was a delay time about 4 to 5 days which increased to the maximum values relative to the typhoon crossing. |
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