Spatial And Temporal Variations And Response Mechanism Of Green Tide And Chlorophyll A Concentrations Based On Remote Sensing In Southern Yellow Sea

The world’s largest macroalgal blooms(MABs)caused by the Ulva prolifera outbreaks have occurred every summer since 2007 in the Southern Yellow Sea,China.Accumulating evidence showed that MABs might deteriorate the regional marine environment and influence the growth of some primary producers such as phytoplankton.In this study,we investigated the spatio-temporal patterns of U.prolifera green tides and the corresponding influence on chlorophyll-a concentration in the Southern Yellow Sea in 2011-2016 using satellite images.The spatial and temporal patterns of U.prolifera green tides were extracted using HJ-1A/1B and MODIS satellite images by means of NDVI(normalized difference vegetation index)and artificial interpretation.And the spatial and temporal distribution and evolution characteristics of chlorophyll a concentration were analyzed using GOCI Level-2A chlorophyll-a concentration data by abnormal pixel removal of original product image and mean fusion in the South Yellow Sea during the outbreak of Ulva prolifera in 2011-2016.The main conclusions of the study are as follows:(1)MODIS data will overestimate the coverage of U.prolifera due to its low spatial resolution,and it is also difficult to monitor the scattered small area of U.prolifera.We built a regression equation about the extracted coverage of U.prolifera by HJ-1 CCD and MODIS in the study area.The purpose of this formula is to effectively coordinate area of U.prolifera extracted from MODIS data and HJ data because they have different spatial resolution.The combination of HJ-1 CCD and MODIS data could provide a sufficient number of images covering both of the large aggregations and sporadic distribution of U.prolifera.(2)From a macro point of view,the spatial and temporal distribution of U.prolifera in 2011-2016 are basically the same.The development of U.prolifera blooms in the Southern Yellow Sea could be featured as "appearance – development – outbreak – decline – disappearance".However,from the characteristics of the outbreak area,the scale of green tide outbreak in 2014-2016 is greater than in 2011-2013.The outbreak of the green tide has a great relationship with the Pyropia aquaculture in the "three sand" sea areas of Zhugensha,Jiangjiasha and Dongsha in southern Jiangsu.The eutrophication of offshore waters,the increase of seawater temperature in early May,and the driving mechanism such as wind field and flow field all promote t he massive growth and multiplication of Ulva prolifera.(3)From April to August,the concentration of chlorophyll-a showed "increase – sharp decline – slow recovery – stabilization" and the concentration of chlorophyll-a in the northern Jiangsu Shoal was higher than that in the southern waters of Shandong Peninsula,and the concentration of chlorophyll-a in the coastal waters was higher than that in the open sea.The chlorophyll a concentration had a great correlation with seawater depth,sea surface temperature and aggregation density of U.prolifera.1)The concentration of chlorophyll-a in the southern Yellow Sea showed a decrease with the increase of the isobaths,but this rule mainly occurred between isobaths 0-40 m.With the increase of the ocean depth,the difference of chlorophyll a concentration is getting smaller and smaller.2)There was a negative correlation between chlorophyll a concentration and sea surface temperature in April-June,and there was a positive correlation in July and August,b ut the overall trend has a negative correlation between chlorophyll-a concentration and sea surface temperature.3)There was a negative correlation between the concentration of chlorophyll-a and the area of U.prolifera during the period of 2011-2016.(4)The concentration of chlorophyll-a had the following relationships with U.prolifera temporally:(1)the concentration of chlorophyll-a increased with the growth of U.prolifera from April to mid-May;(2)the chlorophyll-a concentration decreased sharply with the dramatically increased coverage of U.prolifera in June;and(3)the chlorophyll-a concentration slowly recovered and finally stabilized as U.prolifera decreased in July.Generally,there was a negative correlation between the occurrence of U.prolifera and chlorophyll-a concentration in the Southern Yellow Sea,China.From 2011 to 2016,the chlorophyll-a concentration showed the opposite trend with the area of U.prolifera,that is,the size of the outbreak of U.prolifera affected the reproduction and growth of other microalgae,and the higher the coverage area of U.prolifera,the lower the concentration of chlorophyll-a.