Assessment On The Changes In The Water Clarity At The Yellow Sea And Impacts From Green Tide Using Remote Sensing(2002~2016)

Water clarity(Secci Disk Depth,SDD)is a very important factor for marine ecological environment and function.Since 2008,the world’s largest "green tide" caused by the macroalgal blooms(MABs)of green macroalgae of Ulva prolifera,have occurred every summer in the Yellow Sea,which could have important influence on the SDD of this region.In this study,using a simplified linear-mixing reflectance model and the measured hyper-spectral remote sensing reflectance of sea water and macroalgae,a SDD inversion algorithm for the Yellow Sea was simulated to investigate its responses to the presence of floating macroalgae.Using MODIS images and the derived remote sensing reflectance data,the influence of the existence of floating macroalgae in the images of the SDD and the chlorophyll were verified.On the basis of MODIS remote sensing reflectance data,a long-term average monthly SDD data set from 2002 to 2016 of the yellow sea were generated.The effects of MABs on the water clarity in the west Yellow Sea are presented for the first time.The results show that the mixture of sea water and macroalgae will lead to decreased water clarity with the increased portion of floating macroalgae when the SDD is larger than 1.2 m.Therefore,the ocean color standard products with false values for pure sea water should be used with caution for the regions with large scale of floating macroalgae blooms.The period of 2002~2007 is defined as pre-MAB phase when there was no large-scale "green tide" disaster in the Yellow Sea during this period;And the period of 2008~2016 as MAB phase when large-scale “green tide” have occurred every summer in the Yellow Sea from 2008.The long-term SDD in June and July(2002~2016)over the Yellow Sea are investigated and analyzed with the presence of "green tide",comparing the MAB phase(2002~2007)to the pre-MAB phase(2008~2016).The significant increase in the SDD by 2.6m and with 12544 km2 of sea surface in total in July while no pronouncing changes in June,suggests that the water clarity in the west Yellow Sea have been strongly affected from the period of 2002~2007(the pre-MAB phase)to the period of 2008~2016(the MAB phase).