Fig. 1. Distribution of Ulva prolifera in Yellow Sea area. (a) Location of Yellow Sea, where RGB image shows pseudo-color composite image of GOCI acquired on 26 May, 2017; (b) true-color composite image of GOCI along coast of Qingdao acquired on 26 May, 2017; (c) aerial photo of Qingdao coast attacked by Ulva prolifera taken on 10 July, 2016

Fig. 2. Mean value (solid line) and standard deviation (shadow) of Rayleigh-corrected reflectance spectra of macroalgae, seawater, terrestrial vegetation, and cloud pixels

Fig. 3. Characteristic curves of K-T transformation components of floating macroalgae and seawater at three different periods (error lines represent standard deviation of mean values). (a) 19 May, 2017; (b) 16 June, 2017; (c) 13 July, 2017; (d) three different periods

Fig. 4. Probability and cumulative probability distributions of TCT-GTI values for different target object pixels. (a) Seawater pixels (N=12634); (b) Ulva prolifera pixels (N=7570); (c) mean values and standard deviation of TCT-GTI values for all Ulva prolifera and seawater pixels

Fig. 5. Probability and cumulative probability distributions of brightness values for different target object pixels. (a) Ulva prolifera (N=15078); (b) seawater (N=12633); (c) thick cloud (N=181624); (d) mean values and standard deviation of brightness values for all Ulva prolifera, seawater, and thick cloud pixels

Fig. 6. Confusion matrix and all indexes of accuracy evaluation

Fig. 7. Comparison of results of green-tide extraction using three different algorithms. (a) (e) (i) Pseudo-color images for different regions; (b)-(d) results of green-tide extraction using TCT-GTI algorithm; (f)-(h) results of green-tide extraction using AFAI algorithm; (j)-(l) results of green-tide extraction using IGAG algorithm

Fig. 8. Accuracy evaluation values of TCT-GTI, AFAI, and IGAG algorithms

Fig. 9. Coverage area of Ulva prolifera as a function of time

Fig. 10. Monitoring results of green tide of GOCI images at different time and its drifting trajectory. (a) Distribution of Ulva prolifera on 13 May, 2017; (b) distribution of Ulva prolifera on 21 May, 2017; (c) distribution of Ulva prolifera on 4 June, 2017; (d) distribution of Ulva prolifera on 7 June, 2017; (e) distribution of Ulva prolifera on 26 June, 2017; (f) distribution of Ulva prolifera on 27 June, 2017; (g) distribution of Ulva prolifera on 1 Ju

Fig. 11. Average wind-field distributions for corresponding dates of green-tide remote sensing monitoring. (a) May 13-21,2017; (b) May 21-June 4,2017; (c) June 4-7, 2017; (d) June 6-26, 2017; (e) June 26-27, 2017; (f) June 27-July 1, 2017

Table 1. Band characteristics of GOCI sensor and its main applications

Table 2. Comparison of macroalgae coverage areas for three algorithms