Remote Sensing Monitoring of Green Tide Disaster Using MODIS and GF-1 Data: A Case Study in the Yellow Sea
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors described the dynamics of a green tide occurring in the Yellow Sea by 2021. For this objective, a method for combining satellite images from two platforms was developed and atmospheric and environmental information was gathered. The main conclusion of the manuscript is that the onset of the green tide was favoured by temperature and its development was boosted by rainfall. The manuscript is interesting and presents useful analyses. However, I find some flaws that the authors should consider to produce an improved version.
(1) Lines 28-38. These sentences in the abstract are duplicate.
(2) Lines 76-79. Consequently, it appears that the green tide event occurred in 2021 has been previously analysed even by using satellite images. What is new in this manuscriptIt should be emphasised in the introduction. Additionally, the data obtained should be compared to those ones published.
(3) Lines 80-83. This sentence apparently contradicts the previous one (lines 59-61) asserting that “MODIS data tends to overestimate when extracting the coverage area of green tides”. I guess that the overestimation by MODIS is produced when green tide is detected but detection limit of MODIS (11.2 km2) is higher compared to other satellites. Perhaps a clearer explanation for the reader would be attempted.
(4) Line 94. It is unclear what the authors mean with eutrophication in this context (green tides is a per se eutrophication symptom). Perhaps, it should read “nutrient pollution”.
(5) Lines 128-135. The time period covered by the gathered data should be indicated in this paragraph.
(6) Line 151. The exact wavelength of the spectral bands of the two satellites used to estimate NDVI should be indicated. I wonder if differences in the spectral bands between the two satellites could contribute to biases in the outputs obtained. Please explain which threshold of NDVI was selected to determine presence and/or absence of green tide. Additionally, please indicate the reliability of the NDVI index in detecting the presence of green tides taking into account that it is sensitive to the presence of other aquatic vegetation. Has this accuracy been tested in the study area?
(7) Line 164. I wonder if there is some previous analysis of the GF-1 performance in detecting green tides.
(8) Line 166-171. This explanation about the procedure used to compare MODIS and GF-1 is confusing. The size of pixels from the two satellites is not similar; consequently, choosing blocks of 100X100 pixels implies that you are comparing areas with different sizes. Surely, I am wrong but please, give a clearer explanation.
(9) Line 183. This function is striking. If the problem with MODIS is that the green tide must have a minimal size to be detected, which is lower than it for GF-1, I will expect negative y-intercept. Please, explain.
(10) Lines 195-196. I would like to see some discussion about how this limitation affects the interpretation of the obtained results. Additionally, it should be possible to give a specific value of green tide minimal area with the data gathered.
(11) Line 201. Explain how the error was calculated.
(12) Line 204. The criteria used to discard images affected by clouds should be described.
(13) Lines 210-215. I do not understand what is meant with “leading to the decision to not distinguish Sargassum in the preliminary green tide assessment”.
(14) Line 216. It is not easy to visualise the information in Figure 3. It should be improved. Additionally, please indicate the exact dates of each image.
(15) Lines 238-258. These suggested relationships between atmospheric and environmental variables and dynamics of the green tide must be assessed with some statistical analyses. Furthermore, I wonder which role plays the own growth of the alga in determining the patterns obtained. I guess that alga accumulation at a specific location is due to possible drift of thalli from other next areas and growth in the given location. Winds would contribute to both factors since turbulence may be a relevant factor in avoiding shortage of nutrients for growth.
(16) Line 283. I suppose that the data shown in Table 3 were obtained by different techniques. Consequently, I am not sure that they are directly comparable with the data calculated with satellite images. Please, comment.
(17) Line 305. Support with a reference that optimal salinity for green tide growth is 35.
(18) Line 318. I miss the information about these maps shown in Figure 6 have been obtained. Are they monthly means calculated from daily data? Similar comment with regard to Figure 7.
(19) Line 344. The results of the correlation should be shown. As commented previously, one of the main flaw of the manuscript is that these suggested correlations are not supported with statistical analyses.
(20) Line 352. Section 3 is titled “Results and Discussion”. Please, reorder.
(21) Lines 361-369. The possible role of the nutrients in triggering and/or boosting the red tide should be also discussed.
Author Response
Dear Reviewer,
Thank you for your comments that we have revised and replied as attached.
Best regards,
Yuanzhi Zhang
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe MS represents very interesting study on remote sensing in investigation of the green tides.
Comments:
line 212 - seagrass is a term which means higher marine plants, but green tides are formed by algae. Please correct.
Table 2 - please correct date of sampling and give explanation what means "/" in table
Authors write about possible influence of climatica factors on green tide, however i havent seen any patterns or correlations which would confirm it.
Why authors have chosen linear model for description of green tide? it is known that growth of green tide algae has exponential character (Gladyshev and Gubelit, 2019).
Discussion section is very short and no one point havent been discussed and compared with literature data, as it should be. Moreover i have found no one literature reference in Discussion. Please provide detailed discussion with comparison and support of your data with earlier studies.
At present form MS cannot be accepted for publication and needs major revision
Author Response
Dear Reviewer,
Thank you very much for your comments that we have replied and revised as you suggested.
Best regards,
Yuanzhi Zhang
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsAuthors performed good work and improved paper very well. The last my comments: when mention names of algae: provide species name for Sargassum, if there is not than use Sargassum sp. (name of genera in italics). Species name of Ulva also should be in italics Ulva clathrata. In discussion genera name Enteromorpha is outdated, now it is Ulva, please correct
Author Response
Dear Reviewer,
Thank you very much for your helpful comments to improve the quality of paper.
Yours sincerely,
Yuanzhi Zhang
Author Response File: Author Response.pdf
