Research Trends in the Remote Sensing of Phytoplankton Blooms: Results from Bibliometrics

Round 1

Reviewer 1 Report

TITLE: Research trends in remote sensing of phytoplankton blooms: Results from bibliometrics

AUTHORS: Yuanrui Li, Qichao Zhou, Yun Zhang, Jingyi Li, and Kun Shi.

SUMMARY And COMMENTS

The manuscript uses bibliometrics to perform a comparative quantitative and qualitative analysis in remote sensing of phytoplankton blooms. The authors collected data from SSCI and SCIE databases to conducted descriptive relationships among countries, organizations, and keywords using the LDA model of the VOSviewer software. They also reveal the increasing or decreasing trend of 84 topics.

The topic seems interesting to the readers. However, there are several points that need improvement as shown in the attached file.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The authors present an exciting way of looking at research trends in remote sensing of algal blooms over the past three decades. The analysis reveals interesting facts about the publication trends and draws the big picture explaining the research authors, their institutions, country, and collaboration activities. The manuscript is well-written and well-thought-out. However, I still see some scope of improvement in mostly how the information is collectively presented. For example, the manuscript read like it has two distinct parts, 1) the bibliographic analysis and 2) the literature review. Unfortunately, I did not see any text that justifies the need for the second part. Moreover, it is part of the 'Future directions' section. Perhaps a new section on 'Review' or renaming and expanding the 'Future research direction' section with a new starting paragraph explaining the need for a literature review can help connect the two.

Second, the importance of some of the topics generated based on the "keywords" is not very convincing. E.g., 1) "Topic 43 -Concentration level", 2) Topic - 56-Sample analysis, 3) Topic 80 - Observation sites, to name a few. I would suggest reviewing the topic list carefully and only keeping the meaningful ones. Also, I am not sure why Topic 76-Vegetation Index should be part of this review. Carefully reviewing the topics and merging synonymous ones (E.g. Topics 5 and 7 - satellite data, and watercolor sensors) could make the list of topics more meaningful before looking at their trends.

Additionally, I wonder if there is some scope for getting erroneous trend data presented in terms of the popularity of the research topic through keyword analysis. As the analysis is based on keywords only, a simple change in keywords for the same research topic over time (think, "buzzwords" in place of boring words) may generate erroneous information. At least, there should be a brief discussion on the limitations of the bibliographic analysis in the manuscript.

Finally, there are several statements on 'degree of influence' that need further attention. Please refer to the specific comments below for details.

Specific comments

1. Line 52: "The oxygen depletion caused by microbial degradation after algae death may lead to hypoxia..."

  Consider replacing algae death with 'termination of blooms'

2. Line 62: "Unique absorption spectral characteristics make the use of remote sensing data to monitor phytoplankton blooms possible."

Unique absorption characteristics of what? Make it explicit.

4. Line 68: The new paragraph with entirely different information starts suddenly without any context provided before. Adding a set of transition sentences may help to let the information flow smoothly. I did not see any clear statement on 'Why is this research important?'. Maybe that can be provided as a transitioning statement at the end of the previous paragraph.

 

4. Line 73: "The SSCI and SCIE databases …"

Please expand the abbreviations.

 

5. Line 132: "… the number of various types of publications related to the application of remote sensing in phytoplankton research showed an overall decrease between 1990 and 2020."

 

I see an increasing trend in the figures. Is it a typo?

 

6. Line 145: "The development trend of other countries has also shown steady growth but to a lesser extent. The overall interest in this field is low, but it is still influential."

 

I am concerned with the language 'influential' here. How do the authors define 'influential'? What metric was used to derive whether something is influential or not? Please clarify.

 

7. Line 151: "Countries with higher industrial and agricultural levels are more likely to have issues with aquatic ecosystems, exacerbating the occurrence of phytoplankton blooms, which leads these countries to pay more attention to water environment issues"

 

I believe it is a more complicated issue intertwined with public awareness, political will and support, resource availability, and government funding to research a particular topic of interest. Nevertheless, for the sake of discussion, can it be also because land-locked countries with no large lakes will not be motivated to pursue remote sensing-based bloom monitoring?

 

8. Line 184: "Visual analysis of the number of publications and cooperative relations between countries and research institutions can clarify research hotspots …"

 

"Collaborative relation" may be a better word choice than "cooperative relation."

 

9. Line 192: "Although Germany, England, Spain, Italy, France, and other countries have good research outcomes in this field, they have a lower degree of influence than China and the United States."

 

'Degree of influence' has been used repeatedly. I would suggest defining 'degree of influence' based on some verifiable metric in the method section. Alternatively, avoid using this term as that is very subjective and can be pretty misleading. 

 

10. Line 250: "Against the background of global change, inland water environments are also becoming a research hotspot in this field, as are the bio-optical properties of the water environment, remote sensing monitoring of eutrophication, aquatic vegetation information extraction, and remote sensing estimation of biomass."

 

• Note that 'aquatic vegetation information extraction' may have nothing to do with the scope of this review.
• 'remote sensing estimation of biomass.' 'Remote sensing' is not needed here.

 

11. Line 296: "For example, the limitation of temporal and spatial resolution, the limitation of satellite transit cycles and the limitation of the accuracy of water bloom information extraction have caused research"

 

Consider changing 'water bloom' with 'phytoplankton bloom.'

 

12. Line 301: "The emergence of new remote sensing technologies have broken through the inherent limitations of many traditional remote sensing techniques and provided a great opportunity for phytoplankton research …"

 

New remote sensing technologies such as …? Enrich this statement with some examples of new technologies.

 

13. Line 334: "Although remote sensing technology can provide high-frequency, long-term series, and large-scale data"

 

Long-term time series?

 

14. Line 339: "Thus, the precise identification of phytoplankton blooms is a problem that urgently needs to be solved in the future."

 

VIS/NIR remote sensing challenges such as obfuscating due to cloud cover and lack of night-time imagers (at much needed spectral/spatial resolution) are not unique to only bloom identification. Therefore, the reasoning does not seem quite right.

 

 

15. Line 335: "In the future, we should pay more attention to using new remote sensing technologies to improve the identification accuracy of phytoplankton blooms."

 

I am not sure what the authors mean by the identification of phytoplankton bloom. Is it the detection of bloom presence or identification of 'type of phytoplankton bloom'? If it is later, provide references showing how much SAR can help.

 

16. Line 384:"As the impact of global changes is gradually increasing, the phenology of phytoplankton has become a hot topic."

 

Is it a statement supported by data/reference or an opinion? Rephrase it accordingly.

 

17. Line 393: "With the gradual intensification of phytoplankton blooms, the prediction and management phytoplankton blooms have become key issues"

 

Typo: It should be - the prediction and management of phytoplankton blooms

 

18. Line 425: "In the field of phytoplankton and water environments, long time-series multispectral and hyperspectral sensors have provided breakthrough support for scientific research, but they also have many limitations, such as the transit cycle time of the satellite, the accuracy of the satellite's resolution for identifying algal taxa,"

 

1. What is the transit cycle time of the satellite? Should it be temporal resolution?
2. What is the accuracy of the satellite's resolution? Are the authors referring to geometric accuracy or spatial resolution here?

 

19. "The high-precision atmospheric correction model for water bodies is still in the developmental stage."

 

By who and where?

 

20. Line 444: "Therefore, in the future use of remote sensing in phytoplankton research, special sensors for phytoplankton identification and extraction could speed up the progress of phytoplankton research"

 

What is a special sensor?

 

21. Line 456: "… and UAVs can be equipped with various types of special sensors to perform high-frequency, high-resolution monitoring of water environments…"

 

This sentence sounds vague. Consider adding some specifics to the statement.

 

22. Line 473: "While the application of virtual constellations is also in its infancy, the application of this method in phytoplankton research has demonstrated bright prospects."

 

It sounds counter-intuitive. Provide a reference. 

 

Figure comments

 

23. Figure 3. I can hardly read the figure. Consider using a darker font color for the labels.
24. Figure 5. Same comment as above
25. Figure 6. 

• The font size of the figure labels and the legend and color bar can be more prominent and darker. 
• Figure caption: How was the importance of keywords determined? Briefly explain that in the text.

 

Table comments

 

26. Table 1. Percent of what number? Why do the percentage numbers collectively exceed 100%?

 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Line 73: Provide full name and link for “SSCI” and “SCIE”

Line 97-105: Please provide the formula in a separate block (or paragraph) like the way you present equation. In addition to providing the formula, please explain the different terms (parameters) and give the justification for each technical term. You can be creative here and present this in table or graphical layout for good visualization.

Line 121: Are there other model like LDA? Please explain. If there are, provide reason for choosing LDA and not others.

Line 134: “between 1990 and 2020”, do you mean 2000 instead of 2020?

Table 1: The total of the % is more than 100. Is there an overlap between the discipline? If so, please explain either in the caption or as a note in the table.

Figure 2: Journal name in title case and provide a legend title “Name of the Journal” above the legend.

Table 2 and 3: Provide the names in title case.

Figure 6: The caption for Figure b is little confusing. Please rephrase it.
Also, not sure why few of the labels are very faint gray color. Very difficult to see them.

Line 262: “three topics were removed”: can you expand on this?

Table 4: No need to mention “Topic” for each topic, just provide the corresponding number and the keyword (e.g., “17: Method”) and explain what the number is either on the table heading or in the caption. Please expand the caption for the figure.

Table 5: Please expand the caption and also replace “hot and cold” with more meaningful words (like “linear increase/decreases”?).

Figures 7 and 8:  Rephrase “hot” and “cold” in the caption. Also remove the word “Topic” from each entry and just start with number (e.g., 49: Red tide) and explain the number in the caption or in the legend.

Line 393: missing “of” before phytoplankton?

Line 403: Provide full name for MODIS in its first instance. Same for UAV, although it is common.

Line 428: transit cycle? May be “temporal resolution” is more appropriate here.

Line 466: multisensory? You mean “multisensor”

Line 475, 479: “water environment”? You mean aquatic environment?

 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors carefully addressed and responded to all comments.

I have only one comment as follows:
I would ask the authors to rephrase the newly added statements (Lines 397~406) as they are long and hard to read.

Author Response

Response: We have revised these in accordance with your comments:

“The application of remote sensing technology to analyze the traits of phytoplankton blooms, such as the phytoplankton community structure, temporal and spatial distributions, and secondary metabolites and their long-term responses to environmental factors, can aid in exploring the causes and consequences of phytoplankton blooms. Combining remote sensing technology and HPLC (high-performance liquid chromatography) pigment analysis, the phytoplankton community composition can be deduced [53-54].” (Lines 399~408).

 

Reviewer 2 Report

The authors have addressed all my comments and questions. The revised manuscript looks complete to me and almost ready for publication. However, I only have one edit that needs explanation.

Lines 63-66: "The unique spectral characteristics of phytoplankton blooms (with obvious absorption bands around 500 nm and 675 nm and reflection peaks around 550 nm and 700 nm) enable the use of remote sensing data to monitor phytoplankton blooms [15]."

15. Simis, S.G.; Huot, Y.; Babin, M.; Seppala, J.; Metsamaa, L. Optimization of variable fluorescence measurements of phytoplankton communities with cyanobacteria. Photosynth Res2012, 112, 13-30, doi:10.1007/s11120-012-9729-6.

 Comment

The absorption band description sounds a bit misleading. I am not sure what pigment is referred here to have an absorption peak at 500 nm. Perhaps, the authors mean the primary chl-a absorption peak at ~443 nm and the secondary chl-a peak at 675 nm here? 

As the authors would know, "Phytoplankton bloom" is a broader group and should not be generalized by specific absorption peaks. It is the pigments in the phytoplankton group that defines the absorption properties of the blooms. Hence, it should be specified at the pigment level. Please refer to the "Spectral characteristics of absorption and fluorescence" section in Semis et al. (2012), a very relevant reference, and see how they describe the absorption properties specific to pigments or at phytoplankton genus/species level.

 

Author Response

Comments and Suggestions for Authors:

The authors have addressed all my comments and questions. The revised manuscript looks complete to me and almost ready for publication. However, I only have one edit that needs explanation.

 

Lines 63-66: "The unique spectral characteristics of phytoplankton blooms (with obvious absorption bands around 500 nm and 675 nm and reflection peaks around 550 nm and 700 nm) enable the use of remote sensing data to monitor phytoplankton blooms [15]."

 

15. Simis, S.G.; Huot, Y.; Babin, M.; Seppala, J.; Metsamaa, L. Optimization of variable fluorescence measurements of phytoplankton communities with cyanobacteria. Photosynth Res 2012, 112, 13-30, doi:10.1007/s11120-012-9729-6.

 

Comment:

The absorption band description sounds a bit misleading. I am not sure what pigment is referred here to have an absorption peak at 500 nm. Perhaps, the authors mean the primary chl-a absorption peak at ~443 nm and the secondary chl-a peak at 675 nm here?

 

As the authors would know, "Phytoplankton bloom" is a broader group and should not be generalized by specific absorption peaks. It is the pigments in the phytoplankton group that defines the absorption properties of the blooms. Hence, it should be specified at the pigment level. Please refer to the "Spectral characteristics of absorption and fluorescence" section in Semis et al. (2012), a very relevant reference, and see how they describe the absorption properties specific to pigments or at phytoplankton genus/species level.

 

Response: Thank you very much. You are right, we made an error in the description of "spectral characteristics" in the last revision. Now, we have changed the expression in the manuscript as follow:

“The unique spectral characteristics of phytoplankton pigments (e.g., the obvious absorption peaks of chlorophyll-a near 440 nm and 675 nm, and the absorption peaks of phycocyanin at 615‒630 nm) enable the use of remote sensing data to monitor phytoplankton blooms [15].” (Lines 63~68).