Review Reports - Harmful Algal Blooms in Eutrophic Marine Environments: Causes, Monitoring, and Treatment

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have done a great job and presented an overview of numerous data. The work is necessary and relevant. The article "Harmful algal blooms in eutrophic marine environments: causes, monitoring and treatment" is presented in the review format and requires revision for a full analysis of the data.

General comments.

To navigate in a large array of data, it is necessary:

1. Improve the design of the article.

Lines 50-52, increase the size of "Figure 1. Evolution of the number of publications in indexed journals containing the keywords “Marine eutrophication” and “Global warming” between 2001 and 2024.", also for "Figure 4 Global consumption of nitrogen and phosphate fertilizers" and "Figure 10. Key Steps in using automated in situ sensors"

There are no examples of satellite maps and examples of remote sensing with decoding.

Line 115-116 "Figure 3. Factors leading to marine eutrophication", the figure does not contain information on all the factors influencing eutrophic marine environments. It is necessary to indicate the main pathways for the entry of biogenic elements.

2. Formulation and analysis of criteria for both "eutrophication" and "water bloom", possibly identifying bloom types.

- Analysis of criteria for water "bloom" types, by the composition and biomass of the dominant phytoplankton group. Taking into account the influence of alien species and invasion factors.

- Bloom type in coastal waters or remote waters.

- Seasonal bloom type for a temperate climate.

- Analysis of the dependence of bloom types on hydrochemical factors, nitrogen (N) and phosphorus (P) and silicon (Si) content, as well as other causes - currents, horizontal and vertical circulation of water masses, oxygen content, coastal features, etc. 3. The impact of water blooms on the carbon cycle and carbon accumulation processes during biomass formation.

4. The role of the transition of agriculture to organic farming methods, as well as innovative nitrogen and phosphate fertilizers and mixed organo-mineral fertilizers, which can reduce the removal of biogenic elements from catchment areas

5. The degree of accuracy of remote sensing and the role of using hyperspectral cameras with the most effective range.

Specific comments:

In the figure "Figure 7. Distribution of global marine eutrophication hotspots and coastal cities with a population of more than 3 million worldwide." There are at least 15 such "hotspots" indicated, it seems to me that a brief analysis should be given for each point, there is a degree of eutrophic marine environments, bloom, aquaculture development. In this case, the article will have a more interesting data analysis and comparisons, and the publication will be useful to many specialists. For example, in the Black Sea for a number of temperate zones, the phenomenon of "winter bloom" of Emiliania huxleyi November-February (Black Sea) is described and is associated with its weak consumption by zooplankton compared to diatoms, etc. Regional specificity requires the binding of regulation methods, and if used incorrectly, it can lead to the invasion of alien organisms, with negative consequences for ecosystems. The authors have already provided some information, line 30 in "Table 6. Practical examples of HABs surveillance using remote sensing". I propose to mark these areas in "Figure 7", which will show the degree of elaboration of the material by the authors of the article.

- The impact of toxic bloom processes requires supplementation taking into account the geographical distribution. It is also necessary to indicate the inclusion of toxins in the standardization of water resources. - Measures for the prevention and reduction of biomass, including the analysis of local mechanical collection and processing of algae, for example, into feed or organic fertilizers during thermophilic fermentation with deactivation of toxins (thermal decomposition). - A more detailed analysis of the factors causing "blooms" is needed, taking into account changes in the parameters of the content of hydrochemical elements, for example, the ratio, so American oceanographer Alfred C. Redfield who in 1934 first described the relatively consistent ratio of nutrients in marine biomass samples collected across several voyages on board the research vessel Atlantis, and empirically found the ratio to be C:N:P. = 106:16:1 (Link: https://hahana.soest.hawaii.edu/cmoreserver/summercourse/2012/documents/bronk_05-30-12/Redfield_1934.pdf). The silicon ratio is being studied in a number of inland seas and coastal areas. In many coastal temperate areas, this has been particularly evident in recent decades in the changes in nutrient concentrations involving silicon (Si), nitrogen (N), and phosphorus (P) (Rabalais et al., 1996). The main consequence of these changes in nutrient concentration ratios is a shift in the composition of plankton species in the near-shore zone, with diatoms replacing flagellates, cyanobacteria, and other non-silicon microalgae (Humborg et al., 2000).

Other notes:

Lines 41-42, empty brackets “Over the past two decades, significant progress has been made 41 in understanding marine eutrophication ()."

References:

Humborg, C. Silicon retention in river basins: Far-reaching effects on biogeochemistry and aquatic food webs in coastal marine environments / C. Humborg, D. J. Conley, L. Rahm, F. Wulff, A. Cociasu and V. Ittekkot // Ambio. –2000. – Vol. 29. – P. 45-50.

Rabalais, N. N. Nutrient changes in the Mississippi River and system responses on the adjacent continental shelf / N. N. Rabalais, W. J. Wiseman, R. E. Turner, B. K. SenGupta and Q. Dortch // Estuaries. – 1996. – Vol. 19. – P. 386-407.

Author Response

General comments.

To navigate in a large array of data, it is necessary:

Improve the design of the article.

Response: Thank you for your valuable comments, the size of these pictures was increased.

There are no examples of satellite maps and examples of remote sensing with decoding.

Response: The examples were listed in Table 22.

Response: The revised picture contains 8 pathways for the entry of biogenic elements: (1) atmosphere deposition, (2) wastewater discharges, (3) agriculture runoff, (4) urban runoff, (5) surface water runoff, (6) coastal development, (7) soil erosion, and (8) aquaculture. We have made the corresponding changes. Thank you very much for your valuable comment.

Formulation and analysis of criteria for both "eutrophication" and "water bloom", possibly identifying bloom types.

Response: The criteria for eutrophication and HABs was shown in Table 2, and the bloom types were listed in Table 3.

- Analysis of criteria for water "bloom" types, by the composition and biomass of the dominant phytoplankton group. Taking into account the influence of alien species and invasion factors.

Response: The criteria for eutrophication and HABs was shown in Table 2, and the influence of alien species were listed in Table 17.

- Bloom type in coastal waters or remote waters.

Response: The relationship between location, dominant algal species, geographical distribution, environmental factors, oceanographic features, and the mechanism of influence on toxic bloom processes were listed in Table 18.

- Seasonal bloom type for a temperate climate.

Response: Seasonal bloom type were listed in table 14.

Response: The elemental requirements for various HAB species were listed in Table 8.

The role of the transition of agriculture to organic farming methods, as well as innovative nitrogen and phosphate fertilizers and mixed organo-mineral fertilizers, which can reduce the removal of biogenic elements from catchment areas.

Response: Innovative fertilizer technologies and mixed organo-mineral fertilizers for reducing eutrophication were illustrated in Table 11 and Table 12.

The degree of accuracy of remote sensing and the role of using hyperspectral cameras with the most effective range.

Response: The degree of accuracy of remote sensing were illustrated in Table 22, and the role of using hyperspectral cameras with the most effective range were discussed in the following paragraph.

Specific comments:

In the figure "Figure 7. Distribution of global marine eutrophication hotspots and coastal cities with a population of more than 3 million worldwide." There are at least 15 such "hotspots" indicated, it seems to me that a brief analysis should be given for each point, there is a degree of eutrophic marine environments, bloom, aquaculture development. In this case, the article will have a more interesting data analysis and comparisons, and the publication will be useful to many specialists. For example, in the Black Sea for a number of temperate zones, the phenomenon of "winter bloom" of Emiliania huxleyi November-February (Black Sea) is described and is associated with its weak consumption by zooplankton compared to diatoms, etc. Regional specificity requires the binding of regulation methods, and if used incorrectly, it can lead to the invasion of alien organisms, with negative consequences for ecosystems. The authors have already provided some information, line 30 in "Table 6. Practical examples of HABs surveillance using remote sensing". I propose to mark these areas in "Figure 7", which will show the degree of elaboration of the material by the authors of the article.

Response: The population, and nitrogen and phosphorus discharge in major global cities were illustrated in Table 13, and the "winter blooms" phenomenon were summarized in Table 14 and discussed in line 357-370. Practical examples of HABs surveillance using remote sensing were added to Figure 7, green dots.

Response: The geographical distribution of toxic bloom were illustrated in Table 18. Algae Resource Conversion Technologies from HABs were listed in Table 35. Detailed elemental requirements for various HAB species were summarized in Table 8.

Other notes:

Lines 41-42, empty brackets “Over the past two decades, significant progress has been made 41 in understanding marine eutrophication ()."

Response: Thank you for your valuable comments. This is a empty brackets caused by our negligence. We have filled it.

References:

Response: Thank you for your valuable comments. These references have been cited in the revised article.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

General Comment: I have no suggestion for improvement for the content / reviews in this manuscript. All are well-written and comprehensively reviewed. In my field of microalgae research, reviews made were accurate and there are no misleading facts. However, here are my few suggestions regarding the representation of findings.

1. Page 3, Line 52: Figure 1 - Advise to change either both to column chart or both to line chart, put color according to its respective y-axis. I suggest both data using similar type due to its similar nature: no. of publication.

2. Page 3, Line 56: Remove number of the aims.

3. Page 7, Line 136: If you could plot phosphate on secondary y-axis, that would be better to show its trend.

4. Page 7, Line 150: It is suggested to combine x-axis by merging graph. This suggestion is very favorable due to similar x-axis, this would help to make the figure more appealing and simpler to understand.

5. Page 8, Line 164: Color the arrow to make it more visible.

6. Page 18, Line 338: It is suggested to put a box as text background to make it more readable. Make the line thicker and more contrast.

7. Page 21, Line 373: Suggested to add one more column, reference, to Table 11.

Comments for author File: Comments.pdf

Author Response

Page 3, Line 52: Figure 1 - Advise to change either both to column chart or both to line chart, put color according to its respective y-axis. I suggest both data using similar type due to its similar nature: no. of publication.

Response: Thank you for your valuable comment. Both have now been changed to column charts.

Page 3, Line 56: Remove number of the aims.

Response: Thank you for your valuable comment. The number of the aims were removed.

Page 7, Line 136: If you could plot phosphate on secondary y-axis, that would be better to show its trend.

Response: Thank you for your valuable comment. Global consumption of phosphate fertilizers were shown in column charts on secondary y-axis.

Page 7, Line 150: It is suggested to combine x-axis by merging graph. This suggestion is very favorable due to similar x-axis, this would help to make the figure more appealing and simpler to understand.

Response: Thank you for your valuable comment. The same x-axis has been merged.

Page 8, Line 164: Color the arrow to make it more visible.

Response: Thank you for your valuable comment. The color of the arrows were changed.

Page 18, Line 338: It is suggested to put a box as text background to make it more readable. Make the line thicker and more contrast.

Response: Thank you for your valuable comment. We had put a box as text background to make it more clear.

Page 21, Line 373: Suggested to add one more column, reference, to Table 11.

Response: Thank you for your valuable comment. References were added in the extra column.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The review paper titled "Harmful Algal Blooms in Eutrophic Marine Environments: Causes, Monitoring, and Treatment," provides a thorough examination of the current methodologies and technologies used to monitor and mitigate the impacts of marine eutrophication and harmful algal blooms (HABs). This review is comprehensive, covering a wide range of topics, including molecular techniques, remote sensing, chemical and physical treatments, and the application of advanced technologies like nanotechnology and electrocoagulation. Overall, the review succeeds in compiling a significant amount of information that could serve as a valuable resource for researchers and practitioners in the field.

My suggestions and comments are presented as follows

-line 31-37 : Can the authors provide a more detailed explanation of how specific nutrients contribute to the formation of harmful algal blooms? Additionally, how do these processes differ across various marine ecosystems?

- How does this review differ from or build upon previous reviews on marine eutrophication and HABs? Can the authors specify the unique contributions of this review to the existing literature?

- Could the authors provide a more geographically focused discussion of where marine eutrophication is most prevalent? This could help contextualize the global significance of the issue.

Lines 63-64: Can the authors provide a more precise definition of oxygen depletion and explain the thresholds that define hypoxic and anoxic conditions in marine environments?

Lines 65-67: Could the authors elaborate on how nitrogen and phosphorus specifically contribute to eutrophication and oxygen depletion? Are there differences in their roles or impacts?

Lines 74-77: Can the authors provide more information on the microbial communities involved in the decomposition of algal biomass and how environmental factors influence their oxygen consumption rates?

Figure 2: Could the authors briefly describe the content of Figure 2 within the text and explain how it visually supports the discussion of habitat degradation mechanisms?

Lines 118-123: Can the authors provide more details on how different types of crops or specific agricultural practices influence the amount and type of nutrient runoff?

Lines 124-129 Could the authors expand on the specific challenges faced by developing countries in managing fertilizer use

Lines 192-197: Can the authors provide more detail on the specific mechanisms by which CO2 emissions influence the proliferation of harmful algal species? How do increased sea surface temperatures and changes in ocean chemistry directly impact algal bloom dynamics?

Lines 189-190: Could the authors expand on how the fluctuations in CO2 emissions during the COVID-19 pandemic and subsequent rebound might have impacted marine ecosystems? Were there any observed changes in the frequency or intensity of HABs during this period?

- Figure 8: the text does not adequately explain how this figure relates to the discussion of HABs or the broader impacts on marine ecosystems. Could the authors include a brief explanation of how Figure 8 relates to the discussion of CO2 emissions' impacts on marine ecosystems, particularly in the context of HABs? What insights does the figure provide about future trends?

Lines 251-258: Could the authors expand on how digital PCR (dPCR) differs from traditional PCR and qPCR? how does dPCR achieve absolute quantification, and what are the practical implications of this in monitoring harmful algal blooms?

Lines 307-308: Could the authors include a brief discussion of the case studies presented in Table 6? How do these examples illustrate the effectiveness and limitations of remote sensing technologies in different regions and ecological contexts?

Table 11: Can the authors include a comparative analysis in the text that discusses the relative efficacy, cost, and environmental impact of the different phosphorus precipitation treatments listed in Table 11? What factors should be considered when selecting a treatment method?

Lines 494-495: Can the authors elaborate on the materials typically used for sediment capping and their effectiveness over time? What are the potential risks associated with this method, such as the need for repeated applications or unintended environmental consequences?

Lines 590-593: Could the authors provide specific examples or recommendations on how an integrated approach to managing marine eutrophication might be practically implemented? What combinations of monitoring and treatment methods are most likely to be effective in different environmental contexts?

Lines 593-595: Can the authors identify specific research gaps or technological challenges that need to be addressed to improve the management of marine eutrophication? What are the priority areas for future research?

Lines 600-603 : How can sustainability considerations be better integrated into the selection and implementation of monitoring and treatment methods for marine eutrophication? What criteria should be prioritized to ensure long-term environmental health?

Author Response

My suggestions and comments are presented as follows

Response: Thank you for your valuable comments. The nutrients requirements for various HAB species were illustrated in Table 8. Nutrient contributions to HABs across various marine ecosystems were illustrated in Table 9.

- How does this review differ from or build upon previous reviews on marine eutrophication and HABs? Can the authors specify the unique contributions of this review to the existing literature?

Response: Thank you for your valuable comments. This review discusses the causes of HABs in the ocean, and explores in depth the causes of HABs under different nutrients, marine ecosystems, and global warming trends. It also summarizes and compares the monitoring and governance solutions for HABs. In addition, it also points out the technical difficulties that need to be overcome in the future to control marine HABs..

- Could the authors provide a more geographically focused discussion of where marine eutrophication is most prevalent? This could help contextualize the global significance of the issue.

Response: Thank you for your valuable comments. Geographical distribution of HABs were discussed in Section 3.4. The locations where HABs most frequently break out are marked with red dots in Figures 7 and 8.

Lines 63-64: Can the authors provide a more precise definition of oxygen depletion and explain the thresholds that define hypoxic and anoxic conditions in marine environments?

Response: Thank you for your valuable comments. The definition of oxygen depletion and explain the thresholds that define hypoxic and anoxic conditions in marine environments were provided in section 2.1.

Lines 65-67: Could the authors elaborate on how nitrogen and phosphorus specifically contribute to eutrophication and oxygen depletion? Are there differences in their roles or impacts?

Response: Thank you for your valuable comments. Environmental factors significantly impact oxygen consumption rates in microbial decomposition processes. Higher temperatures typically accelerate microbial metabolic rates, leading to increased decomposition and oxygen consumption, though this can also result in faster oxygen depletion, especially in poorly mixed water bodies. Con-versely, colder temperatures slow microbial activity, reducing both decomposition and oxygen use. Nutrient availability plays a dual role: nutrient-rich environments promote rapid microbial growth and higher oxygen consumption, while excessive nutrients can lead to algal blooms, increasing organic matter and exacerbating oxygen depletion. Oxygen levels are critical; high oxygen supports efficient aerobic decomposition, while low oxygen shifts processes to less efficient anaerobic pathways, resulting in the accu-mulation of partially decomposed matter and production of compounds like hydrogen sulfide and methane. The composition of algal biomass also affects decomposition rates; algae with tough cell walls or harmful toxins may decompose more slowly, influencing microbial activity and oxygen consumption. And this was discussed in line 104-117.

Response: Thank you for your valuable comments. Microbial communities involved in the decomposition of algal biomass in marine environments were listed in Table 4.

Figure 2: Could the authors briefly describe the content of Figure 2 within the text and explain how it visually supports the discussion of habitat degradation mechanisms?

Response: Thank you for your valuable comments. The original Figure 2 does not well describe how HABs lead to habitat degradation, so it is replaced by Table 7, contribution of pathways to habitat degradation by HABs.

Lines 118-123: Can the authors provide more details on how different types of crops or specific agricultural practices influence the amount and type of nutrient runoff?

Response: Thank you for your valuable comments. Cereal crops, as well as Leguminous crops, have a high demand for nitrogen and phosphorus fertilizers and are therefore the source of nitrogen and phosphorus runoff in agricultural production. This was discussed in Lines 253-260, and influence of agricultural practices on nutrient runoff were listed in Table 10.

Lines 124-129 Could the authors expand on the specific challenges faced by developing countries in managing fertilizer use

Response: Thank you for your valuable comments. The specific challenges faced by developing countries in managing fertilizer use were discussed in Lines 261-272. Developing countries face a multitude of challenges in managing fertilizer use, encompassing economic constraints, technical and knowledge gaps, infrastructural difficulties, environmental vulnerabilities, weak regulatory frameworks, and social and cultural barriers. High fertilizer costs and dependence on subsidies make it difficult for smallholder farmers to apply fertilizers efficiently, while limited access to soil testing and agricultural extension services leads to imbalanced fertilization. Poor infrastructure, such as inadequate transportation and storage, further complicates fertilizer distribution. Environmental factors, including vulnerability to extreme weather and soil degradation, exacerbate these issues. Additionally, weak regulations allow low-quality fertilizers to flood the market, and traditional practices often resist modern agricultural methods. Addressing these challenges requires a comprehensive approach that improves education, infrastructure, regulation, and promotes sustainable agricultural practices.

Lines 192-197: Can the authors provide more detail on the specific mechanisms by which CO₂ emissions influence the proliferation of harmful algal species? How do increased sea surface temperatures and changes in ocean chemistry directly impact algal bloom dynamics?

Response: Thank you for your valuable comments. CO₂ emissions cause climate change, leading to rise of sea temperatures and ocean acidification. The mechanism underlying the effect of increased sea surface temperatures and changes in ocean chemistry on algal bloom were discussed in section 3.2.1 and section 3.2.2.

Lines 189-190: Could the authors expand on how the fluctuations in CO₂ emissions during the COVID-19 pandemic and subsequent rebound might have impacted marine ecosystems? Were there any observed changes in the frequency or intensity of HABs during this period?

Response: Thank you for your valuable comments. The COVID-19 pandemic led to a temporary reduction in CO₂ emissions due to decreased industrial activity, transportation, and energy consumption. The temporary reduction in emissions during the pandemic slightly slowed the rate of ocean acidification and global warming, but these changes were short-lived and not substantial enough to re-verse long-term trends. This was discussed in Lines 380-390.

- Figure 8: the text does not adequately explain how this figure relates to the discussion of HABs or the broader impacts on marine ecosystems. Could the authors include a brief explanation of how Figure 8 relates to the discussion of CO₂ emissions' impacts on marine ecosystems, particularly in the context of HABs? What insights does the figure provide about future trends?

Response: Thank you for your valuable comments. Increased CO₂ emissions cause climate change, leading to rise of sea temperatures and ocean acidification. Figure 8 indicated the increased CO₂ emission, This will increase ocean water warming and ocean acidification. How warming ocean waters and ocean acidification lead to HABs are discussed in subsequent sections 3.2.1 and 3.2.2.

Response: Thank you for your valuable comments. The differences between traditional PCR, dPCR, and qPCR were listed in Table 19.

Response: Thank you for your valuable comments. The effectiveness and limitations of remote sensing technologies in different regions and ecological contexts was discussed in Lines 608-617.

Response: Thank you for your valuable comments. The efficacy, cost, and environmental impact of the different phosphorus precipitation treatments were listed in Table 27 now.

Response: Thank you for your valuable comments. Sediment capping is a widely used physical treatment to control nutrient release from sediments and mitigate marine eutrophication. Common materials used for sediment capping and potential risks associated with this method were discussed in Lines 843-864.

Response: An integrated approach to managing marine eutrophication combines multiple strategies to effectively address the complex and multifaceted nature of this environ-mental issue. This approach involves a combination of monitoring, prevention, and remediation techniques tailored to specific environmental contexts. Details were discussed in Lines 953-992.

Response: Thank you for your valuable comments. Research gaps or technological challenges that need to be addressed to improve the management of marine eutrophication were discussed in Lines 1003-1031.

Response: Thank you for your valuable comments. Sustainability must be integral to the selection and implementation of monitoring and treatment methods for marine eutrophication, and this was discussed in Lines 1032-1039.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have made significant changes to the manuscript of the article. The reviewer thanks for the comments taken into account.

Correction:

There are no examples of satellite maps and examples of remote sensing with decoding.

Response: The examples were listed in Table 22.

In this case, the reviewer suggested adding a picture of the red tide to the publication. Indicating the types of zones according to the degree of "seawater bloom"

Comments for author File: Comments.pdf

Author Response

Please see attachment

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The revised manuscript is sufficient. The quality of the manuscript has been improved and it can be published in present form.

Author Response

The revised manuscript is sufficient. The quality of the manuscript has been improved and it can be published in present form.

Respond: Thank you again for your recognition of our work.