the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Oct 2025
Chlorophyll-a Variation Trends in Marginal Seas: Assessing the Impact of Global warming and Anthropogenic Activities Using Time Series Satellite Data (1998–2020)
Abstract. Global warming has been identified as the main cause of the decline of surface chlorophyll-a (Chl-a) concentrations in the oceans. Conversely, an increase in Chl-a concentration has been observed in a number of marginal seas over recent decades due to increasing anthropogenic input of key nutrients. However, with the intensification of global warming, its impact on Chl-a in coastal waters has been rarely studied, with the superimposed effects of human regulation of nutrients emissions. This study utilized time series of oceanic color satellite data from 1998 to 2020 to examine the spatio-temporal distribution of Chl-a in a range of marginal sea areas, and its relationship with environmental factors, particularly with sea surface temperature (SST), photosynthetically active radiation (PAR) and surface wind speed (SWS) are considered as well. The results suggested that the sea areas examined with varying mixing and water exchange characteristics and degrees of human influence have differing responses (in terms of their Chl-a trends) to increasing SST. Specifically, eutrophic closed seas with weak hydrodynamic exchange capacity, like the Bohai Sea, increasing SST did not apparent suppress Chl-a concentration, but instead continuous increase in Chl-a was observed in the central of the sea. In comparison, the open marginal seas examined show strong negative relationships between SST and Chl-a with distance offshore regardless of the degree of pressure from human activities, indicating that expected global warming effects driving reductions in Chl-a are extending to nearshore / marginal sea areas. This trend may be exacerbated due to stricter environment management policies imposed in recent years which have reduced anthropogenic nutrient inputs. Distinct from the above effect of global warming, PAR and SWS shape Chl-a in ways that are strongly modulated by geography and climate. PAR is the dominant positive control only in the Amazon estuary, where equatorial cloudiness and high turbidity create a light-limited regime, so any PAR increase directly stimulates phytoplankton. In mid-latitude open waters, PAR is secondary to SST: its seasonal rise is coupled to SST and therefore correlates negatively with Chl-a once thermal stratification reduces nutrient supply. SWS emerges as a key driver in the three open regimes (East China Sea >US East Coast> Amazon shelf), through injecting nutrient-rich cold deep water and episodically raise Chl-a. Inside the two enclosed seas (Bohai, Gulf of Mexico), correlations with both PAR and SWS are weak (│r│ < 0.2); Thus, PAR and SWS control Chl-a in a complex way, but both are more or less links to SST and nutrients input. This study highlights the complex interaction between primary production, SST, nutrient inputs and exchange, and environmental protection controls under the dual pressures of changes in human activity and coastal development, and global warming.
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Status: open (until 31 Mar 2026)
- RC1: 'Comment on egusphere-2025-4558', Mian Liu, 19 Nov 2025 reply
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CC1: 'Comment on egusphere-2025-4558', Shengqiang Wang, 10 Jan 2026
reply
General comments
This manuscript presents an interesting comparative analysis of long-term Chl-a trends across five representative marginal seas with contrasting hydrodynamic conditions and anthropogenic pressures. The selection of the study regions is appropriate and well justified, covering both enclosed and open systems under different climatic and human influences. The use of multi-sensor satellite time series spanning more than two decades provides a solid data foundation, and the combination of trend analysis, stability assessment, and correlation analysis allows for a comprehensive examination of Chl-a variability. Overall, the manuscript is generally well structured, and the results are clearly presented. The discussion demonstrates a good understanding of regional oceanographic processes and places the findings in a broader context of global warming and anthropogenic nutrient regulation. This paper can be considered for publication after clarifying the following concerns.Specific comments
1. The manuscript briefly mentions that systematic errors between SeaWiFS and MODIS products were eliminated before merging the datasets. It would be helpful to provide a short quantitative description of the inter-sensor bias correction or cross-calibration results in the main text, for example by reporting mean bias or correlation statistics during the overlap period, with detailed information remaining in Appendix A.2. Chl-a, SST, PAR, and SWS datasets have different spatial resolutions. Although this is common in satellite-based studies, a short explanation of how spatial resampling or aggregation was handled would improve methodological transparency.
3. Coastal and estuarine waters often suffer from data gaps due to cloud cover or high turbidity. The authors should briefly describe how missing pixels were handled when calculating regional means and trend statistics.
4. The four-year moving window approach is reasonable. However, the authors may consider adding a brief sensitivity discussion explaining why a four-year window was selected instead of alternative window lengths, such as three or five years.
5. The manuscript states that Chl-a and SST time series were normalized before correlation analysis. It would be useful to specify whether standard z-score normalization or another approach was applied.
6. Terms such as “Eastern Coastal Waters of the US” and “U.S. East Coast” appear to be used interchangeably. Standardizing terminology throughout the manuscript would enhance clarity.
7. The Conclusions section is relatively lengthy. The authors are encouraged to further streamline this section by concisely summarizing the main findings and scientific contributions of the study. In addition, explicitly stating the limitations of the current analysis and outlining potential directions for future research would strengthen the conclusions.
8. In Figure 1a, the blue label “East Sea” should be corrected to “East China Sea”.
Citation: https://doi.org/10.5194/egusphere-2025-4558-CC1 -
RC2: 'Comment on egusphere-2025-4558', Anonymous Referee #2, 10 Mar 2026
reply
The manuscript “Chlorophyll-a Variation Trends in Marginal Seas: Assessing the Impact of Global warming and Anthropogenic Activities Using Time Series Satellite Data (1998-2020)” by Nan Yao and colleagues assesses the impact of sea surface temperature and nutrient concentration in a couple of regional seas and basins on Chlorophyll a concentration timeseries derived from ocean color remote sensing products. The statistical analysis differentiates between enclosed seas and open water regimes, as well as periods of stable or rising temperatures, and as such, tries to disentangle the effects of rising sea surface temperatures among other physical parameters on primary production. The study could benefit from some revisions regarding the sometimes lenghty presentation of results, contrasting the rather short discussion, missing the embedding of the conclusions drawn in the broader scientific frame. I have some suggestions that hopefully the authors can address to improve the manuscript.
The differentiation of open and enclosed seas seems a bit weak because the authors have already stated the influence of ocean currents (and water mass trasnport) in the Gulf of Mexico. However, in the Results and Discussion Sections, the authors first generalize based on the Bohai Sea for statements on the enclosed seas which do not old for the Gulf of Mexico. Maybe the general statements could be somehow toned down?
As in the Discussion Section, the authors still introduce new results, I got a bit lost in the details. I would like to see how the results are discussed along available literature on drivers of Chl-a in the five regions selected in this study. This is partly done in Section 4.2.3 (L. 455-472), the study would benefit from assessing the results/conclusions with literature. Moreover, it would be interesting to broaden the perspective at some point, create a train of thought (like in the Conclusions), and compare, how other regions are shaped (quickly synthesizing other studies).
Quality of plots: I would prefer if you could spell out all abbreviations again (increased readability, also inconsistently done in manuscript) and generally use a larger font size in the labels and annotations.
Appendix A, C, D: Could you integrate these parts in the main manuscript? They are rather short and the Methods Section could profit from including a bit more details about the processing. You could still keep all the resulting equations of Appendix A in an Appendix Table.
Please carefully check grammar and spelling again, I listed some examples below that caught my eye.
Specific comments
Section 1
L. 17: Why do you prefer the term "oceanic color" to "ocean color"?
L. 21: "Specifically, in eutrophic closed seas"
L. 22: apparently/appear to?
L. 23: in the central part of the sea
L. 24 "with distance offshore" please rephrase
L. 33: raising
L. 34: linked
L. 35: exchange of what?
L: 40: I would think the term "biological carbon pump" is more common
L. 51: increasing with distance from shore/from coast?
L. 51: There is a sentence consisting only of citations. What is the message?
L. 64: Maybe you could add the values here?
L. 68-71: I suggest splitting the sentence.
L. 73: including? I suggest re-phrasing and splitting, your point here is really hard to grasp.
L. 76-77: Please rephrase: You argue that global warming induced changes of PAR and SWS might change chl-a, but this is complex to clarify?
L. 81: reference for "The influence of SWS on Chl-a is non-linear and spatially heterogeneous."? Maybe you want to add references to the following statements as well? Is it only Wang and Xiu (2022)?
L. 95: warm water?Section 2
Please reconsider the use of tenses and unify in the whole section. Right now you switch between past and present tense quite randomly.
L. 113: I'd prefer larger font size for the annotations on the world map and for the geographical names in the subsets a-e.
L. 127-129: Please rephrase this sentence and unify tenses, maybe "events which increased"?
L. 138: Add a reference fot the pollutants.
Table 1: Consider horizontal row seperators because the table is hard to read.
L. 165: What are applicable methods? Do you mean you "applied a method"? Maybe you want to shift the processing to the Methods Section 2.3?
l. 170: "The two types of data products have different coverage times" - what are the different types? That's unclear.
L. 203: Please rephrase the reference to Appendix C.
L. 226: Is there a reference for the seasonal cycles or weather events?
L. 243-249: Unlcear how you conducted the PCA, which are the input timeseries and which is the target.
L. 244: Please rephrase. What do you conclude from M+R1993?
Figure 4: It would be much clearer if you could use the same color bar scale for all maps - log10 Chl-a could be considered. Panel f: No difference visible between two red dashed lines (GOM, BS). Please consider using log10 Chl-a and all regions on one y-axis. "monthyl change rate" seems a bit odd here, could you transfer it to the main text?Section 3
L. 253-255: As you have only two closed sea areas, it seems a bit odd to generalize from the Bohai Sea only, as the Gulf of Mexico represents the contrary.
L. 254: "while the Gulf... shows"
L. 259: Consistently higher annual Chl-a concentration? variation?
L. 259: "the other regions"
L. 264: Remove whole line.
L. 272: Please check grammar.
L. 274: "While a slightly increasing tendency"
L. 276: "in the sea area"
L. 279: "continous increase"
L. 279: decline of the chl-a concentration in the East China Sea?
L. 279-280: I don't see where you indicated the "turn point of Bohai Sea" in Figure 5.
Figure 5: Spelling of "red dashed line", first", "a regressive trend line", "four columns to the right". Waht are the colors in the first column representing? Which color - red (blue) - refers to Chl-a (SST)? X-axis ticks missing. COuld you unify the x-axis for all panels? Please consider log10 Chl-a for all panels. Maybe you can specify: it's a linear regression line you added to the timeseries. What about their statistics/sgnificances?
L. 285: What "seasonal Chl-a" do you mean? concentration? It is not clear to me what you are refering to and what is different in the Gulf of Mexico.
L. 288: I don't get what you mean by " the Chl-a in
winter substitute with the summer Chl-a and becomes the second decisive factor".
Table 5: Check capitalization. Add a vertical line to separate open and enclosed sea areas.
L. 296-297: You mean "areas with increasing Chl-a trend"?
Figure 6: A legend is missing - do you use the same boundary norm for all panels?
Caption and panel labels do not match.
Why is the fraction of downward tendency in the Gulf of Mexico so high?
L. 308: " as illustrated in Figure 5d and 6d" - no! pie charts disagree!
L. 313: terrestrial nutrient input
L. 320: What are "significant rivers"?
L. 324-339: Please consider re-writing the two paragraphs and sort by open/enclosed. The first paragraph is quite chaotic, the second provides a bit more structure.
L. 325: What is an "obvious" trend?
L. 329: In which terms are the Chl-a stabilites "significant different"?
L. 330-331: Please rephrase "Chl-a shows lower stability of Chl-a are observed..."
L. 338: "whatever" - probably: "in both open and enclosed seas."?
Figure 7: Panel labels do not match caption.
L. 345: "spatio-temporal", please be consistent in the whole document, also L. 346.Section 4
Section 4.1: This is still presenting results, so shift to Results Section?
Figure 8: I can't distinguish "color-coded data density". Significance levels of panels to not match caption. Please use bigger font size. What do you gain from including this figure in the manuscript compared to a table reporting on the correlation statistics like Table 5?
L. 359: All panels suggest a negative correlation of Chl-a and SST, but not all are significant. I don't get why you highlight the open seas here.
L. 371-383: I suggest to reorder the two paragraphs by regions. Further, it is unlcear to me how you conducted the PCA (see comment to L. 243-249). How did you prepare the data? Did you use spatial or temporal averaging? What avout the two time periods specified in the Methods Section?
L. 380-383: Is there a reference to underline your thoughts?
L. 394: You probably want to say, that your three indices are valuable for predicting Chl-a concentrations? "determining Chl-a concentration" sounds misleading.
L. 395 studied
L. 397: What is "inhibition of Chl-a"?
L. 397 "induced by global warming"
L. 408: "heightened" - increased?
L. 423: The section heading is not really fitting. Here, you report results on SST and refrain from discussing global warming as such.
Figure 9: This is a complex figure, I would like to see it introduced in the Results Section.
Can you explain why the correlation coefficients vary so much in space? I would have assumed that major current patterns would be visible as strucutral element.
In the caption, you specify the years and differentiate between SST rising and stable periods. Did you evaluate more than one period for each panel? Maybe a cross-reference to Table 4/Figure 3 would be nice. There, you only specified one stable/rising period for each region, so I would assume this is also the underlying data here?
Please unify the y-axis in panel group (b), so percentages can be compared more easily.
L. 440-441: Why are open sea ares more affected by SST? The values stated for enclosed seas in L. 447-448 are comparable to the open seas.
L. 455/Section 4.2.3: This section is more about Chl-a timeseries than on anthropogenic nutrient inputs. Could you enrich the section with a discussion on nutrients and their impact on Chl-a, citing relevant literature?
L. 461: "In spring, there is"?
L. 470: "In autumn, there is"?
L. 475-476: reference?
L. 481: "since 4" - something is missing here.
L. 481: "a decreasing rate of Chl-a conc increase", probably easier to rephrase: "a slowdown of Chl-a increase"?
L. 481-484: references?L 548: "fluvial" only appears here, I suggested "terrestrial" (L. 313, but also elsewhere)
L. 560: Github repository: could you provide a commit ID to track which version you used? Some of the comments in the code is in Chinese, it would be great to translate it to English. It would be nice to have a bit more information in the Readme document on github.
L. 753: This is already Appendix A.
L. 838: Which validation steps explictly?
Appendix E: Check capitalization. Could you add the significance levels of the trends?
Appendix F: Check capitalizationTechnical comments
Please unify in whole document:
Tab. or Table? Fig. or Figure?
Check general use of comma, use of Oxford comma
Text alignment within tables (left?)
Consistent use of articles when refering to, e.g., "the Bohai Sea" (L. 278)
Please consider the use of "temporal variation rate" and "trend" (e.g., Figure 6). I suggest to stick to one term and use it throughout the mansucript. Switching back and forth leads to irritations on my side. In the text you even use "annual growth rate" additionally (e.g., L. 300).
Same for "increase"/"upward tendency"/"growth" - the wild mix of these terms confuses me when comparing the text to figures (e.g. L. 308-310 and caption Figure 6).
Tables: Capitalization of first word in each grid cellL. 41: add space after "carbon sink" before citation, also L. 45, 49, 53, 75, 80, 86, 127, 196, 239, 244, 337, 477
L. 60: please re-order citations
L. 66: two dots in "e..g."
L. 71: anthropogenically enhanced processes
L. 78: use em-dash
L. 96: describe (plural), or "the D&M section describes", also for introduction of Results, Discussion, and Conclusions sections
L. 105: five (as in the rest of the manuscript)
L. 133: Please unify in the manuscript: HABs events or HAB events?
L. 140: acts
L. 153: Consider use of Oxford comma, reorder citations for consistency
L. 158: Please unify in whole document: Tab. or Table?
Table 2: Please add date of last access of websites, consider using horizontal lines to separate rows
L. 176: "to the availability"
Figure 2: Please unify capitalization.
Table 3: "Significantly"
L. 214: "in five different locations"
L. 257: sea area
L. 371: Split sentences correctly.
L. 404: steadily
L. 405: However
L. 418: currents are
L. 424: Pearson
L. 487: surplus ")"
L. 562: Missing space after link. Please add last access to links.
L. 750: Overlapping
L. 829: dailyCitation: https://doi.org/10.5194/egusphere-2025-4558-RC2
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- 1
General Comments
This manuscript presents a comprehensive analysis of chlorophyll-a (Chl-a) trends in five marginal seas with varying hydrodynamic and anthropogenic characteristics, using satellite-derived data from 1998 to 2020. The study addresses a timely and important topic—the interplay between global warming and human activities in shaping coastal phytoplankton dynamics. The methodology is robust, combining trend analysis, stability assessment, and correlation/PCA to disentangle the effects of SST, PAR, and SWS on Chl-a. The findings highlight the contrasting responses of enclosed vs. open seas to warming, which is a valuable contribution to the field. The manuscript is generally well-structured and clearly written, though some sections could benefit from greater clarity and methodological detail.
Major Comments
1. Data Integration and Harmonization:
The use of multiple satellite sensors (SeaWiFS and MODIS) is appropriate for long-term trend analysis. However, the methodology for merging and harmonizing these datasets (Appendix A) is not sufficiently detailed in the main text. The authors should clarify how they addressed potential biases between sensors, especially for Chl-a and PAR, and justify the selection of specific months for correction.
2. SST Period Identification:
The identification of SST warming and stable periods using a 4-year moving window is innovative. However, the rationale for choosing this window length and its suitability for capturing climate-scale signals versus interannual variability (e.g., ENSO) should be more thoroughly explained.
3. Interpretation of Correlation vs. Causation:
While the study identifies correlations between Chl-a and environmental factors, the discussion often implies causation. The authors should more carefully distinguish between correlation and causation, especially when attributing Chl-a changes to global warming versus anthropogenic nutrient inputs.
4. Spatial Heterogeneity and Scale:
The study covers large and diverse regions, but some conclusions (e.g., about “open” vs. “enclosed” seas) may oversimplify intra-regional variability. More nuanced discussion of sub-regional processes (e.g., upwelling, river plumes) would strengthen the interpretation.
5. Anthropogenic Proxy:
Population density is used as a proxy for anthropogenic pressure, which is reasonable but indirect. The authors should acknowledge the limitations of this approach and consider discussing other potential indicators (e.g., fertilizer use, wastewater discharge) where data are available.
Specific Comments
1. Abstract:
The abstract clearly summarizes the main findings but could better highlight the novel aspects of the study, such as the contrasting responses of enclosed vs. open seas to SST rise.
2. Introduction:
Lines 65–70: The discussion of upwelling and its role in Chl-a dynamics is well-placed, but the transition to anthropogenic effects seems a bit abrupt and could be smoother.
3. Methods:
Section 2.2: Please specify how the missing data were handled, especially in coastal areas with high turbidity or cloud cover.
Section 2.3.1: The M-K trend test is appropriately applied, but the justification for using both M-K and least squares trend analysis should be briefly explained.
Results:
Figure 4: The spatial patterns are informative, but the color scales for Chl-a are not always intuitive. Consider using a consistent scale across panels for easier comparison.
Figure 9: The correlation maps are useful, but the classification of correlation strength (e.g., “high negative”) should be clearly defined in the caption or legend.
Discussion:
Lines 420–430: The interpretation of PAR in the Amazon Estuary is insightful, but the mechanism behind the weak SST-Chl-a relationship could be elaborated (e.g., barrier layer effects).
Lines 530–540: The conclusion that Chl-a in the Bohai Sea is less impacted by warming due to high nutrient levels is compelling but should be tempered with the acknowledgment of potential lag effects or nonlinear responses.
Conclusions:
The conclusions are well-supported but could be more forward-looking. For example, what are the implications for coastal management under future warming scenarios?
References:
The reference list is comprehensive and up-to-date, though a few recent key studies on marginal sea Chl-a trends (e.g., from the past 2–3 years) could be included.
Format adjustments：
Line 51: The references cited are divided into two parentheses. Please correct.
Line 75: a missing space between the end of the sentence and the cited references
Line 80: Similarly, see line 75
Line 86: Similarly, see line 75
Line 145: It is recommended that the complete form be placed on one page, and the title of the tableshould be placed in the center. (Similar situations will not be listed one by one)
Line 161: The title of the table should come before the table, also the complete form should be placed on one page.
Line 210: The title of the tableshould be placed in the center. (The following tables all have similar situations and will not list them one by one.)
There might be format errors that the reviewer have or omitted or not yet discovered. Please check carefully
Overall Recommendation:
This manuscript presents a valuable and well-executed study that advances our understanding of Chl-a dynamics in marginal seas under dual pressures of climate change and human activity. With minor to moderate revisions—particularly in methodological transparency and causal interpretation—it will be suitable and worthy for publication on Ocean Science.