the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Aug 2025
Meta-analytical insights into organic matter enrichment in the surface microlayer
Abstract. The surface microlayer (SML), the uppermost ~1 mm water layer at the air-water interface, plays a critical role in mediating Earth system processes, yet current knowledge of its composition and organic matter enrichment remains scattered across disciplines. Here, we present the first known meta-analysis of SML studies that quantitatively assesses the distributional characteristics of selected organic compounds, including organic carbon and nitrogen, amino acids, fatty acids, transparent exopolymer particles, carbohydrates, lipids and proteins, through probability density estimates, central tendency metrics and correlations analyses. Our results confirm a preferential enrichment of nitrogen-enriched, particulate organic matter in the SML, highlighting the significance of compound-specific accumulation and selective enrichment patterns. We also observe that the enrichment of a given compound may exhibit notable variability that depends on distinct internal and external conditions. Our evaluation of enrichment factors (EFs) of various measurable compounds provides updated estimates for their typical values and ranges. While delving into the ability of EFs to reflect the partitioning of organic matter within the SML, we also critically examine their limitation in capturing trophic conditions. Based on these findings, we propose that future SML research should incorporate both absolute concentration changes and enrichment capacities in the SML, alongside their relative changes (as denoted by EFs), to more accurately interpret ecological implications. Additionally, our meta-analysis demonstrates the value of logarithmic data transformations and robust central tendency estimates, as essential tools for improving the statistical reliability, comparability, and representation of SML enrichment patterns.
Competing interests: Authors A. S., T. B., A. E. and M. S. are affiliated with the same institution as H. B., who serves as an overseeing editor for the special issue “Biogeochemical processes and air-sea exchange in the sea-surface microlayer”. Authors A. S., T. B., A. E., H. H., M. P., O. W. and M. S. are collaborators with H. B. on an ongoing research project. These potential competing interests have been fully disclosed to the journal. The authors declare no other competing interests relevant to the submitted work.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-4050', Anonymous Referee #1, 24 Oct 2025
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RC2: 'Comment on egusphere-2025-4050', Anonymous Referee #2, 02 Dec 2025
Manuscript: egusphere-2025-4050
General Comments
This manuscript presents a novel and comprehensive meta-analysis of organic matter enrichment in the sea surface microlayer (SML). The study synthesizes a large dataset (2055 data points from 30 publications) to provide a statistically robust, cross-compound assessment of enrichment factors (EFs). The manuscript is generally well-structured, the methodology and the statistical treatment (e.g., boot-strapped KDE, log-transformation) is technically sound. The findings on preferential enrichment of nitrogen-enriched and particulate OM, alongside insights into EF limitations, are valuable for advancing SML biogeochemistry and climate-related models.
However, while the study effectively describes SML enrichment patterns (e.g., N-rich > C-rich, particulate > dissolved), it lacks a deeper mechanistic explanation for their existences. The discussion primarily correlates findings with previous studies but does not fully leverage the power of the meta-dataset to synthesize and propose unified physicochemical or biological mechanisms governing the observed selective enrichment. A more in-depth exploration of the underlying drivers (e.g., molecular hydrophobicity, particle buoyancy and aggregation, microbial biofilm formation, photochemical processing) would significantly enhance the conceptual contribution of this work.
In addition to the major issues mentioned above, moderate revisions are needed to enhance clarity, statistical rigor, and contextualization of results.
- The methodology for the literature search and study selection is briefly mentioned. A more detailed description (e.g., search databases, keywords, inclusion/exclusion criteria) would strengthen the reproducibility and rigor of this meta-analysis. The provided Supplementary Table S1 is crucial, but its description in the main text is minimal.
- The discussion is thorough in comparing results with past work, but the connection between the observed enrichment patterns and the underlying physicochemical or biological mechanisms should be more explicitly developed. The manuscript would benefit from a dedicated paragraph that synthesizes potential mechanisms for the dominant patterns (N-rich, particulate enrichment) revealed by the meta-analysis.
- The manuscript states that "fixed optimal bandwidth was applied" for log KDEs but does not specify the value or justification. Similarly, the 67% subsampling rate for bootstrap resampling lacks rationale—was this based on sample size distribution or statistical convention? These details are critical for reproducibility.
Specific Comments:
Page 2, Line 31: The definition of SML thickness as "1-1000 μm" is very broad. It would be helpful to briefly mention that this range reflects different operational definitions and sampling techniques. Consider adding: "...which has an operationally defined thickness typically ranging from 1-1000 μm, depending on the sampling method used (e.g., screen, drum, plate)."
Page 3, Line 87: The definition of EF is clear. However, the sentence "This equation proposes that..." is slightly awkward.
Page 5, Line 131-132: The use of PlotDigitizer and GraphClick is noted. It would be good practice to state the estimated error or uncertainty associated with digitizing data from figures, or to mention that data points were cross-checked for accuracy where possible.
Page 6, Line 156: The bootstrap resampling approach using 67% of the data as a solution for small sample sizes. However, the choice of 67% is not justified.
Page 13, Line 270: “FA … fail to display conspicuous consistent trends” – awkward; rephrase to “FA exhibit no clear trend”.
Page 16, Line 353-365: The analysis of factor-specific variability (e.g., location, season, method) is a highlight of the discussion. This section could be strengthened by more explicitly stating the main takeaway from each panel in Fig. 8. For example, for panel (b), what is the key difference in DOC enrichment between oceans, coasts, estuaries, and freshwater?
Page 20, Line 441-447: The terms "trophic conditions," "trophic status," and "ecological setting" are used frequently in the discussion of EF limitations. However, the manuscript does not explicitly define what is meant by these terms in this context.
Page 25, Line 552-554: The statement that "computational codes... are available upon request" is no longer considered best practice in scientific publishing. To ensure full transparency and reproducibility, the authors should deposit the code in a permanent, publicly accessible repository.
Citation: https://doi.org/10.5194/egusphere-2025-4050-RC2
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I have reviewed this manuscript as a non-expert in meta-analysis; therefore, I am not in a position to fully assess the methodological rigor or potential limitations associated with the meta-analytical approach and related statistical procedures. My comments should therefore be considered primarily from the standpoint of subject-matter expertise rather than methodological specialization.
General Assessment:
This study presents a meta-analysis-based approach to previously published sea surface microlayer (SML) research, aiming to quantitatively assess the distributional properties of selected organic carbon and nitrogen compounds. Overall, this manuscript presents a valuable effort to integrate existing SML data, with potential relevance for future modeling studies aimed at elucidating the role of the SML in biogeochemical cycles and climate-related processes. Overall, the manuscript aligns well with the interdisciplinary aims of Biogeosciences, providing findings and perspectives that would likely be of interest to the diverse journal's readership.
The results indicate that nitrogen-rich compounds and particulate organic matter are generally more enriched in the SML compared to carbon-rich compounds and dissolved organic matter. The authors further demonstrate that enrichment levels for individual compounds vary under different internal and external conditions. By examining enrichment factors (EFs) for a range of measurable compounds, the study provides updated estimates of their typical values and ranges. The discussion also offers a critical evaluation of EFs, underscoring their utility in describing the partitioning of organic matter within the SML while acknowledging their limitations in reflecting trophic conditions and absolute concentrations of either SML or ULW (“actual” enrichment).
However, despite the application of a novel data-processing approach, the study does not appear to yield substantially new findings beyond those already established in previous research. Overall, the results largely confirm existing knowledge regarding SML properties and organic matter enrichment patterns, without providing significant new insights. Therefore, I suggest that the authors revise the concluding statements throughout the manuscript, including Abstract, to more clearly reflect the study’s contributions and its relationship to existing knowledge. This would help to strengthen the manuscript’s impact and clarify its significance for the field.
Introduction
In the introduction, the authors provide a thorough overview of the specific characteristics of the sea surface microlayer (SML) and the general tendency for organic matter to accumulate within this layer. The focus on surfactants as a key organic fraction—both as components forming surface films and as important contributors to gas exchange—is appropriate and well justified. However, the introduction does not provide sufficient context regarding previous research on the surface activity, selective transport, and enrichment of specific organic compounds within the SML, which are later discussed in the manuscript. Additionally, the authors do not address prior findings on the contribution of these compounds’ surface activity to the overall SML surfactant pool, despite the strong emphasis on surfactants in the introduction. Consequently, it is unclear why certain compounds or organic fractions were selected for study, and why surfactants themselves—a highly relevant and previously measured organic fraction—were not included among the investigated compounds. Clarification of these points would strengthen the rationale for the study and better situate it within the context of existing literature.
Methodology
The authors should clearly define their search strategy, including the databases consulted, as well as the inclusion criteria, such as the specific keywords used, to ensure transparency and reproducibility of the study.
The manuscript mentions extracting “secondary data” (e.g., environmental variables, sampling factors) when reported. This implies that metadata coverage is inconsistent across the dataset. I recommend quantifying the proportion of records with complete metadata and discussing potential selection bias in analyses that rely on the subset of data with full metadata coverage.
Conclusion
L491-494 Considering previous work (as discussed by authors L304-L309), the concluding statements should be revised to explicitly emphasize the unique contributions and novel findings provided by this study in comparison to earlier work.