This manuscript presents a comprehensive field study on the biomolecular composition of the sea surface microlayer (SML) in the Central Baltic Sea, with a particular focus on the role of cyanobacteria and phytoplankton size structure. The dataset is rich and combines multiple complementary approaches, including microscopy, flow cytometry, detailed biomolecular analyses, and surfactant measurements. The study has clear potential to advance our understanding of how phytoplankton community structure imprints on SML chemistry and potentially affects air–sea exchange processes.

However, several issues need to be addressed before the manuscript can be considered for publication. Most importantly, the classification scheme based on HPA/LPA and phytoplankton size classes is conceptually confusing and, in its current form, hampers the interpretation of results and discussion. The conflation of size range and abundance category, as well as the use of cross-definitions (e.g. HPA>20 μm corresponding to LPA<20 μm), should be clarified or simplified, ideally by adopting more explicit ecological descriptors.

The manuscript contains numerous punctuation and formatting inconsistencies throughout the text, which should be carefully checked and corrected by the authors.

Overall, this is a potentially impactful contribution, but substantial revisions are required to improve conceptual clarity.

1. Specific scientific comment

Line100: The manuscript discusses surfactants, amino acids, and carbohydrates in the SML in relation to cyanobacteria. However, it remains unclear whether these compounds are produced directly by cyanobacteria or whether they primarily result from bacterial processing and degradation of cyanobacterial biomass and exudates. The relative roles of cyanobacteria versus heterotrophic bacteria in shaping the SML biomolecular composition require clearer clarification.

Line103-105: The SML was sampled using a glass plate or Garrett Screen, whereas the ULW was sampled using nets and/or discrete water samples at ~1 m depth. Given the use of different sampling methods, it is unclear to what extent the observed molecular differences between the SML and ULW may be influenced by methodological biases, particularly with respect to particulate versus dissolved fractions. In addition, Table A1 suggests that ULW samples were collected using a Niskin bottle, which raises some ambiguity as to whether both net- and bottle-based approaches were used for ULW sampling. The authors should clarify the sampling strategy and discuss potential methodological effects on the SML–ULW comparison.

In Section 2.2, net samples containing the >20 μm particulate fraction were diluted with filtered seawater at ratios ranging from 2:1 to 1:2 prior to subsampling. It is unclear whether this dilution procedure may have altered the structure of colloids or extracellular polymeric substances (EPS), potentially affecting aggregation state and surface activity measurements. The authors should briefly discuss whether dilution could influence EPS integrity and surfactant analyses.

In the microphytoplankton microscopy analysis, community composition and biomass estimates were restricted to the eight most dominant species. While these taxa likely dominate in abundance, it remains unclear whether less abundant species with potentially large cell volumes may still contribute disproportionately to biomass, EPS production, or surface-active compounds.

In Section 2.7, the manuscript states that triplicate 18 mL SML and ULW samples were prepared, but only one of the three replicate samples was analyzed. It is unclear why only a single replicate was measured and what the purpose of the remaining replicates was.

In Section 3.1, the classification of phytoplankton abundance is confusing. The low-abundance category of phytoplankton >20 μm is labeled as “LPA<20 μm”, which is identical to the low-abundance category of phytoplankton <20 μm. This notation conflates size class and abundance level and may mislead readers to interpret the low-abundance >20 μm group as belonging to a different size fraction. Low-abundance categories should retain the same size designation (i.e. >20 μm) and differ only in abundance. In addition, Aphanizomenon sp. is listed twice in the species list. Finally, the notation “300 x 10³ cells mL⁻¹” does not follow standard scientific formatting and should be written as “300 × 10³”.

In the second half of this section, the logic of the HPA/LPA classification becomes increasingly difficult to follow and begins to affect the interpretation of the results. Statements such as “under HPA>20 μm (which corresponds mostly with LPA<20 μm conditions)” and “LPA>20 μm conditions (which corresponds to HPA<20 μm conditions)” introduce cross-definitions that obscure which ecological states are actually being compared.

In Section 3.2, there is an inconsistency between the statistical significance and the descriptive language used to interpret the results. For TAA, terms such as “tendency,” “slightly higher,” and “more pronounced” are repeatedly used, while the corresponding statistical tests are either not significant (e.g. p = 0.076) or not explicitly reported.

In Section 4.2, the manuscript repeatedly emphasizes that enrichment factors (EFs) are close to 1, while at the same time using relatively strong wording such as “significantly higher,” “substantially higher,” or “highest average EF” to describe differences. For example, the mean TAA EF of 1.2 ± 0.4 largely overlaps with the previously reported range of 0.8–1.2, raising the question of whether this truly represents enhanced enrichment in a statistical or process-based sense. It may be more appropriate to describe these values as being at the upper end of the historical range or as consistent with, but slightly elevated relative to, previous studies, rather than implying a clearly enhanced enrichment.

4.3 Section, In the discussion of POC composition, the authors state that cellular biomass accounts for only ~13.5% of POC>20 μm, with the remainder attributed to heterotrophic plankton, detritus, and extracellular material. This interpretation is conceptually sound and well supported by the literature. However, the subsequent statement that “POC>20 μm was related to cyanobacterial cellular biomass >20 μm (HPA>20 μm: 1.6 ± 1.7; LPA>20 μm: 0.7 ± 0.7%)” is unclear. It is not evident whether these percentages refer to correlation strength, explained variance, or relative contribution to POC. The authors should clarify the meaning of these values and how they were derived. In addition, “Synecococcus sp.” appears to be a typographical error and should be corrected to “Synechococcus sp.”.

1. Other comments and technical corrections

Line19: Please define the abbreviation “HPA” at its first occurrence.

Line36: Please add a space after the citation “(Engel et al., 2017)”.

Line53: The phrase “A large-scale oceanic in the Atlantic demonstrated” appears incomplete. A noun such as “study”, “survey”, or “investigation” seems to be missing.

Line 121: In “>20μm”, a space is missing before “μm”. Please check and correct similar formatting issues throughout the manuscript.

Line 123: “Garret” should be corrected to “Garrett”.

Line 130: The units “knt” and “knots” are used interchangeably. Please standardize the unit notation.

Line 131: In “~14min”, please add a space before “min”.

Line 146: “within ≤2 h” is recommended to be revised to “within 2 h”.

Line 153: “Bottle” should be “bottle”.

Line 185: Please provide a reference for the chlorophyll a measurement method.

Line 215: “sample” should be corrected to “samples”.

Line 223-224: Thirteen amino acids are stated, but only twelve are listed. In addition, please revise the punctuation in the references according to journal style.

Line 225: In “(Dittmar et al., 2009.; Engel & Händel, 2011)”, there is an extra punctuation mark after “2009”.

Line 231: “Sample were measured” should be corrected to “Samples were measured”.

Line 236: “the samples ionic strength was standardized” should be revised to “the sample’s ionic strength was standardized”.

Line 327, In the sentence “Cylindrotheca closterium and Chaetoceros sp. occurred at HPA>20 μm only with < 0.2 μg C L⁻¹)”, there is an extra closing parenthesis.

Line 407: The term “later amino acids” is incorrect and should be replaced with “latter amino acids”.

Line 419: “glucosamin” is missing the final “e” and should be corrected to “glucosamine”.

Line 435: The phrase “excluded … due to their low and abundance” is incomplete and should read “low abundance”.

Line 698: “Moring” should be corrected to “Morning”.

1) General assessment

The manuscript presents a detailed investigation of biochemical characteristics of the sea surface microlayer in the central Baltic Sea, with a particular focus on potential links to cyanobacterial blooms. The study addresses an important and understudied interface in marine biogeochemistry, and the authors do a good job of placing their results within the context of existing literature. However, aspects of the manuscript would benefit from improved clarity and clearer framing. Additionally, some conclusions appear to be stated more strongly than is supported by the presented data, and certain interpretations would benefit from additional clarification. In several sections, especially in the Discussion, more explicit references to figures, supplemental material, and the specific datasets being discussed would substantially improve readability and help guide the reader through the results. These issues appear readily addressable through focused revisions of the text and its organization.

2) Comments on the Abstract

The abstract generally reads well. However, unlike other acronyms, POC is not defined and should be introduced explicitly.

Regarding the sentence in the abstract: “While SML enrichment patterns and carbohydrate concentrations were comparable to those previously reported for the Western Baltic Sea, concentrations of total amino acids (TAA) and surfactants were substantially higher, suggesting enhanced production by cyanobacteria.” The sentence reads well stylistically, but the concluding phrase “suggesting enhanced production by cyanobacteria” appears too early in the abstract. At this point, the abstract has not yet established the mechanistic/observational links between surfactants and cyanobacterial presence. The following sentences provide information that would support this inference, but as currently written this conclusion seems insufficiently justified at this stage. Consider either softening this statement or restructuring the abstract so that the supporting context precedes this statement.

3) Comments on the Body of the Manuscript (substantive)

General comment: When trends or results from this study are discussed in the Discussion section, it would be very helpful to consistently reference the corresponding figures or supplemental material. The discussion frequently refers to data generated here, but it is not always clear where the reader can quickly locate those results. Aside from this, the manuscript does an excellent job of engaging with and referencing related literature.

Lines 84–86: “Biochemical studies in the Baltic Sea have shown that the SML is enriched in amino compounds and carbohydrates relative to the underlying water (Van Pinxteren et al., 2012). In mesocosm experiments, proteinaceous gels (CSP) abundance was five times higher than that of carbohydrate-rich gels in the SML (Galgani and Engel, 2013).”

• The first sentence compares enrichment between the SML and the underlying water, whereas the second sentence compares proteinaceous versus carbohydrate-rich material within the SML. Since the first sentence already establishes that both compound classes are enriched in the SML, it would be helpful if the second sentence either (i) explicitly stated how CSP abundance compared between the SML and underlying water (if available), or (ii) provided clearer justification for why the relative abundance of proteins versus carbohydrates is important in this context. For example, how does this comparison advance understanding of phytoplankton bloom dynamics or surfactant production?
• More broadly, the paragraph spanning lines 84–98 contains interesting and relevant material, but the narrative does not clearly guide the reader toward a central takeaway. The key point appears to be articulated later in line 100 (“cyanobacteria represent major producers of extracellular organic matter”). If this is indeed the main message, consider introducing this idea earlier in the paragraph to better frame the supporting evidence.

Lines 97 – 98: “Seasonal records further indicate that surfactant concentrations peak during summer months following the spring bloom and reflecting intensified solar radiation (Laß et al., 2013).”

• A slightly weak statement to have as the final sentence of a paragraph. Also is not entirely clear what the implication of intensified solar radiation is here. For example, is this meant to link to climate warming, enhanced biological activity, or altered near-surface physical processes as mentioned in the previous sentence (“The release of such compounds can result in visible surface slicks, which locally warm the upper sub-millimeter ocean skin and modify nearsurface physical processes”)? Clarifying this connection and providing a stronger closing sentence would improve the paragraph’s coherence.

Lines 266–268: “Accordingly, phytoplankton >20μm abundance was grouped into high (HPA>20μm) and low (LPA<20μm) categories (Fig. 2b), and phytoplankton <20μm abundance into high (HPA<20μm) and low (LPA<20μm) categories (Fig. 2c). A summary of this classification is provided in Table A2.”

• There may be an inconsistency here: should the first low category be LPA>20μm rather than LPA<20μm?
• More generally, I found the rationale for distinguishing between HPA and LPA categories difficult to follow. If stations with ULW concentrations above the median are classified as “high” and those below as “low,” it seems that each station would belong to only one category (either HPA or LPA). The purpose of this classification, and how it captures variability across stations, would benefit from clearer explanation.

Line 345: “Despite the increase, EFs were close to 1 across stations (1.1 ± 0.1).”

• This sentence refers to enrichment in TOC levels. It would be helpful to try and explain why TOC concentrations differ significantly while enrichment factors do not.

Lines 354–357: “During HPA<20μm conditions, surfactant concentrations (Fig. 5c) were elevated in the SML (0.56 ± 0.09 mg L⁻¹ TX-100 equiv.) relative to the ULW (0.52 ± 0.14 mg L⁻¹ TX-100 equiv.), corresponding to an EF of 1.1 ± 0.2 (median = 1.1). In contrast, surfactant EFs were lower under LPA<20μm conditions (1.1 ± 0.2; median = 1.0).”

• This comparison is somewhat confusing, as the mean EF values are identical and the median EFs are very similar. The concentrations are also not statistically different between the two groups, making the use of terms like “elevated” and “lower” a bit of an overstatement.

Line 505: “Our data showed that Chl a concentration increased over time”

• This statement seems potentially inaccurate. As I understand it, concentrations were measured at different stations, with each station sampled only once during the cruise (with two stations sampled daily). If I am understanding the sample collection methodology correctly, then these concentrations of ~1.63 μg L⁻¹ and ~2.47 μg L⁻¹ would represent different sampling locations, correct? Because the current statement reads as though Chl-a levels at a set location/ station were increasing over time during the duration of the cruise.
• If I am misinterpreting the sample collection methodology, please add some clarifying information on how many times the stations were visited and at what time.

this seems inaccurate since you were measuring concentrations in different stations, correct? Each station was only measured once, with two stations measured every day? If not, then where is the temporal data shown, since this statement does not contain that information. And if it is temporal data then during what time period did the concentrations measure 1.63 ug/L and when did they measure ~2.47 ug/L?

Lines 538–539: “TAA concentrations in the SML peaked in autumn with a maximum EF of 2.4 (Dreshchinskii and Engel, 2017), which is comparable to the highest EF reported in this study”

• This highest EF appears to be shown in the supplemental material (likely Fig. S7 based on line 362). Please clarify where and when this highest EF was observed in the present study and provide the corresponding value.

Lines 547–548: “The enrichment of surfactants in the SML differed in dependence of phytoplankton abundance, as the EF during HPA<20μm (1.1 ± 0.2; median = 1.1) exceeded the EF during LPA<20μm (1.1 ± 0.2; median = 1.0).”

• This statement is awkward given how similar the EF values are. It is unclear whether these differences are statistically or mechanistically meaningful, and this should be addressed explicitly.

Lines 629–630: “Data separated only into the defined categories of HPA and LPA<20μm along PC2, which explained considerably less variance of the TAA data”

It is not clear to me what new information this sentence is intended to convey. Additionally, while the amino acids associated with PC1 are listed (line 625), a similar description is not provided for PC2. Clarifying this would improve interpretability.

4) Other Minor / Editorial Comments that were noticed

Line 53: “A large-scale oceanic in the Atlantic demonstrated…” This sentence appears to be missing a word (e.g., “study” or similar), which disrupts readability.

Lines 352–354: “When samples were pooled across categories (i.e., not distinguished by HPA/LPA), TAA concentrations differed significantly between depths….resulting in an average EF of 1.2 ± 0.4 (median = 1.1).”

• It is unclear whether this statement refers to >20μm or <20μm conditions. Based on the preceding sentence, it appears to refer to <20μm, but clarification would be helpful.

Line 406: GABA could be redefined here or in the Figure 7 caption/legend, as it was last mentioned much earlier in the introduction.

Line 470: Instead of stating that correlations “appeared,” a more definitive term such as “were observed” may be more appropriate.

Line 573: “summerly” does not appear to be a standard usage word.

Line 590: “Central Baltic Sea and in spite the absence of a bloom.” This sentence appears to have an extra “and” which is causing some confusing grammar.

Figures:

Figure 5 caption: Regarding the statement “surfactants (c) and total combined carbohydrates (TCCHO) (d) were differentiated for high and low phytoplankton <20μm abundance (HPA<20μm and LPA<20μm).”

• On initial reading this caption was confusing because the lowercase “s” made it appear to continue the previous sentence. This also led to confusion because the x-axis (at least on the left side of the figure) refers to >20μm categories. Consider leading with a clearer framing, for example: “High and low phytoplankton <20μm abundance groups (HPA<20μm and LPA<20μm) were compared for differences in surfactant concentrations (c) and total combined carbohydrates (TCCHO) (d).”