| 07 Nov 2025
Wind-induced collapse of the biopolymeric surface microlayer induces sudden changes in sea surface roughness
Abstract. All exchange between the ocean and atmosphere has to cross the sea surface microlayer (SML), yet the SML impact on modulating air-sea exchange rates remains poorly understood. Surfactants, including biopolymers, can influence exchange rates by altering the rheological properties of the SML, damping surface turbulence, and capillary wave formations. We investigated the impact of wind speed on SML biopolymer enrichment, surface roughness and interfacial surfactant coverage at the Heidelberg ‘Aeolotron,’ a large annular wind-wave facility filled with 18.000 L seawater. Our results show that biopolymer enrichment, specifically the enrichment of polypeptides and polysaccharides, in the SML declined sharply at wind speeds above 6 m/s, coinciding with a sudden increase in the Mean Square Slope (MSS) of waves by 2–3 orders of magnitude. At wind speed <6m s-1, biopolymer enrichment in the SML reduced MSS values by up to two orders of magnitude compared to non-enriched or clean, i.e. freshwater, surfaces, indicating a substantial impact of biopolymers in the SML for air-sea exchange at lower wind speed. Selective SML enrichment was observed, particularly for the amino acids arginine and glutamic acid and the amino sugar galactosamine. Amino acid and carbohydrate monomers in the SML also exhibited significant and compound-specific wind-induced variability. Our findings suggest that biopolymers, particularly those derived from bacterial production accumulate in the SML act as powerful biosurfactants. Unlike artificial surfactant films, natural SML components were more susceptible to wind-induced disruption and to microbial production and decomposition. Our findings reveal that ecological processes actively regulate the chemical and physical properties of the SML, thereby potentially modulating air–sea heat and mass exchange.
Title: Wind-induced collapse of the biopolymeric surface microlayer induces sudden changes in sea surface roughness
Author(s): Anja Engel et al.
MS No.: egusphere-2025-5375
MS type: Research article
Special issue: Biogeochemical processes and Air–sea exchange in the Sea-Surface microlayer (BG/OS inter-journal SI)
General comments:
Thank you for presenting this interesting study on the collapse of the SML at wind speeds above 6 m/s. The writing style and the figures are generally clear. Some of the figures need a bit more work (see specific comments).
One could think about moving details in the sections 2.4.1-2.4.3 into the supplementary material.
The results are sufficient to support the conclusions.
Following an extensive discussion, the conclusion is very short. It could be expanded by adding a summary of the main points from the discussion, to highlight the substantial and novel conclusions from this work.
I recommend for publication after minor revisions.
Specific comments:
l. 3 “sea surface roughness“: As this term occurs in the title, it should be explained in a bit more detail in the text (eg. l. 365-366).
l. 27 “2–3 orders of magnitude“: This claim should also be formulated somewhere in the main text.
l. 35-36 “Our findings reveal that ecological processes actively regulate the chemical and physical properties of the SML, thereby potentially modulating air–sea heat and mass exchange.“ If you want to make this claim, I think you should discuss potential effects on the physical properties of the SML more.
l. 59/60 Do they really reduce turbulent energy dissipation or do they shift where the dissipation occurs?
l. 73/74 But the variability of surfactants in the SML is not the only reason for inaccuracies in wind-speed only based parameterizations (see e.g. Wanninkhof 2009). You probably do not want to state that, but your phrasing sounds a bit like it.
l.76-78 e.g. Pereira (2018) analysed water samples in an automated, closed air–water gas exchange tank. Was this effect also measured in the field (although challenging, as you mention later)?
l. 91 I think there are also some measurements reported using different Triton surfactant concentrations in Krall (2013).
l.114: Are there publications to cite here about the advantages of the Aelotron?
l.122-123: “The key mechanisms governing air-sea gas exchange“: What about wave breaking, stokes drift and Langmuir turbulence (see e.g. Belcher (2012))? I think I understand what you are trying to say, but perhaps you could phrase it more carefully.
l. 154-155: How accurate is this conversion? Can you estimate uncertainties, maybe based on Edson (2013)?
l. 325 Figure 2A: If I understand the color scheme correctly, GABA is listed in the legend, but not shown in the plot. If sc or c*/c_max cannot exceed 1, the scale should reflect it.
l. 361 Figure 3: Figure 3 and Figure 4 could be combined into one, or maybe Figure 3 could be dropped.
l. 373 Figure 4: Nice result. You should come up with a way to estimate uncertainties on MSS values or comment why you do not estimate such uncertainties.
l. 392-393 “likely due to enhanced mixing and rising of film-covered bubbles after wave-breaking (Figure 5I)“. Did you observe enhanced mixing or film-covered bubbles after wave-breaking?
l. 416 Figure 6: Why not choose a similar plotstyle in Figures 2 and 6 for sc and c*/c_max?
l. 454 Figure 6: The image quality seems to be low. Maybe use logarithmic axes to e.g. make the structures for EF_TCHO < 2 more visible.
l. 492-494 “and is referred to as the Marangoni effect (McKenna and Bock, 2006)“: This should be explained in more detail in the introduction.
l. 503 “wind-wave tank experiments with natural seawater and, hence, natural surfactants and surface films remain scarce.“ This sentence should be placed as close as possible to the sentences citing these other studies (eg. l. 511-513).
l. 611 I think a sentence similar to this one “However, the damping effect largely vanished at >6 m s-1 when the biopolymeric SML collapsed“ (l. 496-497) should make it into the conclusion, as it is a very nice main result.
Technical corrections:
l. 54. Dot missing.
l.100 18000\,L
l.112 1.4\,m. You should probably check all the physical units for correct spacing.
l.134: Do the black and white rectangles indicate no light source/light source? If so, they do not exactly match the time periods in the text.
l.173 U10 subscript missing.
l. 190-191 ULW sampling is not marked in Figure 1C.
l. 270 Dot missing.
l. 287 If it contains only the definition of the enrichment factor, there should not be an extra subchapter “2.5 Data Analysis“.
l. 306-307 This probably should not be two sentences.
l. 322 In, particular
l. 325 Figure 2: You should either use a,b or A,B.
l. 353 mu is missing, same for the following lines.
l. 403 Figure 5: Please increase the size of the axis tick labels, particularly those on the x-axis. Perhaps try rearranging the subplots in an upright orientation (e.g. 3x5 instead of 5x3). It looks like ULW are white rectangles (grey rectangles in the caption).
l. 423-424 According to the table, ISO has p<0.005.
l. 479 Different citation style.
l. 495 Space missing.