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 formation. 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−1, coinciding with a sudden increase in the Mean Square Slope (MSS) of waves by 1–2 orders of magnitude. At wind speed < 6 m s−1, biopolymer enrichment in the SML was accompanied by high surfactant surface coverage and strongly reduced MSS values compared to non-enriched or essentially surfactant-free clean freshwater surfaces, indicating a substantial impact of biopolymer enrichment 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 and 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, including surfactant surface coverage, and thereby potentially modulate air–sea heat and mass exchange.