Selective accumulation of dissolved organic matter in the sea surface microlayer: Insights from CDOM and FDOM characterisation at a Mediterranean coastal site

Abstract. The sea surface microlayer (SML) is an extremely thin (~1 mm) boundary between the ocean and the atmosphere, forming a dynamic microenvironment that regulates air–sea gas exchange. Owing to its unique position, the SML is enriched with surface-active and hydrophobic organic compounds, typically more concentrated than in the underlying water (ULW), and can therefore modulate gas transfer across the interface. Despite its importance in ocean–atmosphere interactions, the processes governing its functioning remain insufficiently understood. This study investigates the biogeochemical coupling between the SML and ULW by examining the dynamics of dissolved organic matter (DOM)—including its chromophoric (CDOM) and fluorescent (FDOM) fractions—at a coastal Mediterranean site from 2016 to 2017. Twenty-two paired SML–ULW samples and fourteen rainwater samples were analyzed for dissolved organic carbon (DOC), UV–visible (250–700 nm) absorption spectra, and 3D fluorescence excitation emission matrices (EEMs). The SML was consistently enriched in DOC, CDOM, and FDOM relative to the ULW throughout the study. Enrichment factors (EFs) for long-wavelength absorption coefficients (a₃₀₀, a₃₇₀) exceeded 5. Relationships between a₃₀₀ and the spectral slope (S_275-295) indicated photodegradation in both layers, though more pronounced in the ULW. In the SML, photodegradation effects appeared to be partially counterbalanced by in situ production or aggregation of hydrophobic, optically active, higher-molecular-weight material. Parallel Factor Analysis (PARAFAC) identified four FDOM components: two humic-like (A/C, A/M) and two protein-like (T, B). Terrestrial humic-like (A/C) and tryptophan-like (T) fluorophores were dominant and strongly enriched in the SML (EF > 4). By introducing an FDOM/CDOM index, a decoupling between fluorescent and non-fluorescent chromophoric organic fractions was revealed: the SML exhibited higher fluorescence in the UV-C/UV-B regions (A, B, T peaks) but lower fluorescence in the UV-A/visible region (C peak), suggesting selective accumulation of absorbing but non-fluorescent CDOM. Potential drivers of this decoupling include biological transformations, rapid microlayer reorganization, and atmospheric inputs. Rainwater showed DOC concentrations and absorption features comparable to the SML but distinct fluorescence characteristics. PARAFAC modeling of rainwater did not resolve the tryptophan-like fluorophore and revealed blue-shifted humic-like components, consistent with photochemically aged, low molecular weight DOM of mixed marine–terrestrial origin. Overall, the results indicate that photodegradation, biological activity, rapid molecular reorganization, and atmospheric deposition collectively shape the wavelength-dependent enrichment of CDOM and FDOM in the SML, highlighting its active role in air–sea biogeochemical exchange.