Selective accumulation of dissolved organic matter in the sea surface microlayer: integrated multi-spectral CDOM and FDOM characterization and a novel FDOM/CDOM index at a Mediterranean coastal site

The aims of this work are: (i) To understand potential transformations and dynamics within the broad chromophoric dissolved organic matter (CDOM) pool in the sea surface microlayer (SML) and the underlying water (ULW) at a Mediterranean coastal site using high-resolution (biweekly, one-year) temporal data and by applying established absorbance indices across multiple spectral regions along with the novel fluorescent dissolved organic matter (FDOM) to CDOM, FDOM/CDOM index. (ii) To evaluate the relative contributions of in situ processes, upward flux from the ULW, and atmospheric (rainwater) inputs in the shaping of the dissolved organic matter (DOM) and CDOM pools in the SML. 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 indicating a preferential accumulation of high molecular, aromatic absorbing DOM. Relationships between DOC, a300 and the spectral slope (S275−295) indicated that specific processes in the SML modulate the abundance and optical quality of DOM beyond bulk DOC quantity. Photodegradation was apparent 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). However, humic-like and tryptophan-like components exhibited SML enrichment linked to both a300 absorption and layer, while tyrosine – like component was enriched independently of layer effects and mainly reflected a300 fluctuations. By introducing a new 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 excitation regions (A, B, T peaks) but lower fluorescence in the UV-A/near-visible excitation region (C peak), suggesting selective accumulation of absorbing but non-fluorescent CDOM in longer wavelengths. 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 wavelength-dependent enrichment of CDOM and FDOM in the SML is primarily driven by photodegradation, biological activity, rapid molecular reorganization, and atmospheric deposition, rather than upward DOM flux from the ULW.