High-resolution observations on enrichment processes in the sea-surface microlayer

For decades, researchers assumed that enrichment of dissolved organic matter (DOM) in the sea surface microlayer (SML) is solely controlled by changes in the DOM concentration at this uppermost thin boundary layer between the ocean and the atmosphere. We conducted high-resolution observations of fluorescent-DOM (FDOM) at 13 stations in the coastal and open Atlantic Ocean to understand the enrichment processes. Results show that FDOM enrichment in the SML varied between 0.8 and 2.0 (in comparison to the concentrations in the underlying water; ULW), and FDOM enrichment is a common feature of the SML despite the varied distances to the terrestrial sources. At six stations, the FDOM concentration in the SML was less variable over the sampling period (>5 h) compared to FDOM concentrations in the ULW characterized with sudden changes. Even so we observed slightly lower enrichments with increasing wind speeds and solar radiation, changes in ULW concentrations forced the enrichment to change. In addition, we found evidences for the occurrence of photochemical degradation of FDOM in near-shore SML with implications on coastal carbon cycling. Overall, the results show that the processes leading to the enrichment of DOM in the SML are more complex than previously assumed. Given the importance of the organic-rich SML as a diffusion layer in the air-sea exchange of climate-relevant gases and heat, understanding the layer's enrichment processes is crucial.