Abstract
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.
Highlights
The sea surface microlayer (SML) is defined as the uppermost boundary layer of the ocean and is enriched with a complex pool of surface-active substances, including carbohydrates, proteins, and lipids[1]
Even though we observed slightly lower enrichments with increasing wind speed and solar radiation, sudden changes in the concentration in the underlying water (ULW) can be sufficiently high to control the enrichment of fluorescent dissolved organic matter (FDOM) in the SML
Researchers have stated that the ULW is a major source of organic material for the SML15
Summary
The sea surface microlayer (SML) is defined as the uppermost boundary layer of the ocean and is enriched with a complex pool of surface-active substances, including carbohydrates, proteins, and lipids[1]. The effects of the SML on climate change and the marine food web are based on the enrichment of organic material in the SML. For this reason, it is crucial to understand the processes leading to enrichment, which is defined as ratios of concentrations or abundances between the SML and the underlying water (ULW). By changes in concentrations in the SML, or to what extent and which processes in the ULW may be involved For this purpose, we made observations from a state-of-the-art research catamaran and high-resolution in situ measurements of the fluorescent dissolved organic matter (FDOM), temperature, and salinity in the SML and the ULW, as well as meteorological parameters
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