Distribution of surfactants in the sea surface microlayer across a tropical estuarine system in Caribbean Colombia

The sea surface microlayer is the ocean–atmosphere interface (≤1000 μm thick), is ubiquitous on the ocean surface, and is characterized by the accumulation of surfactants that influence the exchange of climate-relevant gases, such as CO₂. This study on the sea surface microlayer in the Gulf of Urabá, Caribbean Sea, intended to determine the influence of fluvial input on the surfactant concentration distribution and the enrichment factor in a tropical estuarine system. For this purpose, samples were collected in the fluvial and marine zones of the Gulf of Urabá. A glass plate was used to sample the sea surface microlayer. No differences were found in the concentration of surfactants and their enrichment factor between zones. The sea surface microlayer was enriched in surfactants, whose concentration was significantly higher in the sea surface microlayer than in the underlying water. All stations had surfactant concentrations higher than 200 μg Teq L⁻¹, a value above which reductions of up to 23 % in the rate of ocean–atmosphere CO₂ transfer was found. About 55 % of the concentrations corresponded to the high regime of surfactants and 28 % to slick zones, and together with the enrichment factor, these were higher than those of other studies conducted in coastal and oceanic areas. Our results offer valuable insights into the ocean–atmosphere interface in tropical biogeochemical cycles and provide new information about the sea surface microlayer in tropical regions where such data are scarce.