Surfactant distribution can impact air-sea exchange in a Tropical Estuarine System in the Caribbean.

The sea surface microlayer (SML), the critical interface between the ocean and atmosphere (≤ 1000 μm thick), plays a vital role in regulating the exchange of climate-relevant gases, such as CO₂. This study provides the first evaluation of the SML in a tropical estuarine system, covering over 80 km of the Gulf of Urabá in Caribbean Colombia. It investigates the distribution and influence of surfactants, focusing on the effect of fluvial inputs during the rainy and dry seasons. Samples were collected from fluvial and marine zones, revealing no significant differences in surfactant concentrations or enrichment factors. However, surfactant concentrations were significantly higher during the rainy season (1011.63 ± 745.21 μg Teq L⁻¹, August 2021) than the dry season (428.34 ± 189.44 μg Teq L⁻¹, April 2022). Notably, all sampling stations exhibited surfactant concentrations exceeding 200 μg Teq L⁻¹, a threshold associated with reductions of up to 23% in the rate of ocean-atmosphere CO₂ transfer. Approximately 55% of the recorded concentrations represented a high surfactant regime, while 28% corresponded to slick zones. These values and enrichment factors were higher than those reported in other coastal and oceanic studies. Our findings underscore the significant role of surfactants in tropical biogeochemical cycles and provide valuable new insights into the SML in tropical regions where data is scarce. This research highlights the potential impact of surfactants on CO₂ exchange in coastal tropical environments, enhancing our understanding of the ocean-atmosphere interface in such regions.