The organic sea-surface microlayer in the upwelling region off the coast of Peru and potential implications for air–sea exchange processes

Abstract

Abstract. The sea-surface microlayer (SML) is at the uppermost surface of the ocean, linking the hydrosphere with the atmosphere. The presence and enrichment of organic compounds in the SML have been suggested to influence airsea gas exchange processes as well as the emission of primary organic aerosols. Here, we report on organic matter components collected from an approximately 50 µm thick SML and from the underlying water (ULW), ∼ 20 cm below the SML, in December 2012 during the SOPRAN ME-TEOR 91 cruise to the highly productive, coastal upwelling regime off the coast of Peru. Samples were collected at 37 stations including coastal upwelling sites and off-shore stations with less organic matter and were analyzed for total and dissolved high molecular weight (> 1 kDa) combined carbohydrates (TCCHO, DCCHO), free amino acids (FAA), total and dissolved hydrolyzable amino acids (THAA, DHAA), transparent exopolymer particles (TEP), Coomassie stainable particles (CSPs), total and dissolved organic carbon (TOC, DOC), total and dissolved nitrogen (TN, TDN), as well as bacterial and phytoplankton abundance. Our results showed a close coupling between organic matter concentrations in the water column and in the SML for almost all components except for FAA and DHAA that showed highest enrichment in the SML on average. Accumulation of gel particles (i.e., TEP and CSP) in the SML differed spatially. While CSP abundance in the SML was not related to wind speed, TEP abundance decreased with wind speed, leading to a depletion of TEP in the SML at about 5 m s −1 . Our study provides insight to the physical and biological control of organic matter enrichment in the SML, and discusses the potential role of organic matter in the SML for air-sea exchange processes.

Effect of wind speed on the size distribution of gel particles in the sea surface microlayer: insights from a wind–wave channel experiment

“…In addition, after adding the E. huxleyi seed culture, no influence of wind speed on slopes of CSP SML (2-16 µm) was detected. These results indicated that the influence of wind speed on size distribution of gel particles may be more pronounced for TEP SML than for CSP SML (Prieto et al, 2002;Engel and Galgani, 2016).…”

“…Also, proteinaceous gels and amino acids can be enriched in the SML and in sea spray aerosols (SSAs) (Kuznetsova et al, 2005). Since gel particles with fractal scaling provide a relatively large surface-to-volume ratio, they are assumed to act as barriers at the interface between air and sea, potentially reducing molecular diffusion rates (Engel and Galgani, 2016). Thus, the enrichment of organic matter, including gels, in the SML could modulate the air-sea gas exchange at low and interPublished by Copernicus Publications on behalf of the European Geosciences Union.…”

“…One explanation for this is that at higher wind speed, aggregation of solid particles with TEP results in aggregates becoming negatively buoyant and sinking out of the SML. For proteinaceous gels, Engel and Galgani (2016) observed that their enrichment was not inversely related to wind speed. However, an inverse relationship between the slope of the CSP size distribution in the SML and wind speed was observed, indicating larger CSP in the SML at low wind speed.…”

“…C.-C. Sun et al: Variation in gel particles in the SML as a function of wind speed mediate winds (Calleja et al, 2009;Mesarchaki et al, 2015;Wurl et al, 2016;Engel and Galgani, 2016).…”

“…TEP enrichment in the SML has been described as inversely related to wind speed greater than 5-6 m s −1 (Wurl et al, 2009(Wurl et al, , 2011Engel and Galgani, 2016). One explanation for this is that at higher wind speed, aggregation of solid particles with TEP results in aggregates becoming negatively buoyant and sinking out of the SML.…”