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 air–sea 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 METEOR 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.
Highlights
The sea-surface microlayer (SML) is the uppermost layer of the water column and the interface between the ocean and the atmosphere
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, total dissolved nitrogen (TDN)), as well as bacterial and phytoplankton abundance
In our study we focused on the upper micrometers of the water–air interface that we operationally define as SML, whose compositional changes and accumulation of organic matter may influence two air–sea interface processes necessary to understand oceanic feedbacks on the atmosphere: sea-spray aerosol (SSA) emission and air–sea gas exchange (Cunliffe et al, 2013)
Summary
The sea-surface microlayer (SML) is the uppermost layer of the water column and the interface between the ocean and the atmosphere. It has been suggested that the SML has a gel-like nature (Cunliffe and Murrell, 2009; Sieburth, 1983) of varying thickness (20–150 μm, Cunliffe et al, 2013) with dissolved polymeric carbohydrates and amino acids present as well as gel particles, such as transparent exopolymer particles (TEP) of polysaccharidic composition, and Coomassie stainable particles (CSPs) of proteinaceous composition These gelatinous compounds originate from high molecular weight polymers that are released form phytoplankton and bacterial cells by exudation and cell break up (Chin et al, 1998; Engel et al, 2004; Verdugo et al, 2004). Gel particles can promote microbial biofilm formation (Bar-Zeev et al, 2012) and mediate vertical organic matter transport, either to the atmosphere (Leck and Bigg, 2005; Orellana et al, 2011) or to the deep ocean (Passow, 2002)
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