Miranda, ML, 2018 Influence of solar radiation on biogeochemical parameters and fluorescent dissolved organic matter (FDOM) in the sea surface microlayer of the southern coastal North Sea. Elem Sci Anth, 6: 15. DOI: https://doi.org/10.1525/elementa.278 IntroductionThe sea surface microlayer (SML), defined here as the upper 50-100 µm of the water column, is an important habitat of the ocean where air-sea exchange and transformation processes take place (Wurl et al., 2016). Compounds from natural and anthropogenic sources accumulate within the SML enhancing their concentrations in comparison with the water column. The relative abundance of organic matter within the SML provides a diverse range of substrates for the growth of surface-dwelling heterotrophic microorganisms (Reinthaler et al., 2008).Among the sources of organic substrates in the SML is the dissolved organic matter (DOM; (Cunliffe et al., 2013). DOM is a large mix of organic compounds, mainly the products of biodegradation processes, and is comprised of carbohydrates, proteins, lignin, organic acids, and humic substances (Zhi et al., 2015). Two optically active components of DOM are known: the colored dissolved organic matter (CDOM) and the fluorescent dissolved organic matter (FDOM). CDOM includes materials that are capable of absorbing light, while FDOM refers to the mix of compounds that can absorb light, emitting fluorescence at certain wavelengths according to the presence of diverse chemical functional groups (Coble, 1996;Stedmon and Nelson, 2015). The SML is a highly dynamic and complex habitat in which several processes transform the DOM. These transformations are triggered by the environmental factors of wind, solar radiation, and light availability, as well as by microbial activity (Cunliffe et al., 2013).Light availability in the water column is controlled by two physical processes, absorption and scattering (Kirk, We investigated the influence of solar radiation on biogeochemical parameters of the sea surface microlayer (SML), including the spectroscopic composition of FDOM, and biotic and abiotic parameters. We calculated the humification index, biological index, and recently produced material index from the ultraviolet spectra to characterize the dynamic environment of the SML. The humification index ranged from 4 to 14 in the SML and 14 to 22 in underlying water (ULW). An inverse relation for this index as a function of solar radiation was observed, indicating photochemical decomposition of complex molecules present in fluorescent dissolved organic matter (FDOM). The biological index (along Leg 2) ranged from 1.0 to 2.0 for the SML and 1.0 to 1.5 for ULW. The index for recently produced material ranged from 0.25 to 0.8 for the SML and 0.5 to 1.0 for ULW. The FDOM enrichment process of the SML was influenced by the photochemical decomposition of highly aromatic-like fluorophores, as indicated by the calculated indices. Fluorescence intensity increased for humic C peaks (>0.5 Raman units) in the North Sea samples and for humic M peaks (>1....
Abstract. Transparent exopolymer particles (TEPs) are a major source for both organic matter (OM) and carbon transfer in the ocean and into the atmosphere. Consequently, understanding the vertical distribution of TEPs and the processes which impact their movement is important in understanding the OM and carbon pools on a larger scale. Additionally, most studies looking at the vertical profile of TEPs have focused on large depth scales from 5 to 1000 m and have omitted the near-surface environment. Results from a study of TEP enrichment in the sea surface microlayer (SML) in different regions (tropical, temperate) has shown that, while there is a correlation between TEP concentration and primary production (PP) on larger or seasonal scales, such relationships break down on shorter timescales and spatial scales. Using a novel small-scale vertical sampler, the vertical distribution of TEPs within the uppermost 2 m was investigated. For two regions with a total of 20 depth profiles, a maximum variance of TEP concentration of 1.39×106 µg XG eq2 L−2 between depths and a minimum variance of 6×102 µg XG eq2 L−2 was found. This shows that the vertical distribution of TEPs was both heterogeneous and homogeneous at times. Results from the enrichment of TEPs and Chl a between different regions have shown TEP enrichment in the SML to be greater in oligotrophic waters, when both Chl a and TEP concentrations were low, suggesting the importance of abiotic sources for the enrichment of TEPs in the SML. However, considering multiple additional parameters that were sampled, it is clear that no single parameter could be used as a proxy for TEP heterogeneity. Other probable biochemical drivers of TEP transport are discussed.
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