Abstract
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 (>1.0 Raman units) for Jade Bay. Spearman analysis for FDOM enrichment in the SML as a function of PAR (for Leg 2) showed a weak positive correlation (Rho = 0.676, n = 11, p = 0.022). Abundance of small photoautotrophic cells (Rho = 0.782, n = 11, p = 0.045) and of bacteria (Rho = 0.746, n = 11, p = 0.0082) also showed a positive correlation as a function of PAR. Overall, we found positive trends between the intensity of available light and the response of the constituents within the SML, highlighting the role of the surface microlayer as a distinctive habitat characterized by unique photochemical processes.
Highlights
The 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)
A general trend for increased fluorescent intensity indicative of the humic A-like fraction of fluorescent dissolved organic matter (FDOM) can be inferred for the study area, from the North Sea to Jade Bay, and the comparative fluorescent signatures between the SML and underlying water (ULW) show the enrichment of this fraction in the SML (Figure 3)
The same potential increase in microbial activity described for ULW appears to be stronger for the SML, as reflected in the biological index (BIX) values shown in Figure 8, We find that these results indicate that the stronger the solar radiation, the greater the increase in the BIX
Summary
The 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). Among the sources of organic substrates in the SML is the dissolved organic matter (DOM; (Cunliffe et al, 2013). Two optically active components of DOM are known: the colored dissolved organic matter (CDOM) and the fluorescent dissolved organic matter (FDOM). 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)
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