Abstract
The sea surface microlayer (SML), created by surface active organic molecules (called: surfactants), is a highly active interface between the sea and the atmosphere. In this study we used the dissolved organic matter absorption and fluorescence properties for description of changes in molecular size and weight distribution of organic molecules as well as in their composition in the SML compared to subsurface water layer (SS), of 1 m depth. Data were collected during three research cruises in the Baltic Sea. Spectral values of the CDOM absorption coefficient were higher in the SML about 29 % (in the UV light) to 17 % (in a blue spectral range), while spectral slope coefficients at different spectral ranges, S_Δλ, were lower in SML by 2.3 and 10.5% compared to SS in 275-295 nm and 350-400 nm, respectively. The fluorescence intensities of main Excitation Emission Matrix spectra peaks belonging to the main fluorescing components of marine organic matter, called: A, C, M, T, were higher in SML by 41, 43, 41 and 14% compared to SS. The ratio of fluorescence intensities, ((M+T))⁄((A+C) ) and humification index, HIX, values in SML were, respectively higher by 17.9% and lower by 10.7 % compared to SS. These two parameters where coupled with inverse non-linear relationship. This relationship indicated faster removal of humic components of high MW in the SML compared to SS. We have observed decreasing trend of spectral slope ratio, S_R, with increasing salinity. The S_R decreased 33.5 and 23.6 % in the SML and SS, respectively in the salinity gradient from 4.5 to 7.94 (of practical salinity ). The fluorescence intensity of all FDOM peaks decreased by in the same salinity gradient. The decrease rate was higher in SML for all fluorescing peaks by 34, 36 and 26 % for A, C and M, respectively than in the SS. Decrease rate indicated the susceptibility to photochemical degradation of respective peaks. This effect was strongest for C while T peak was almost unbleached. The fluorescence intensity decrease rate was smaller in SS what indicated shielding effect of the SML.
Highlights
The sea surface microlayer (SML), commonly defined as the upper 0–1 mm of the surface ocean (Liss and Duce, 1997) is described by different physical properties than the underlying layers due to the molecules that form the surface film (Hardy, 1982)
The curves of the spectral resolution of the CDOM absorption coefficient for the samples collected at the very mouth of the river have less inclined shape, what means a presence of a greater amount of components absorbing at longer wavelengths, 300– 400 nm (Grzybowski, 2000; Twardowski et al, 2004; Helms et al, 2008)
The CDOM absorption coefficients as well as the fluorescence intensity of FDOM components decrease with increase of salinity
Summary
The sea surface microlayer (SML), commonly defined as the upper 0–1 mm of the surface ocean (Liss and Duce, 1997) is described by different physical properties than the underlying layers due to the molecules that form the surface film (Hardy, 1982). The SML is created by the surface active organic molecules, called surfactants, characterized by an amphiphilic structure, i.e., with hydrophobic and hydrophilic heads. The SML interacts with the surface accumulation of organic matter produced by biological processes in the underlying water column (Galgani et al, 2016; Kurata et al, 2016). The SML accumulates a variety of colloidal and particulate organic matter that may be substrates for bacterioneuston, and the SML is called as a complex hydrate “gel” of macromolecules and colloidal material (Sieburth, 1983)
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