The fluorescence and absorption measurements of the samples collected from a surface microlayer (SML) and a subsurface layer (SS), at a depth of 1 m, were studied during three research cruises in the Baltic Sea along with hydrophysical studies and meteorological observations. Several absorption (E₂ : E₃, S, S_R) and fluorescence (fluorescence intensities at Coble classified peaks: A, C, M, T the ratio M + T∕A + C, HIX (humification index)) indices of colored and fluorescent dissolved organic matter (CDOM and FDOM) helped to describe the changes in molecular size and weight as well as in composition of organic matter. The investigation allowed the assessment of a decrease in the contribution of two terrestrial components (A and C) with increasing salinity ( ∼ 1.64 and ∼ 1.89 % in the SML and ∼ 0.78 and ∼ 0.71 % in the SS, respectively) and an increase in components produced in situ (M and T) with salinity ( ∼ 0.52 and ∼ 2.83 % in the SML and ∼ 0.98 and ∼ 1.87 % in the SS, respectively). Hence, a component T reveals the biggest relative changes along the transect from the Vistula River outlet to Gdansk Deep, in both the SML and SS, although an increase was higher in the SML than in the SS ( ∼ 18.5 and ∼ 12.3 %, respectively). The ratio E₂ : E₃ points to greater changes in the molecular weight of CDOM affected by a higher rate of photobleaching in the SML. The HIX index reflects a more advanced stage of humification and condensation processes in the SS. Finally, the results reveal a higher rate of degradation processes occurring in the SML than in the SS. Thus, the specific physical properties of surface active organic molecules (surfactants) may modify, in a specific way, the solar light spectrum entering the sea and a penetration depth of the solar radiation. Research on the influence of surfactants on the physical processes linked to the sea surface becomes an important task, especially in coastal waters and in the vicinity of the river mouths.

This paper presents results of characterization of Dissolved Organic Matter (DOM) using fluorescence spectroscopy in the surface microlayers (SML) and subsurface layers (SS) in the Baltic Sea. Samples for spectroscopic measurements were collected during five research cruises in April/May and October 2013 and 2014 in a surface microlayer and a subsurface layer at a depth of 1 m along two transects from the river outlets to the open sea. The first transect was located from the Vistula River outlet to the Gdańsk Deep and the second transect was located from the Łeba River outlet to Słupsk Furrow. Results indicated that DOM fluorescence intensity in the SML is higher by 20% compared to the SS. The Humification Index, HIX values were lower in SML than SS by 13%. That indicates that SML is depleted in molecules with high molecular weight and higher aromaticy. The inverse relationship of fluorescence intensity of dominant peaks with salinity both in SML and SS suggests that FDOM variability is regulated mostly by terrestrial DOM input.

SummaryIt is well known that surfactants in the southern Baltic Sea constitute the organic matter from riverine waters discharges as well as the secondary degradation products of marine phytoplankton excretion. They reach the surface microlayer by the upwellings and turbulent motions of water and in the membranes of the vesicles as well as from the atmosphere. To assess concentration and spatial distribution of marine surfactants in the southern Baltic Sea, the steady-state spectrophotometric and spectrofluorometric measurements of water samples taken from a surface film and a depth of 0.5m were carried out. Water samples were collected during windless days of the cruise of r/v ‘Oceania’ in November 2012, from the open and the coastal waters having regard to the vicinity of the Vistula and Łeba mouths. In the present paper, fractions of dissolved organic matter having chromophores (CDOM) or fluorophores (FDOM) are recognized through their specific spectroscopic behavior, i.e., steady-state absorption, fluorescence excitation and fluorescence spectra. The steady-state spectroscopic measurements revealed the CDOM and FDOM molecules characteristic to both the land and marine origin. Moreover, the concentration and spatial distribution of marine surfactants significantly depend on the distance from the river mouth. Finally, higher values of absorbance and fluorescence intensity observed in a surface film in comparison to these values in a depth of 0.5m clearly suggest the higher concentration of organic matter in a marine film. On the other hand, our results revealed that a surface microlayer is composed of the same CDOM and FDOM as bulk water

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 absorption and fluorescence analysis of organic matter collected in the SML and in subsurface layer, of 1 m depth, to describe the changes in molecular size and weight and the composition of surfactants. Data were collected during three research cruises in coastal zone and open waters of the Baltic Sea. The values of the CDOM absorption coefficient were higher in the SML about 29% (in the UV light) to 17% (in a blue spectral range), that reveal dominance of low molecular weighted CDOM molecules, absorbing in the UV light, in the SML. The spectral slope coefficients at different spectral ranges, S_Δλ increased with salinity, while the slope coefficient for 350–400 nm reach lower values by 10.5% in SML compared to SS, caused by an effect of irradiation on the SML. The fluorescence intensities of the main peaks at Excitation Emission Matrix spectra 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, in the SML were, respectively, higher by 17.9% and lower by 10.7% compared to SS. These relationships reveal more intensive process of in situ produced components in the SML as well as faster removal of humic components of high MW in the SML. We have observed an increase of spectral slope ratio, S_R, (S_275–295 > S_350–400) with increasing salinity (from 4.5 to 7.94 of practical salinity), being proof that the samples acquire more marine in character. The S_R increased with salinity 33.5 and 23.6% in the SML and SS, respectively, and their maximal values in open water were still maintained. The fluorescence intensity of all FDOM peaks decreased in the same salinity gradient. The decrease rate was higher in SML for the 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.