Articles | Volume 13, issue 5
Research article
31 Aug 2017
Research article |  | 31 Aug 2017

Study on organic matter fractions in the surface microlayer in the Baltic Sea by spectrophotometric and spectrofluorometric methods

Violetta Drozdowska, Iwona Wrobel, Piotr Markuszewski, Przemysław Makuch, Anna Raczkowska, and Piotr Kowalczuk

Abstract. 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,  SR) and fluorescence (fluorescence intensities at Coble classified peaks: A,  C,  M,  T the ratio M + TA + 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.

Short summary
The studies on the absorption and fluorescence properties of the organic molecules included in surface microlayer (SML) and subsurface (SS) waters confirm that (i) the process of the structural changes in molecules of HMW to LMW, due to effects of photo- and biodegradation, occurs faster in the SML than in the SS; (ii) the organic molecules contained in the SML have a smaller molecular mass than in the SS. Hence, SML can specifically modify the physical processes associated with the sea surface.