Search

Boundary layers between different environmental compartments represent critical interfaces for biological, chemical and physical processes. The sea surface microlayer (SSM) as a top layer of the sea surface represents natural interface between the atmosphere and ocean. Although < 1 mm in thickness the SML plays a key role in the global biogeochemical cycling because all gaseous, liquid and particulate materials must pass through this interface when exchanging between the ocean and the atmosphere. The SSM thus represents a very important driver enhancing air–water exchange processes. A variety of natural and anthropogenic organic compounds, particularly those which are surface active (SA) are generally enriched in the SML. It is widely acknowledged that the SSM is complex matrix of SA organics as carbohydrates, proteins, lipids and humic substances. Although lipid material is much less abundant than carbohydrates and proteins in the SML, their contribution to surface activity may be disproportionately large. The surfactant films at the air–sea interface change its physicochemical properties reducing air–sea exchange possesses by impeding molecular diffusion across the interface and influencing the hydrodynamic characteristics of water motion at the interface. Various biological, chemical and physical processes lead to the alteration of the film chemical composition, surface physical properties, surface concentration and spatial distribution of film-forming components. Instead of analyzing its chemical composition, it should be possible to scale the SML surface pressure-area ( -A) isotherms in terms of structural parameters which appear to be a sensitive and quantitative measure of the film physicochemical composition, surface concentration and miscibility of its film-forming components. We will present a large data set obtained by electrochemical and monolayer techniques, accompanied with the novel scaling approach for physicochemical characterization of SA substances of the natural microlayers from coastal Middle Adriatic stations including saline Rogoznica Lake and Krka river estuarine station. Higher primary production during late spring-early autumn is reflected in the presence of microlayers of higher surfactant activity containing on average molecules of lower molecular masses (Mw=0.65 0.27 kDa) and higher miscibility(y=6.46 1.33) and elasticity(E=18.33 2.02 mN/m) modulus in comparison to structural parameters (average Mw=2.15 1.58 kDa ; y=3.51 1.46 ; E=6.41 1.97 mN/m) obtained for microlayers from period of lower organic matter production. Higher inhibition effect on the reduction process of cadmium ions is observed for natural microlayers abundant with SA material from more productive period. This kind of distribution is explained as the consequence of competitive adsorption of hydrophobic lipid-like substances of lower Mw which highly influence the surface structural properties of natural air-water interface forming there segregated surface films during more productive period. This study will offer different perspective on contemporary SML concept taking into account the lipids that act as end-members highly influencing seasonal change of SA concentration and surface structural properties of natural films at the air-water interface.

The sea surface microlayer (SML) as a top layer of the ocean surface represents a natural interface between the atmosphere and the ocean. Although < 1 mm in thickness, the SML is central to a range of global dynamic exchange processes between the atmosphere and the aquatic environments where striking variety of biogeochemical and photochemical interactions and feedbacks occur. A large fraction of the SML organic matter has a particularly strong interfacial affinity. Due to their molecular structures, such surface active substances (SAS) are adsorbed at the air-water interface forming natural surface films which change the physicochemical properties of the interface and influence the air-sea exchange processes. A new perspective to the physicochemical characterization of the SML properties regarding the composition and the structure of dominant surface active material has been recently studied on the SMLs from the coastal Middle Adriatic locations, including the seawater Rogoznica Lake and Martinska station situated at the Krka river estuary. Our work has shown that chemical composition and distribution of the SML surfactants are influenced by the presence of certain end-member material of hydrophobic, lipid-like surfactants which strongly influenced the surface properties of natural air-sea interface. Due to their extremely high surface affinity, the presence of lipids at the interface is the result of their competitive adsorption and segregation from other macromolecular constituents which is closely related to the periods of primary production.

The sea surface microlayer (ML) forms the interface between the atmosphere and oceans covering about 71% of the Earth’s surface. An improved knowledge of the MLs distributions under different forcing conditions, its chemical composition and rheological characteristics are of crucial importance for better insight into the structural changes and mechanisms of ML formation as well as for modelling the phenomena and processes at the air-sea interface. An understanding of the processes and physical mechanisms governing the exchange of gases between air and sea is essential when considering coupled models of atmosphere-ocean interactions and global climate, the interactions which are of particular concern as they may affect or be affected by global changes. The chemical composition of natural films is largely undefined, although significant enrichment of many specific classes of compounds in the ML has been demonstrated. Instead of analysing the chemical composition, it should be possible to scale the MLs π-A isotherms in terms of structural parameters, reflecting the natural film morphology, and resulting from the generalized physical formalisms adopted to multicomponent films of surface active substances (SAS) (Pogorzelski and Kogut, 2003). In particular, a set of the structural film parameters appears to be a sensitive and quantitative measure of the film physicochemical composition (molecular mass Mw, specific limiting area Alim and elasticity modulus Eisoth), surface concentration (πmax, SAS), and the miscibility (y) of its film-forming components. This concept has been tested for marine coastal waters of the Adriatic Sea during longer period time. The MLs were studied by using monolayer technique and structural state scaling approach of -A isotherms as original samples and as ex-situ reconstructed films. In order to address the problem of solvent polarity effect on the surface structural parameters of ex-situ reconstructed films, the surface isotherms obtained from the material extracted from original ML with different solvents (n-hexane and dichloromethane) spread onto water surface were also analyzed by means of the novel scaling approach. The parameters can be novel indicators of surface-active source-specific organic matter production, migration and degradation status tracing physicochemical characterization of the surface active organic matter of the ML with the emphasis to the role of the source-specific lipids as film-forming components or end-members which may dominate the static surface properties of the ML. The normalization of the film structural parameters, based on the physical states similarity theory, provides a universal measure of surface properties evolution, applicable to a wide variety of the original and different ex-situ recovered films met in nature, giving an insight to the additional organic matter characterization of the natural films.