engleski

Characterization of surface-active organic substances in the North Adriatic Sea

Surface-active properties of marine organic matter play a key role in adsorption at natural phase boundaries, aggregation of organic matter, marine snow formation and evolution of mucilagenous aggregates. Surface-active substances (SAS) have specific structures, possessing hydrophobic and hydrophilic moieties of the molecules. Surfactant activity (SA) of some hydrophilic substances originates from their high molecular mass as in the case of polysaccharides1. Chemical composition of natural SAS comprises polysaccharides, proteins, lipids and refractory substances like humic and fulvic acids. As previously reported2 on the basis of different behaviour of DOC seasonal changes and mucilage occurrences in the North Adriatic it seems that the qualitative characteristics of the dissolved and colloidal organic matter are probably more important than their quantitative concentrations in controlling the partitioning between dissolved and aggregate matter. Here we report on the spatial distribution and seasonal variability of SAS of surface and bottom seawater layers in the Northern Adriatic sea studied by the electrochemical method3,4. Electrochemical measurements of SAS using o-nitrophenol as an electrochemical probe enable direct determination of characteristics, especially the hydrophobicity and relative acidity of the natural organic matter. A general characteristic of the SAS seasonal variability in the Northern Adriatic is the sinusoidal change of SA within the period of one year, with decreasing amplitude toward the bottom layer4. Experiment with selected model substances showed great variability of hydrophobic properties and contribution to the acidity of the organic matter pool. Humic and fulvic acids have the largest contribution to the acidity per surfactant activity of organic matter in the seawater sample. On the contrary, biogenic surface active organic substances as proteins, polysaccharides and lipids have lower contribution to the acidity per surfactant activity. Characterisation of SAS in the Northern Adriatic seawater samples showed higher contribution of polysaccharides and proteins in the upper layer and in the period of phytoplankton blooms, with major contribution of refractory substances in the bottom layer. SA of the particulate fraction was estimated as the difference between the SA of the nonfiltered and filtered seawater samples. The obtained values for SA of particulate fraction were relatively low, less than 10 %, with exception in April (55 %) and in October (app. 20 %).

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