Surface-active properties of organic matter in the North Adriatic Sea

Characterization of surface-active organic substances in the North Adriatic Sea, in surface and bottom layers, during the year 2000 has been performed. Surface-active substances (SAS), their hydrophobicity and acidity, were estimated with alternating current voltammetry (in-phase mode) using o-nitrophenol as an electrochemical probe. Depending on hydrophobicity and acidity of SAS present, the electrochemical characteristics of the o-nitrophenol probe change in a specific way. Normalization of the change of prepeak height as an indication of organic matter acidity to shift of peak potential as a measure of hydrophobicity gives an indication of relative acidity of the SAS present. Experiments with model substances showed that humic and fulvic acids contribute the most to the relative acidity, while biogenic surface-active organic substances such as proteins, polysaccharides and lipids contribute less to the relative acidity. The surface water layer is characterized by higher phytoplankton excretion during the whole year, resulting in a higher contribution of freshly produced biogenic organic substances to the surfactant activity (SA). Humic and fulvic substances dominate in SA in the bottom layer. Proteinaceous material is found in a heterodispersed fraction during supposed phytoplankton blooms that occurred in March, July, September and even December, in the northern Adriatic, in connection with unexpectedly high temperatures. SA of the particulate fraction was found to be mainly less than 10%. Exceptions were obtained in April and October when SA of the particulate fraction contributed up to 55 and 20%, respectively.