Variation in Colloidal Organic Matter Composition and Aggregation in Coastal Waters (Gulf of Trieste, Northern Adriatic Sea)

Large volumes of seawater were sampled monthly from December 2011 to October 2012 in the southeastern part of the Gulf of Trieste (northern Adriatic Sea) in order to study the seasonal changes of colloidal organic matter (COM) concentrations, its origin and composition. Isolation of COM was performed by ultrafiltration with molecular weight cut-off membranes of 5kDa and final desalinization by dialysis. COM was characterized using H-1 NMR and FT-IR spectroscopy, elemental C and N and C-13 and N-15 isotope analyses and high-pressure size exclusion chromatography. COM represents about 1/4 of dissolved organic carbon (DOC) and appears to be mainly of marine origin. COM is mainly composed of polysaccharides, lipids, proteins and humics. FT-IR analysis indicated the presence of OH (carbohydrates and lipids), COOH (proteins and lipids) and NH (proteins) containing functional groups. Using the NMR technique, it was possible to semiquantitatively determine variations of four main biochemical constituents present in our COM samples. In the late spring - summer an accumulation of COM was observed, reaching up to 25 mu mol L-1 C-org. and representing about 1/3 of DOC in the Gulf waters. The lipid fraction increased up to 2-fold and the polysaccharide fraction remained nearly constant while protein fraction decreased, reflected in a higher C/N (28) molar ratio. Also, higher concentrations of humics were observed in late spring - early summer probably due to local freshwater discharges in spring. An increase of lipid fraction and nearly constant polysaccharide content in late spring - early summer, in parallel with agglomeration of high molecular weight (>200kDa) macromolecules, indicates the possible formation of macroaggregates, which has been periodically occurred in the northern Adriatic Sea in the past.