Seasonal variations of dissolved organic matter in Northern Adriatic Sea

Concentrations and seasonal variability of dissolved organic carbon DOC., total dissolved carbohydrates TDCHO., and dissolved free amino acids DFAA., were measured in two frontal regions of the northern Adriatic Sea during two surveys June, 1996 and February, 1997.. Colloidal )1 kDa. and truly dissolved -1 kDa. fractions of DOC were also determined. DOC, TDCHO and DFAA concentrations were higher in summer than winter confirming previous findings on DOC seasonality in northern Adriatic waters. Average values from both frontal regions were 151"40 mM DOC; 30"18 mM TDCHO-C; 1.40"0.56 mM DFAA-C in June compared to 82"18 mM DOC; 13"8 mM TDCHO-C; 0.86"0.30 mM DFAA-C in February. The accumulation of dissolved organic matter in summer presumably involves an enrichment in biorefractory compounds discharged by riverine inputs or produced by abiotic transformations and an inability of bacteria to consume all of the DOC produced. Increases in biological productivity and freshwater residence time and the onset of strong water stratification which occur in summer also favour this accumulation. The average contribution of TDCHO to DOC ranged from 20% in June to 15% in February, while that given by DFAA was about 1% in both seasons. The estimated contributions to DOC by both these classes of compounds rank in the high range of values reported for marine environments. Results on DOC molecular weight distribution showed that the largest fraction of the overall DOC pool was colloidal ;60%., with truly dissolved compounds being a minor contribution. The observed accumulation of dissolved organic matter in summer along with an increased role of carbohydrates and colloidal matter may be critical factors for the formation of mucilaginous aggregates in this basin.