The dynamics of DOM in the northern Adriatic Sea

The variability of dissolved (DOC) and colloidal (COC) organic matter and o f organic biochemical components and their possible relationships with hete rotrophic activity have been investigated in the northern Adriatic Sea duri ng four surveys (June 1996, February 1997, June 1997 and February 1998). Sa mples were collected in two frontal regions, one in the north under the dir ect influence of the Po River and the other in the south close to the borde r-line between the northern and the middle Adriatic. The overall set of data ranged from 53 to 281 muM for DOC, 44 to 98 muM for COC, 6 to 72 muM for total dissolved carbohydrates (TDCHO-C), 0.1 to 2.4 m uM for free amino acids (DFAA-C), 1.2 to 9.4 muM for total dissolved amino acids (TDAA-C) and 0.01 to 7.1 mug C 1 (1)h (1) for bacterial carbon produc tion (BCP). COC and TDCHO data showed tight relationships with DOC values, which were in turn negatively dependent on salinity. Organic carbon showed increases in surface waters with respect to the background DOC value of 76 +/- 10 muM averaged from bottom data in February surveys. Seasonal changes were more or less marked depending on the riverine discharge curve and rela ted trophic dynamics in the investigated year. The most marked increases we re observed in June 1996 and, based on average values, amounted to 104 and 62 muM DOC in the north and south frontal regions, respectively; in the suc cessive June 1997, the corresponding increases were 47 and 20 muM. COC conc entrations gave a high contribution to DOC and showed a large fraction of t he high molecular weight class. Contrary to the increases in DOC and COC concentrations which were stronger in 1996 than 1997, mucous macroaggregates showed a much stronger occurrenc e in June 1997 compared to the June 1996 survey. Considering that the aggre gation of colloids is one of the main processes leading to the formation of macroaggregates, the different behaviour of DOC seasonal changes and mucil age occurrences in the two years may suggest that the qualitative character istics of the DOC and COC pools are probably more important than their quan titative concentrations in controlling the partitioning between dissolved a nd aggregate matter. Bacterial production rates showed a large variability during the study peri od which reflected concentration gradients of inorganic and organic nutrien ts, differences in the lability of the organic substrate and nutrient limit ations. Bioassays carried out on samples collected in different seasons and at various salinities suggested an apparent strong limitation of the bacte rial activity by phosphorous. (C) 2001 Academic Press.