Major inputs into the northern Adriatic Sea and critical problems affecting this basin are revised strengthening the need for a thorough knowledge of organic carbon cycling. Results gathered in the context of the PRISMA 2 project by our research group give information on the distribution and composition of organic matter in terms of major biochemical components, molecular weight sizes and biodegradability characteristics. Seasonal accumulation of dissolved organic matter in summer along with an increased role of carbohydrate and colloidal matter (being on average 15-20 and 60-65% of DOC, respectively) and restricted bacterial activity due to P deficiency are all priority subjects to be further investigated.