Seasonal fluctuations of DIN/DIP and DON/DOP ratio in the northern Adriatic Sea

Within the frame of PRISMA 1 "Biogeochemical cycles" research project (April 1995 - January 1996) the quantities and the compartments of dissolved nitrogen and phosphorus have been studied in the northem Adriatic basin, considering also the organic pools. The research aimed to provide a better understanding of nutrient availability and to investigate the possible factors which promote the phenomenon of mucilage formation. For this purpose, the availability and the ratios between dissolved nitrogen and phosphorus considering both inorganic and organic fractions have been studied in relation to variations of river outflow and of biological activities. The results obtained reveal the large importance of organic nitrogen (annual average 16 YM) and phosphorus (annual average 0.13 PM) in contributing to the total nutrient availability (annual average total dissolved nitrogen: 29 YM and phosphorus: O. 1 8 gM) and the pronounced seasonal variability mainly ascribable to biological processes of uptake and remineralization. Furthermore, beside the well documented unbalanced ratio between inorganic nitrogen and phosphorus, the results obtained point out, for the first time, the unbalance also in the organic compartment (ratio between organic nitrogen and phosphorus ranges between 50 and 530), whose consequences might be important in relation to the phenomenon of mucilage formation. Key words: nitrogen. phosphorus, N/P ratio. northern Adriatic Sea, dissolved organic matter.