Seasonal fluctuations of DIN/DIP and DON/DOP ratios as a consequence of biological activity

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 North Adriatic basin, considering, for the first time, also the organic pools. The research aimed to provide a better understanding of nutrient availability in this area which is strongly influenced by continental inputs, and also to investigate the possible factors which promote the phenomenon of mucilage formation. To approach this purpose, the availability, the seasonal variations 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 in seawater. The results obtained during four seasonal campaigns in two fixed stations located near the Po river outflow and at a farther distance from the coast, respectively, reveal the large importance of organic nitrogen (annual average 16 ?M) and phosphorus (annual average 0.13 ?M) in contributing to the total nutrient availability (annual average total dissolved nitrogen: 29 ?M and phosphorus: 0.18 ?M) and the pronounced seasonal variability mainly ascribable to biological processes of uptake and remineralization. Furthermore, beside unbalanced ratio between inorganic nitrogen and phosphorus, already well documented for this area, 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.