ANNUAL NUTRIENT EXCHANGES BETWEEN THE CENTRAL LAGOON OF VENICE AND THE NORTHERN ADRIATIC SEA

The exchange rates of nitrogen and phosphorus compounds between the Adriatic Sea and the inner parts of the central lagoon of Venice, highly populated by macroalgae, have been estimated by monitoring monthly, for 1 year, a number of stations located both along the main channels and in shallow areas of the lagoon as well as in the sea. Total inorganic nitrogen (TIN), between February and September, was 3-4 times higher in the sea station than in the lagoon. During the same period, the lagoon inflow of TIN through the two channels (Lido and Malamocco) that connect the central lagoon with the sea was 1.2 times higher than the total loads entering the whole lagoon by freshwater sources. In contrast, phoshorus concentrations were higher (15-45%) only in April-June. Four seasonal campaigns were also carried out both in flood and ebb tide at the lagoon mouths. Results confirm the TIN and phosphate inflow from the sea to the lagoon between March and September. On a yearly basis, nutrient concentrations were negatively correlated with chlorinity. The nutrient concentrations monitored concurrently at two stations located on a freshwater stream entering the lagoon (Osellino river) were approximately 1 order of magnitude higher than those of the lagoon and sea stations. The annual mean phosphate concentration of the Osellino river, monitored in these campaigns, was approximately 50% of that measured in 1982.