GROSS PRIMARY PRODUCTION AND NUTRIENT BEHAVIOR IN A SHALLOW COASTAL ENVIRONMENT

The estimation of the gross primary production (GPP) through measureme nts of a conserved element, such us phosphorus (P) in macroalgal bioma ss, surface sediment and settled particulate material (SPM) is propose d. The central Venice lagoon is covered by a biomass ranging from 1 to 20 kg m(-2), wet wt, with macroalgae accounting for almost the whole production. The total phosphorus GPP (P-GPP) was more than sixfold the P involved in the net primary production (P-NPP). The GPP also provid ed the fluxes of nutrients at the water-sediment interface and allowed the estimation of losses of nonconserved elements, such as nitrogen ( N) and carbon (C), by denitrification and respiration-decomposition pr ocesses, respectively. In the Venice lagoon the Nand C losses were eva luated to be 51% and 63%, respectively, of the nutrients involved in t he total GPP. From a comparison of these results with the total nutrie nt loadings entering the lagoon through external inputs, it was sugges ted that the sea strongly supported the GPP occurring in the lagoon. T he release of N from the lagoon to the atmosphere was equivalent to ab out 4.3 times the N introduced yearly from external inputs.