The estimation of the gross primary production (GPP) through measurements of a conserved element, such us phosphorus (P) in macroalgal biomass, surface sediment and settled particulate material (SPM) is proposed. 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 provided 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 processes, respectively. In the Venice lagoon the Nand C losses were evaluated to be 51% and 63%, respectively, of the nutrients involved in the total GPP. From a comparison of these results with the total nutrient loadings entering the lagoon through external inputs, it was suggested that the sea strongly supported the GPP occurring in the lagoon. The release of N from the lagoon to the atmosphere was equivalent to about 4.3 times the N introduced yearly from external inputs.