ABSTRACTThe Venice lagoon has been the subject of many studies during recent years directed towards the assessment of its ecosystem and to its environmental restoration, as well as to finding a solution to the problem of the high water condition which affects the historical city centre. This paper presents input and output data of the ecosystem and the present situation concering eutrophication and the pollution of the environment.The analysis of nutrients shows that the biological processes do not occur at the same rate. In fact the variation of phosphate concentrations is wider than that of nitrogen because of the greater biological reactivity of phosphate. The analyses show that it is impossible to determine the dominant eutrophication element in the lagoon, therefore its environmental protection needs action to reduce both nitrogen and phosphorus input.