Managing the nutrient loads of the Venice Lagoon Watershed: are the loads external to the watershed relevant under the WFD River Basin District framework?

Water policy in the EU needs to be increasingly coordinated under the integrated management structure required by Water Framework Directive. In the WFD framework, all river catchments are assigned to administrative River Basin Districts (RBDs) that produce river basin management plans (RBMPs). RBMPs need to define appropriate Program of Measures (PoMs) based on the knowledge of the pressures and impacts affecting the water environments. However, pressures may also be external to the administrative RBDs, and still play a significant role. This study presents the case of the Venice Lagoon Watershed (VLW) and analyses the apportionment of the nutrient loads discharged to the lagoon by apportioning the external and the internal nutrient sources. VLW is included in the River Basin District of the Eastern Alps, although due to the complex framework of legislations and authorities that have managed the watershed to date, it is the object of a separate planning. The source apportionment assessment is assessed at the watershed scale by combining direct measurements and modeling results. The US Soil and Water Assessment Tool is used to simulate the expected improvements due to the application of "within watershed" measures. Model simulations pointed out the significant contribution of the loads external to the watershed, which seriously affect the effectiveness of the "within watershed" PoMs which are revealed largely ineffective if not coordinated with PoMs of boundary RBDs.