Defining Maximum Ecological Potential for heavily modified lowland streams of Northern Italy

In relation to hydromorphological alteration the Water Framework Directive (WFD), a major piece of European legislation, has introduced the concept of Heavily Modified Water Bodies (HMWB). In water bodies falling in this category, hydromorphological modifications are permanent, significantly alter the character of the river and cannot be removed without compromising the use of the water body. In HMWBs a dedicated approach to the evaluation of their status is set, and their Ecological Potential must be assessed. Crucial to the process is the definition of Maximum Ecological Potential (MEP) as the reference conditions for HMWB. In the present paper we aim to define MEP conditions for Italian heavily modified lowland rivers, affected by strong bank protection (i.e. levees or bank reinforcement) in reason of flood protection and land drainage uses. The approach applied to identify MEP conditions follows the one considered for natural (not heavily modified) rivers in Italy and large part of Europe and bases on the identification of 'reference sites' representative for the river category and alteration. For the selection of MEP sites environmental features representing mitigation measures and/or expected natural features were considered. The ability of such features in discriminating MEP and disturbed sites was verified by multivariate analyses run on abiotic features (Principal Component Analysis) and biological communities (non-metric multidimensional scaling). We demonstrated differences both in terms of invertebrate community and biological metrics used to assess ecological status (and potential) between MEP and impaired river stretches. Finally, we recognized relevant habitat features able to clearly separate MEP reaches from nonMEP reaches with indication on the type and quantity of measures significant for benthic invertebrates and applicable in lowland Heavily Modified Water Bodies.