Odonates as indicators of the ecological integrity of the river corridor: Development and application of the Odonate River Index (ORI) in northern Italy

The assessment of the ecological conditions of rivers is crucial for their appropriate management andrestoration. Bioindicators commonly used to evaluate the river status (i.e. diatoms, aquatic macrophytes,benthic macroinvertebrates and fish) detect alterations of water quality, but are not particularly sensitiveto hydromorphological degradation, which is another relevant pressure in river systems. Furthermore,those bioindicators are usually applied only to flowing channels. We developed a new multimetric index,the Odonate River Index (ORI), to assess the conditions of the whole corridor in alluvial rivers. The ORIis a development of an evaluation system proposed in Austria, and based on the Odonate Habitat Index(OHI). Odonates were chosen as bioindicators for the ecological integrity of the river corridor, since thistaxon provides information on the conditions of their aquatic breeding sites, as well as on the surroundingterrestrial areas, due to its amphibiotic life cycle. We used a case study of 18 reaches from six Italian Alpinerivers, characterized by different morphological conditions and level of human impact. Within each studyreach, we selected four sites, both lotic and lentic sites. Dragonfly surveys consisted in field observationof adults, and collection of larvae and exuviae. To define the best sampling strategy, we compared theresults of the ORI metrics obtained varying the input data by combining different sampling methods: thebest compromise between effort and exhaustiveness was obtained coupling the observation of adultswith the collection of exuviae. We found the ORI to be a robust and reliable tool to assess the status ofthe river corridor in a wide range of environmental conditions and river morphology, being particularlysuitable to detect hydromorphological degradation and alterations of the structure of aquatic and riparianvegetation. We identified two limiting factors for the applicability of this index: low water temperaturesof the main channel (i.e. mean annual value below 10◦C) and river reaches with no or scarce aquatic andriparian vegetation. In addition to the assessment of river conditions, the ORI could also be applied formonitoring the effects of river restoration actions.