Habitat indices for rivers: quantifying the impact of hydro-morphological alterations on the fish community

The impact of water abstractions and hydro-morphological alterations on the aquatic ecosystems can be measured either by the status of the biota or by its habitat. can be measured by assessing the effects on the biota or on its habitat. The former is difficult to accomplish because of the following: (i) the natural biological variability can increase the field data uncertainty; (ii) collection of field data for certain biotic omponent, such as fish, con be difficult due to the spatial and temporal mobility of such target organisms the difficulties of biotic observation can be related to their spatial and temporal mobility (iii) after an hydro-morphological disruption, the time-lag required for populations recruitment can be long; and (iv) the instream hydro-morphological characteristics may not be the only drivers of species distribution. Therefore, the use of a spatial unit of physical habitat suitable for the desired aquatic community can be seen as a more pragmatic and accurate metric in describing the impact of water abstractions and alterations on biota. Habitat simulation models are effective tools and can be used to estimate the spatial and temporal habitat availability in rivers, and to design habitat restoration actions. The present work aims at proposing habitat indices developed with the support of a mesohabitat simulation model (i.e. MesoHABSIM) and the habitat time series analysis. Such indices represents metrics which quantify the impact of hydro-morphological alterations on the fish community. The MesoHABSIM model refers to mesohabitats or hydro-morphological units (HMU, such as pools, riffles or rapids) to integrate system-scale assessment of ecological integrity in flowing waters with quantitative information on physical habitat distribution and to simulate habitat changes over a range of discharges. The habitat time series represent how physical habitat changes through time and their statistical analysis can be useful to identify stress conditions created by persistent limitation in habitat availability. Two habitat indices are defined to evaluate the spatial and temporal alteration of instream habitats. Firstly, the habitat quantity index (HQI) is used to describe the relative amount of habitat loss due to a particular water abstraction and, secondly, the Habitat Stress Days Index (HSDI) is used to measure the increase of continuous duration of events when habitat bottlenecks create stress to the fauna. Case studies from the mountainous areas of Northern Italy are presented to provide example of applications. The obtained results indicate that (i) the meso-scale can be considered as an appropriate resolution to link fish habitat requirements to fluvial morphological characteristics and (ii) the proposed indices can be considered as flexible tools: they can capture both spatial and temporal alteration of habitat structure and can be developed for different kind of water withdrawals.