A comparison of macroinvertebrate and diatom responses to anthropogenic stress in small sized volcanic siliceous streams of Central Italy (Mediterranean Ecoregion)

The European Water Framework Directive (WFD 2000/60/CE) requires the use of different biological indicators, supporting physicochemical and hydromorphological elements, in the assessment of the ecological status of rivers: in each member state rivers should be at least in a good condition (or good ecological potential if heavily modified) by 2015, and those failing this criterion should be restored. Given that the need for restoration is primarily based on the results from ecological classifications, precision of these classifications is essential. The present study analyzed the relationships of two different biological communities (diatom and macroinvertebrate) when they are used to characterize a set of sites (from reference sites to human-impacted sites) within a single stream type (Mediterranean small sized volcanic-siliceous streams). In particular, we examine the issue of concordance among these communities and the anthropogenic stressors detected, using both multivariate and multimetric approach. Mantel test based on Bray–Curtis distance showed that assemblage dissimilarity between diatom and macroinvertebrate was weakly correlated, dissuading the use of one taxa group as surrogate of the other. The two biological indicators similarly reflected the overall river integrity, but provided complementary information on the specific stressors. Particularly, in accordance to previous studies, our results show that macroinvertebrate assemblage was influenced by water quality (nutrients enrichment and organic pollution) and habitat degradation. On the other hand, same as for benthos, diatom assemblage was regulated by chemical factors (especially nutrients), while did not show strong responses to morphological degradations. In addition, the ecological status of volcanic streams was evaluated by means of the two Intercalibration Common Metrics Indices (ICMi), based, respectively, on macroinvertebrate and diatom communities. The two ICM indices, developed for the Italian biomonitoring program (sensu WFD), were strongly correlated and provided a final classification mainly related to the water quality (nutrient enrichment and organic pollution). However, both indices were less sensitive to the hydromorphological alterations, highlighting the need for the development of stressor specific indices to achieve a complete evaluation of the anthropogenic stress present in the area. Studies of this kind across different disturbance gradients and focused on a particular river type (e.g. Mediterranean small-size volcanic-siliceous streams), will enable the relative merits of the indicators to become apparent and greatly facilitate their appropriate application.