Deliverable I2d4 - Relazione tra la ritenzione dei nutrienti e la distribuzione e l'abbondanza degli invertebrati acquatici, i ruoli trofici e la classificazione dello stato ecologico nelle aree di studio

In riverine environments, nutrients such as mineralized compounds of phosphorus and nitrogen play an essential role in defining ecosystems functioning, acting in particular as limiting factors. Within nutrients cycling, the fundamental role of bacterial communities has been widely recognized; however, only relatively recent studies demonstrate the importance of animal communities in such processes. In particular, among the aquatic communities, benthic biocoenoses play part of the process in the accumulation, transformation and removal of nutrients in a water body, defining nutrient retention, that is one of the measures of freshwater ecosystem functionality. Among the activities considered in INHABIT project, the present deliverable considers the relationships between the dynamics of nutrients retention and aquatic macrobenthic community structure, considering in particular distribution and abundance of taxa, trophic roles and classification of ecological status based on benthic invertebrates. The activity has considered the investigation of 71 river stretches located in Sardinia; on 19 of them (also called 'addition sites') the experiment of nutrient addition was performed, in order to assess dynamics of nutrient retention. Two distinct sets of multivariate analyses were carried out with the aim, in the first case (N=71), to contextualize river stretches considered for the nutrient addition experiment in a wider regional framework and, in the second case, to more specifically characterize such traits with an analysis carried out at microhabitat level (single macroinvertebrate sampling units, N=380). Axes resulting from multivariate analyses were interpreted by means of correlation (R- Pearson) with approximately 200 variables, that included geographic, physical-chemical, mesohabitat- and microhabitat- related, habitat quality, nutrient retention and biological variables. Direct linear correlations between selected biological metrics, descriptive of trophic roles and ecological quality, and retention metrics were also considered. Results of the analyses have confirmed for 'addition sites' a macrobenthic community structure comparable to that of other sites in Sardinia, considered in a wide regional area, confirming the general soundness of the experimental assumptions. Within considered river stretches, the lenticlotic character has resulted the main environmental feature defining macrobenthic community, as already shown in other activities of INHABIT project. Particularly interesting were the results observed for the relations between the ammonium 'coefficient of mass transfer' retention metric and selected biological metrics, descriptive of trophic roles (ratio of Grazers / Passive Filterers) and ecological status (STAR_ICMi index). In both cases positive linear correlations were observed suggesting, on one hand, the importance of the association between Grazer taxa and nutrient retention dynamics, presumably in relation to the development of periphytic biofilms and highlighting, on the other hand, the increase of retention efficiency at increasing environmental quality. From these results it is possible to infer that, in riverine environments, the protection of a high ecological status can simultaneously ensure the activation of the processes reducing nutrients accumulation.