Influence of seasonal multiannual drought events on the resilience of ecosystem processes and macroinvertebrate assemblages in a Central Apennine stream (Aterno River, Italy)

This research provides a rare field experiment on the resilience of ecosystem processes and macroinvertebrate assemblages by multiannual drought events, in a Mediterranean river. Here, we studied the influence of multiannual drought events on the decomposition process of leaf detritus and on the recovery of macroinvertebrate assemblages, both in the leaf-bag and in the bottom samples. Two sites, Acciano and Tione, located in a third order branch of a Central Apennines river (Abruzzo, Italy) were monitored form 2007 to 2009 with the aim to record the drought events during summer and bottom samples of macroinvertebrate assemblages were collected in the flowing period of the summer. In the monitored years, Acciano was exposed to drought events during the summer of each monitored year, while Tione was always flowing. In the 2010, we performed an experimental design to analyse the effect of progressive desiccation on leaf detritus decomposition process and on macroinvertebrate assemblages from the leaf-bags and bottom samples. Unexpected, during the 2010 year Acciano was always flowing and sampling was carried out from June to September. Since Tione was right, the unexpected event was a great fortune for us because we were able to analyze the recovery of both functional process and macroinvertebrate community structure and traits in Acciano site respect to Tione site, after multiannual seasonal drought disturbance. Decomposition process was analyzed in both sites and in 4 periods (from June to September, monthly) using the leaf-bag technique; 12 leaf-bag containing 3 g of dry Phragmites australis (Cav.) Steudel leaves were submerged at each site and period, and retrieved after 30 days. Macroinvertebrate assemblages were collected from both leaf-bag samples and bottom samples, and the main structural descriptors and traits were compared between sites and among sampling periods. Bottom samples were available also in the previous three years. The obtained results showed no difference between sites of the decomposition process of Phragmites australis leaves, while significant differences of the structural descriptors and traits of macroinvertebrate assemblages were rarely observed between sites, just in some month were significant and main linked to trophic traits. Finally, this study showed that also in correspondence of climate change drought disturbance become more unpredictable mainly in Mediterranean type rivers, moreover functional processes are not affected by dryining memory, while the descriptors of trophic traits seem to be more affected by dryining memory.