The beaver dam as an ecological factor determining the composition and functionality of the benthic copepod assemblages of the Tiber River

With few exceptions, copepods dominate over meiofaunal taxa in freshwater benthic habitats. Beaver dams induce changes to stream morphology that could alter the composition and functionality of benthic invertebrate assemblages, affecting food webs and ecosystem function. However, there are no studies investigating the effects of beaver activities on benthic invertebrates, at least to our knowledge. This study aims to monitor the benthic copepod assemblage of a beaver-dammed segment of the Tiber River, where the dam is the first evidence of the activity of the Eurasian beaver Castor fiber L. ever reported in Italy since the XVI century. The dam altered the granulometry of the segment by creating a pond upstream of the dam (with a prevalence of gravel, sand, and silt) and a lotic reach downstream of the dam (with a prevalence of small pebbles and gravel). From February to September 2022, we monitored the benthic copepod assemblage of the dammed segment of the Tiber River to test the hypothesis that the composition and functionality of the benthic copepod assemblage of the pond would differ from that downstream of the lotic reach. At the end of September 2022, the dam was swept away by a flood. The granulometry of the stream segment was highly altered, and most of the sand and silt were washed out. In November 2022, we monitored the copepod assemblage to test the hypothesis that the composition and functionality would differ among habitat types (undammed vs. dammed stream segments). We also analyzed the hydrochemistry (pH, conductivity, major ions and nutrients) of the pond, lotic reach, and undammed segment. Overall, the Tiber River in the analyzed segment showed a good water quality, , with no significant differences in the chemical composition of the pond and lotic reach and of the dammed and undammed segments. We observed no differences in copepod taxonomic composition, life traits and biomass between the pond and lotic reach. On the contrary, the taxonomic composition, life traits and biomass of the undammed segment differed significantly from the dammed one. The dammed segment showed 60% fewer specimens on average, a higher number of nauplii, lower biomass, and a lower number of ovigerous females, adults and copepodids, than the undammed segment. Nitokra hibernica hibernica (Brady, 1880), which was more abundant in the undammed segment, explained 33% of the differences, followed by Bryocamptus (Echinocamptus) echinatus(Mrázek, 1893). Our preliminary results suggest that the recolonization of beavers in Italy may be altering stream morphology and benthic copepod assemblages. However, the abnormally high temperatures recorded in November 2022 might have altered the cycles of the species whose abundance usually peaks in summer. Further monitoring and studies involving other functional traits, such as locomotion, the relationship with the substrate and feeding habits, are necessary to attain any definitive conclusions.