| Summary: | Ammonium has detrimental effects on the survival and physiology of freshwater invertebrates, however, the effects of ammonium on the functionality of hyporheic invertebrate assemblages have never been investigated. In this study, we applied a mixed (taxonomy- and trait-based) approach to explore the taxonomic and functional trait variation along a gradient of ammonium contamination in the hyporheic zone of a Mediterranean stream. We analysed fifty-one trait modalities of thirty-eight invertebrate taxa. We tested a priori hypotheses on the functional traits that should be affected. We observed a severe erosion of taxonomic and functional diversity with the loss of 12 taxa and 11 trait modalities related to size and body form, fecundity and reproduction, resistance forms and respiration, diet, locomotion and feeding habits. The effect on taxonomic and trait modality abundances was also relevant; in particular, the decrease in detritivores seemed to have led to a low content in dissolved organic matter. Synergic detrimental effects of ammonium and the summer season were also observed. Despite some uncertainties, we found out that the mixed approach responded consistently along a gradient of ammonium contamination. Since functional traits link community organization to ecosystem goods and services, the results of our study could be prodromal to the inclusion of ammonium as indicator of functional stress in a health index of hyporheic zones in Mediterranean rivers. © 2021
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