Detecting the structural and functional impacts of fine sediment on stream invertebrates

Fine sediment is one of the major sources of stream physical and ecological impairment worldwide. We assessed the ecological effects of fine sediment in an otherwise undisturbed catchment (the Isábena, NE Spain). Using data from sites across the catchment we describe the spatial variability and nestedness of invertebrate assemblages and evaluate the effectiveness of compositional (taxon-based) and functional (trait-based) metrics for detecting sediment impacts on these assemblages. Invertebrate assemblages were relatively taxon poor and had low densities in those locations with high fine sediment content. Assemblages showed significantly nested patterns, with those in sediment-rich locations consisting of a subset of those in locations with little fine sediment. A number of biological traits appeared to promote this nestedness, particularly those conferring resistance and resilience to fine sediment (polivoltinism, short live cycles and small body sizes). Generalised Additive Models indicated that most metrics were able to detect ecological responses to sedimentation. Some taxon-based metrics (%EPT and evenness) performed less well, with values showing only a weak relationship with fine sediment. Results are consistent with previous studies which have highlighted the limitations of taxon-based metrics and suggest that indices of functional diversity are capable of detecting sediment related impairment. Overall, the study suggests that fine sediment in the Isábena was selecting for specific life-history traits, and that this selection resulted in clear differences in assemblage structure across the catchment. The use of biological traits in studies of sediment related disturbance may help identify extinction-prone species (e.g. those with univoltine and/or long life-cycles), while trait-based monitoring and assessment metrics, because they reflect the ecological mechanisms underlying observed patterns, should prove useful to help guide management in catchments subjected to excessive fine sediment. More broadly, the study indicates that nestedness in assemblage structure can be driven by local habitat changes, and not only by large scale biogeographical processes.