Trait-based structure of invertebrates along a gradient of sediment colmation: Benthos versus hyporheos responses

Streambed colmation by fine sediment, e.g. the deposition, accumulation and storage of fines in the substrate, is known to have severe effects on invertebrate assemblages in both the benthic and hyporheic zones but the changes in biological attributes of invertebrate assemblages related to colmation have never been considered simultaneously for these two zones. We studied the effects of colmation on the invertebrate assemblages of three rivers, testing a priori hypotheses on the biological attributes that should be more selected in communities subjected to different levels of colmation in both zones. Only the proportion of organisms with high fecundity increased and the proportion of small-sized organisms decreased along the colmation gradient in both zones simultaneously. As expected, a higher number of traits were significantly modified with colmation in the benthic vs. hyporheic assemblages. Most of the biological attributes impaired were different in the two zones. In the benthic zone, colmation mainly selected particular physiological or trophic characteristics of species and features related to their resistance or resilience capacities. In contrast, the morphological attributes of species were much more impaired by colmation in the hyporheic zone than in the benthic zone. In clogged benthic habitats, traits seemed to be more impaired by an increase in physico-chemical constraints (e.g. the reduction of oxygen availability) and a reduction of potential exchanges (including exchanges of food resources) due to a decline in stream bed conductivity. The morphological attributes of the hyporheic species were probably more influenced by changes in interstitial space characteristics. A potential indicator of the effects of colmation on river health may be based on the functional traits of benthic communities because they (i) satisfy the WFD recommendations, (ii) respond consistently along a colmation gradient and (iii) are comparable among assemblages even across ecoregions that differ in their taxonomic composition.