Influence of Naturally High Fine Sediment Loads on Aquatic Insect Larvae in a Montane River

Sedimentation of fines associated with human activity is an important source of ecological impoverishment in rivers. However, there is a paucity of information on whether fine sediment associated with natural processes results in similar impairment. We assessed the effect of fine sediment on the taxonomic composition and abundance of insect larvae in a river which receives high inputs as a result of naturally erodible badland areas. An adjacent subcatchment, which does not experience high sediment inputs but is otherwise similar, was used as a reference. There were marked differences in insect density and taxon richness between the two subcatchments. The abundance of a number of insect taxa decreased as the proportion of bed material comprising fines increased. A similar decrease was observed in the EPT metric. Fine sediment exerted a dominant and homogenizing influence on community structure in the patches of streambed where it occurred in high concentrations. Between 24 and 36% of the overall variability in community structure could be explained by sedimentary conditions. In locations where when fine sediment content was limited, other habitat variables influenced insect community structure, with patch assemblages generally more diverse and heterogeneous.