EXPERIMENTAL EFFECTS OF SEDIMENT DEPOSITION ON THE STRUCTURE AND FUNCTION OF MACROINVERTEBRATE ASSEMBLAGES IN TEMPERATE STREAMS

Fine sediment in streams and rivers is one of the most globally widespread of all freshwater pollutants. However, the ecological implications are still poorly quantified, and field experiments to assess likely functional and structural effects are scarce. We assessed the response of stream invertebrates to fine sediment (i.e. inert sand) added to trays (n = 65) containing otherwise natural substrata over a three-week period in three replicate streams in the Usk catchment, Wales. Sediment addition to 0.6-18 kg m(-2) affected both the structure and functional composition of invertebrate assemblages while also reducing overall abundance and in some common species (Baetis rhodani, Ecdyonurus sp. and Leuctra geniculata). Sediment also reduced richness and overall trait diversity (TD), while different life-history traits were either favoured (polivoltinism, tegumental respiration and burrowing behaviour) or disfavoured (swimmers, attached taxa, gill respiration). Moreover, sediments appeared to promote a nested subset pattern in species composition, with generalists favoured at the expense of specialists either through exclusion or impaired colonization. Effects were due largely to the loss of five taxa that contributed to the significant nestedness across the sand gradient: B. rhodani, Ecdyonurus sp., Leuctra geniculata, Simuliidae and Ephemerella ignita. This short-term experiment supports recent surveys in indicating how sedimentation can change the structural and functional composition of stream invertebrate assemblages even at low to moderate rates of deposition. In revealing direct effects on trait adversity, trait representation and nestedness, the data are also consistent with survey data in indicating that sediments have potentially important ramifications for conservation by removing organisms systematically according to life-history features. Copyright (C) 2010 John Wiley & Sons, Ltd.