Reach and catchment scale influences on invertebrate assemblages in a river with naturally high fine sediment loads

Stream invertebrate distribution patterns reflect local sedimentary and hydraulic conditions, which in turn are influenced by a range of factors operating at larger scales. We assessed whether spatial variation in invertebrate assemblages across a meso-scale catchment is best understood in terms of the characteristics of the study reaches themselves or the characteristics of respective upstream catchment areas. The study river experiences naturally high fine sediment loads as a result of the extraordinary supply of sediment from high erodible marls in its catchment. We hypothesized that between-reach variation in the volume of fine sediment stored within the channel results from a combination of reach and upstream catchment characteristics, and that these characteristics help explain variation in invertebrate assemblages. The storage of fine sediment in study reaches correlated with a number of upstream catchment characteristics, as well as reach-scale hydraulic conditions. Variability in invertebrate assemblages correlated most strongly (62% of variance explained) with the characteristics of the catchment upstream from each reach (area of contribution), with the characteristics of the reaches accounting for only 35% of the variability. The explanatory power of the reach-scale habitat variables was reduced when the effect of upstream catchment conditions was removed. This suggests inbuilt effects of larger scale conditions on reach habitat and invertebrate assemblages. Results lend support to theories of scale hierarchy within river systems and help emphasize the need to target management at upstream catchment areas.