Overs and unders. fine sediment in the riverbed or transport through a reach: Relationships with macroinvertebrates

It is widely accepted that an increase in fine sediment (<2mm particle size) transport over a riverbed and accumulation within a riverbed changes macroinvertebrate communities. Here we determine how macroinvertebrate communities relate to field measures of fine sediment accumulation and fine sediment transport and accumulation predicted by the Sediment River Network model (SedNet) for 5-10km river reaches in south-east Australia. Macroinvertebrates were sampled from riffles using kick nets, and 15cm³ baskets filled with 20mm gravel buried in riverbed. Field measurements of fine sediment accumulation included sediment depth measures, the Wolman walk, bulk sediment samples and the weight of fine sediment impregnating each basket. Macroinvertebrate communities showed a greater response to measured and predicted accumulation than predicted fine sediment transport. If predicted transport was high but predicted accumulation low, there was little effect on the macroinvertebrate communities. River reaches with high predicted accumulation had a greater mass of fine sediment within the riverbed and less fine sediment sensitive taxa. Furthermore, the impact of fine sediment accumulation on macroinvertebrate communities was more pronounced when observed accumulation cover was 100% of bed area. Therefore, accumulation within the riverbed may have a greater effect on macroinvertebrate communities than fine sediment transported over the riverbed.