Scale-dependent effects of fine sediments on temperate headwater invertebrates

1. Anthropogenic activities can increase fine sediment supply to streams over multiple spatial and temporal extents. Identifying the processes responsible, and the scale at which any effects on stream organisms become evident, are key management needs, but appropriately scaled surveys are surprisingly few. 2. We surveyed macroinvertebrates and superficial fine sediments at two spatial resolutions (reach- and patch-scale) in tributaries of the River Usk, a temperate, montane catchment in rural Wales (U.K.). Land use, habitat and geomorphological character were measured on-site or derived from an existing database (=Fluvial Audit). We aimed to identify: (i) how in-stream sediments varied with land use and associated geomorphology; (ii) likely consequences for macroinvertebrates and (iii) any scale-dependence in relationships between macroinvertebrates and sediment character. 3. At both the reach- and patch-scales, bed cover by fine sediment was related directly to the extent of eroding banks 500 m upstream. In turn, sedimentation and bank erosion were negatively correlated with catchment or riparian woodland extent. 4. At the reach scale, macroinvertebrate composition varied with catchment land use and stream chemistry, with richness declining as rough grazing or woodland was replaced by improved grassland. There was no response to deposited sediment except for weak increase in the relative abundance of oligochaetes. 5. By contrast, at the patch scale, fine sediments were accompanied by pronounced changes in invertebrate composition, and we ranked the 27 most common taxa according to their apparent sediment tolerance. General estimating equations showed that total and Ephemeroptera, Plecoptera and Trichoptera richness decreased significantly by 20% and 25% at the most sediment impacted sites (30% cover) by comparison with sediment-free sites. 6. We conclude that sediment deposition in the upper Usk system mostly reflects local bank erosion, with riparian woodland likely to mediate this process through bank stability. Fine sediment release had marked ecological effects, but these were detectable only at patch-to-patch scales. We suggest that investigation of localized sediment release in streams will benefit from scale-dependent or scale-specific sampling, and some effects could go undetected unless sample resolution is selected carefully