Evidence of a threshold level of fine sediment accumulation for altering benthic macroinvertebrate communities

When land use practices alter natural hydrologic and sediment delivery regimes, the effects usually are negative to macroinvertebrates. We hypothesized a threshold level of fine sediment accumulation in the substrate may exist where benthic macroinvertebrate abundance and diversity will be significantly reduced. We surveyed seven Appalachian streams with different levels of substrate fine sediment twice yearly from fall 1998 to spring 2000. Three riffles (with 2 replicates each) were sampled with a 0.25 mm Surber sampler in each season and stream. Simple linear regression was used to test relationships between substrate size classes and metrics, and nested ANOVA was used to test macroinvertebrate differences among streams. Consistent negative relationships with the finest substrate particles (<0.25 mm) were observed with EPT (Ephemeroptera, Plecoptera, and Trichoptera) taxa richness. In seasons of normal hydrology, EPT taxa richness significantly decreased (p < 0.05) in streams where fine substrate particles (< 0.25 mm) exceeded 0.8-0.9% of riffle substrate composition. In drought seasons, fine sediment (< 0.25 mm) exceeded 0.8-0.9% in most surveyed streams, lowering macroinvertebrate diversity in all streams. In these streams, a threshold for EPT diversity appears to be in excess of 0.8-0.9% fine sediment (< 0.25 mm) substrate accumulation. We suggest similar threshold levels exist in other streams where macroinvertebrate taxa are altered with potential effects on trophic webs and nutrient processing.