The effects of suspended sediment on aquatic community structure and detritus processing

Increased suspended sediment inputs to streams as a result of human activities and disturbance have been identified as a principle source of stream impairment. Allochthonous energy inputs to riverine food webs, in the form of terrestrial leaf litter, is an important nutrient resource for benthic macroinvertebrate assemblages. Breakdown of CPOM in these systems by the feeding activity of invertebrate shredders is preceded by microbial conditioning. Instream conditions that impair leaf litter conditioning and subsequent feeding by shredders was examined by comparing leaf packs placed in four different streams in the South Fork of the Eel River in Humboldt County, California. Decomposition rates and leaf pack community structure were compared between two sediment-impacted streams and two unimpacted (reference) streams in autumn 2005. Results indicate that leaf litter decomposition in sediment impaired streams is carried out by mechanical abrasion from the suspended particles in the water. In addition, microbial conditioning of leaf litter may be inhibited creating an unsuitable food resource for shredders. Benthic invertebrate community structure changes in response to the availability of food resources.