Development of a biotic index using stream macroinvertebrates to assess stress from deposited fine sediment

Abstract

Summary Detrimental impacts of excessive fine‐grained sediment inputs to streams and rivers are well established. What is less well understood is the susceptibility of different elements of the freshwater biota to such perturbations and how such knowledge of their susceptibility could aid in identifying where excessive fine‐grained sediment is impairing ecological condition. Following the collection of biological and sediment data from 179 streams across England and Wales, representative of a range of river types over a gradient of fine sediment loading, objective statistical approaches were applied to establish relationships between the macroinvertebrate assemblage and fine‐grained sediment inputs to river channels. Having factored out that portion of the biological variation associated with natural environmental gradients, a model comprising mass of organic sediment in erosional areas of the stream bed [predominantly associated with the first axis of the partial canonical correspondence analysis (pCCA)], and mass of fine‐grained sediment in the surface drape of depositional areas and % organic content in erosional areas (associated with the second axis of the pCCA) as explanatory variables best accounted for the residual variation in the macroinvertebrate assemblage. The relative position of taxa along both axes of the pCCA, provided a ranking of taxa in relation to the two gradients of fine‐grained sediment and provided the basis for a new empirically derived diagnostic index for fine‐grained sediment stress in rivers. Two sub‐indices were derived to capture the assemblage responses to both the gradient of organic sediment in erosional areas and the gradient of total fines in depositional areas. The two sub‐indices were then combined to derive the new combined fine sediment index (CoFSIsp). The index was tested on an independent test data set (comprising 127 samples from 83 sites) and was found to provide a robust indication of benthic fine‐grained sediment conditions (Spearman's rank correlations ρ = −0.519 to −0.703). The strength of correlation with the total fine‐grained sediment gradient was always greater than that for other routinely used indices, confirming that CoFSIsp offered additional explanatory power when assessing this stressor of aquatic environments.