Hydraulic constraints on habitat suitability for benthic invertebrates in gravel‐bed rivers

Abstract The primary components of the physical habitat in a stream are water depth, velocity and substrate size, and these appear to be the best predictors of benthic invertebrate distribution within a stream. Substrate stability and fine sediment deposition also influence benthic invertebrate abundance, with reduced abundance where substrates are frequently disturbed or where fine sediment accumulates. Substrate stability and deposition of fine sediment provide hydraulic constraints on habitat suitability and their limits can be predicted by hydraulic formulae. Hydraulic conditions near or at the surface of the stream bed may have a more direct influence on benthic invertebrates than either water depth or mean velocity in the water column above them. If benthic invertebrates have ‘preferred’ near‐bed conditions, hydraulic considerations suggest that optimum mean water velocities for benthic invertebrates should increase with average stream depth and substrate size. Examination of three sets of field data from rivers of varying size showed that benthic invertebrate abundance was strongly related to mean velocity and substrate size, with large differences in habitat use between species. However, although there were large differences in preferred mean velocities between rivers of different size and depth, there was little evidence that this occurred within rivers of similar size and hence little support for the hypothesis that near‐bed hydraulics can provide a generalized method of predicting invertebrate habitat suitability between rivers of different size. Copyright © 2003 John Wiley & Sons, Ltd.