Hydraulic habitat use with respect to body size of aquatic insect larvae: Case of six species from a French Mediterranean type stream

Macroinvertebrates play a key role in lotic ecosystems, as fish prey and processors of organic material. Therefore, their hydraulic preferences have to be integrated in instream habitat models for ecological stream management. This study characterized physical habitat use in terms of shear velocity for the larvae of three Ephemeropteran (Ephoron virgo, Oligoneuriella rhenana, and Serratella ignita), two Trichopteran (Cheumatopsyche lepida and Hydropsyche exocellata) and one Dipteran species (Blepharicera fasciata) in a Mediterranean stream at a relatively low water discharge. O. rhenana, C. lepida, H. exocellata, and B. fasciata larvae were mainly found in high shear velocity conditions, whereas E. virgo and S. ignita larvae were found in low shear velocity conditions. Knowing that habitat preferences should vary during ontogenesis (with respect to changes in biological requirements and/or morphological abilities to withstand high flow, for example), our second objective was to characterize differences in the hydraulic habitat use (in terms of shear velocity) for different size classes of these six species.Larvae of H. exocellata and B. fasciata mainly colonized high shear velocity conditions and numerous individuals of these species also used medium shear velocity conditions, independent of size class.The use of high shear velocity conditions increased with larval size for C. lepida and O. rhenana, whilst the use of low shear velocity conditions increased for larger larvae of E. virgo and S. ignita. Various hypotheses are proposed to explain these different strategies of habitat use during ontogenesis. We point out the lack of knowledge about physical habitat shifts during the larval growth of freshwater invertebrates.These results highlight the interest to consider the respective habitat requirements of different size classes of invertebrates in instream habitat models. Population bottlenecks should be overcome if hydraulic conditions are kept suitable for all size classes by stream managers.