Reach‐scale benthic substrate composition shapes hyporheic invertebrate communities and physicochemical characteristics under baseflow conditions

Understanding the processes structuring ecological communities is vital to underpin research, management and monitoring efforts. Historically, research has focused on the role of the habitat template in structuring benthic riverine communities, with limited consideration of subsurface hyporheic habitats. Here we investigated the role of reach‐scale benthic substrate composition (cobble, pebble or granule) in controlling hyporheic physiochemical characteristics and macroinvertebrate communities at 19 sites across England and Wales under baseflow conditions. We observed that different benthic substrate compositions supported distinct hyporheic physicochemical characteristics and macroinvertebrate communities under baseflow conditions. Hyporheic functional richness and divergence were found to vary by substrate type, whilst taxonomic richness, abundance and EPT richness were relatively consistent. Hyporheic beta diversity was found to differ, with greatest community heterogeneity observed in reaches characterised by intermediate substrate grain size composition (e.g. pebble) which was driven by equal contributions of richness‐differences and replacement. In marked contrast, replacement was found to structure differences in community composition in coarser (cobble) sites relative to fine grained (granule) sites. We observed that despite strong differences in benthic substrate composition, this did not extend to differences in the hyporheic substrates with grain size distributions being comparable across the three benthic substrate groups. Our results therefore suggest that under stable base flow conditions, reach scale benthic substrate context may play an influential role in structuring hyporheic physicochemical conditions and macroinvertebrate communities through effects beyond grain size alone (e.g. hydrological exchange and habitat conditions). This study provides vital insights into the mechanisms and processes that structure hyporheic communities, and we call for further studies that investigate environmental controls temporally to reflect the dynamic hydrological conditions experienced in lotic systems.