Habitat-specific invertebrate responses to hydrological variability, anthropogenic flow alterations, and hydraulic conditions

© 2019 The Authors. Freshwater Biology Published by John Wiley & Sons Ltd. Quantifying ecological responses to river flow regimes is a key scientific approach underpinning many environmental flow (e-flow) strategies. Incorporating habitat-scale influences (e.g. substrate composition and organic matter cover) within e-flow frameworks has the potential to provide a broader understanding of the causal mechanisms shaping instream communities, which may be used to guide river management strategies. In this study, we examined invertebrate communities inhabiting three distinct habitat groups (HGs—defined by coarse substrates, fine sediments, and the fine-leaved macrophyte Ranunculus sp.) across four rivers (each comprising two study sites) within a single catchment. We tested the structural and functional responses of communities inhabiting different HGs to three sets of flow-related characteristics: (1) antecedent hydrological (discharge—m3/s) variability; (2) antecedent anthropogenic flow alterations (percentage of discharge added to or removed from the river by human activity); and (3) proximal hydraulic conditions (characterised by the Froude number). The former two were derived from groundwater model daily time series in the year prior to the collection of each invertebrate sample, while the latter was collected at the point of sampling. While significant effects of hydrological and anthropogenic flow alteration indices were detected, Froude number exerted the greatest statistical influence on invertebrate communities. This highlights that habitat-scale hydraulic conditions to which biota are exposed at the time of sampling are a key influence on the structure and function of invertebrate communities. Mixed-effect models testing invertebrate community responses to flow-related characteristics, most notably Froude number, improved when a HG interaction term was incorporated. This highlights that different mineralogical and organic habitat patches mediate ecological responses to hydraulic conditions. This can be attributed to HGs supporting distinct taxonomic and functional compositions and/or providing unique ecological functions (e.g. flow refuges), which alter how instream communities respond to hydraulic conditions. While the individual importance of both flow and small-scale habitat effects on instream biota has been widely reported, this study provides rare evidence on how their interactive effects have a significant influence on riverine ecosystems. These findings suggest that river management strategies and e-flow frameworks should not only aim to create a mosaic of riverine habitats that support ecosystem functioning, but also consider the management of local hydraulic conditions within habitat patches to support specific taxonomic and functional compositions.