Species depletion profiles as an alternative to streamflow alteration thresholds in a hydroecological risk assessment

Tradeoffs between human demands and ecological consequences of water resources necessitate setting limits to the hydrologic alteration of rivers - these limits or thresholds are required to prescribe environmental flows to maintain the ecological integrity of river ecosystems. Ideally, water managers could derive these thresholds empirically from hydroecological studies. Unfortunately, recent synthesis studies suggest that clear thresholds for terrestrial and aquatic ecosystems are elusive - if they exist at all. The field of ecological risk assessment (ERA) offers many approaches to describe and manage the prevalence and threat of an environmental stressor and establish limits of that stressor. This study introduces a hydroecological risk assessment (HERA) to identify thresholds of hydrologic alteration using an approach from ecological risk assessment and a stressor derived from publicly available hydrologic data. Rather than a single threshold value for ecological responses to hydrologic alteration, this study found that species exhibit variable patterns of response to depletion across their ranges - what we term Species Depletion Profiles (SDPs). These SDPs correlate with population health of the species in question, with similarities in conservation status and distribution evident among species with similar SDPs. Our results suggest that a well-used approach common to aquatic ecotoxicology has potential application in devel-oping environmental flow guidelines for protection and remediation of riverine systems subject to anthropogenic influences where specific flow-ecology relationships are unknown.