Biological indices to characterize community responses to drying in streams with contrasting flow permanence regimes

Many river networks include temporary reaches that stop flowing and may dry during unpredictable droughts (near-perennial) or more frequently (intermittent). A few biological indices have been developed to assess invertebrate community responses to hydrological variability, including the instream conditions associated with drought, but their performance in temporary streams remains poorly known. We evaluated the ability of two such indices, the Lotic-invertebrate Index for Flow Evaluation (LIFE) and the Drought Effect of Habitat Loss on Invertebrates (DEHLI), to predict responses to flow cessation and drying in temporary streams with contrasting flow permanence regimes. We used a 26-year dataset comprising spring-season invertebrate community samples and daily discharge measurements from 46 sites in a cool, wet temperate region, to examine relationships between hydrological variables and changes in index scores. We also identified taxon-specific thresholds at which occurrence changed with increasing drying and flowing durations. Both indices effectively characterized responses to increasing no-flow durations. DEHLI also reflected community changes following flow resumptions, identified differences in responses among flow permanence groups, and was particularly able to predict community responses at near-perennial sites. DEHLI scores at near-perennial sites took on average three years after a drying event to return to values typical of perennial sites, whereas responses to increasing flow duration were more erratic at intermittent sites. Lotic specialists declined whereas lentic and semi-aquatic taxa increased in occurrence with no-flow duration after summers with < 50 days without flow, due to changes in the availability of preferred habitat types. Community responses to drying events were less predictable among intermittent than near-perennial sites, likely because differences in habitat conditions and connectivity may lead intermittent communities to harbour contrasting pools of species with strategies that promote persistence during and/or recolonization after drying. We identify DEHLI as an index that can characterize community responses to drying in temporary streams with contrasting flow permanence regimes. We also recommend the development of new indices that include lentic, semi-aquatic and terrestrial as well as lotic taxa, to more comprehensively describe and predict community responses to changing instream conditions.