Influence of mild to severe flow alterations on invertebrates in three mountain streams

Invertebrates were compared upstream versus downstream from diversions on three small, headwater streams in the central Rocky Mountains, USA. Flow alteration of these streams varied from mild (some aspect of all natural flow components was transferred downstream) to severe (nearly 100% of the flow was diverted for 10–11 months of the year). The analysis was separated into periods of frequent (diversion gates often opened and closed; April–October) and infrequent flow fluctuations (stable low flows due to constant diversion; November–March). Invertebrates appeared resilient to mild flow alterations as neither the abundance, diversity, nor spatial and temporal variation in abundance and diversity differed upstream versus downstream during either period. In severely diverted streams, however, total invertebrate density downstream from the diversion was only 50% of upstream. Invertebrate diversity was also reduced; ten taxa abundant upstream were absent in downstream sections. Chironomids, ostracods and Ameletus spp. comprised 80% of total invertebrate density during constant, low flow conditions in the severely diverted streams. Although all taxa in the severely diverted streams recovered (drift) during the period of frequent flow fluctuations, spatial and temporal variation (coefficient of variation) in both density and the number of taxa was significantly greater downstream. Depending on the frequency with which free-flowing conditions were re-established, many invertebrate populations (especially mayflies and some stoneflies) declined or were even locally extirpated (e.g. Hesperoperlapacifica, Megarcyssignata, Neothremmaalicia, Polyceliscoronata). Downstream communities in severely diverted tributaries appeared to fluctuate between two stable endpoints; a depauperate low-flow community dominated by chironomids and ostracods and a more abundant and diverse natural-flow community dominated by mayflies, chironomids, ostracods, stoneflies and caddisflies. Water abstraction (extent and timing of diversion) could be managed to minimize risks to downstream ecological resources.