Benthic and interstitial habitats of a lentic spring as invertebrate refuges during supra-seasonal drought

Instream refuges are places where invertebrates persist during disturbances due to reduced adverse impacts. During droughts, low flows may be accompanied by elevated temperatures, and potential refuges including subsurface sediments and spring-fed headwaters are therefore characterized by hydrological and thermal stability. This study examined invertebrate use of benthic and interstitial habitats (analogous to the hyporheic zone) in a groundwater-dominated, perennial limnocrene spring during a supra-seasonal drought. Although exceptionally high air temperatures occurred as flow declined, environmental conditions in the spring were relatively stable, and refuge-seeking vertical migrations into interstitial habitats did not coincide with peak temperatures. However, maximum benthic abundance of two amphipods (Gammarus pulex and Crangonyx pseudogracilis) occurred shortly after the period of elevated temperatures. It is suggested that this temporary increase in the abundance of these mobile taxa reflected upstream migrations triggered by a combination of refuge-seeking behaviour and thermally-stimulated activity. In addition, the spring provided a passive refuge for many lotic invertebrate taxa. A conceptual model is presented, which illustrates the potential contribution of multiple refuges to invertebrate persistence in drought-impacted ecosystems