Contrasting use of hyporheic habitat by benthic invertebrates during spates and low flows

Abstract

Spates are common disturbance events in lotic ecosystems, with high flows mobilising bed sediments and displacing biota. During spates, invertebrates inhabiting benthic sediments are typically exposed to higher flow velocities than those in the hyporheic zone, which may therefore act as a refugium. Between spates, declining discharge typically reduces the area of submerged sediments, concentrating populations of mobile taxa into a smaller inhabitable space. In such cases, the hyporheic zone has the potential to provide refuge from increasing biotic pressures in the surface stream. We compared the effects of a spate and flow recession on the abundance of benthic invertebrates in the hyporheic zone of a groundwater-dominated river. During flow recession, a reduction in the area of submerged sediments was associated with an increase in the population densities of the dominant amphipod Gammarus pulex. Subsequent increases in the hyporheic abundance of G. pulex and in the proportion of the total (benthic + hyporheic) population inhabiting the hyporheic zone indicated that these sediments acted as a refugium from biotic pressures in the benthic zone during low flows. During the spate, a decline in invertebrate abundance was most pronounced in the surface sediments, but significant reductions were also recorded in the hyporheic zone, indicating that the refugial capacity of these sediments was limited.