Sediment characteristics influence benthic macroinvertebrate vertical migrations and survival under experimental water loss conditions

Subsurface riverine sediments can provide instream refugia for benthic macroinvertebrates during drying events in intermittent streams. The permeability of sediments may strongly influence the ability of fauna to migrate vertically through benthic and subsurface substrates. We examined the vertical movement of four common taxa with different levels of mobility (Gammarus fossarum, Ephemera danica, Pisidium spp. and Tubificinae spp.) in a laboratory experiment using substrates composed of three different grain sizes (2 7 mm, 7-10 mm and 20-32 mm). Experimental substrates were placed in perforated mesocosms so that the deeper layers of the substrate remained saturated while the upper layers were subject to gradual drying. The mesocosms were inspected on five occasions over 32 days to determine the number of live specimens. We hypothesized that substrates comprising smaller particles (with smaller interstitial spaces) would be more difficult for macroinvertebrates to move through than substrates with larger interstices. We observed significant differences in the vertical distribution among substrate types for all taxa. We also hypothesized that the deeper saturated sediment layer would facilitate higher survival rates than the upper layers with reduced moisture content. The results indicated the substrate moisture content had a significant effect on the survival of G. fossarum and E. danica, but the effect was less clear for Pisidium spp. and Tubificinae spp. Our study demonstrates that sediment characteristics influence the availability of stream bed refugia for benthic macroinvertebrates and that its use depends on taxon specific abilities to access subsurface habitats.