Effects of a Simulated Spate on Water Mites in the Hyporheic Zone of an Australian Subtropical River

Abstract

Posed in 1974, the 'hyporheic refuge hypothesis' predicts that the hyporheic zone, the saturated sediments below and alongside rivers and streams, acts as a refuge from the scouring effects of spates for many aquatic invertebrates including water mites. We tested this hypothesis in a lateral cobble bar of a subtropical river in Australia by collecting triplicate samples of water and water mites (6L) from the hyporheic zone at two depths (10 and 50 cm) and three sites at two 'pre-flood' sampling times before diverting water across the bar for 14h to simulate a spate. Samples were collected twice during the 'spate', and again soon after flow was returned to normal. We were also able to sample shortly after a fortuitous natural spate of equivalent magnitude to the experimental one. There were significant transient changes in subsurface hydrology and dissolved oxygen at some of the sites during the experimental spate but these did not persist and were not evident two days after the natural spate. Taxon richness was dominated by the Prostigmata (26 spp.) with at least 11 new species being discovered from this single site. Oribatids were common, and consistently occurred in all habitat-depth combinations before, during and after the spates except at the end of the long flow path where they were absent from the shallow wells before the artificial spate. Overall, our water mite data failed to support the flood refuge hypothesis. During and shortly after the artificial spate, we did not find more epigean water mites in downwelling zones where surface streamwater enters the hyporheic zone. There was no evidence for a 'wash out' effect because total hyporheic mite densities did not significantly decline late in the spate, even at the end of the short and long flowpaths where hyporheic water entered the surface stream. Multivariate analysis indicated that any effects of the artificial spate on water mite assemblage composition were only transient. For hypogean water mites, floods of the low magnitude modelled in this study apparently do not pose a lasting disturbance. Similarly, epigean water mites probably find refuge in backwaters or behind rocks near the sediment surface and do not seem to migrate actively into the hyporheic zone to avoid increased surface flows.