The recovery of benthic invertebrate communities following dewatering in two braided rivers

The recovery of benthic invertebrates was investigated in the braided Tukituki and Waipawa Rivers ( North Island, New Zealand) following extended channel dewatering of sites for 6, 10 or 14 weeks during summer 1997/1998. After rewatering, invertebrates rapidly colonised each denuded site and, although some invertebrates were numerically dominant at different times, 95(+)% of the taxa list were present after 7 days of rewatering. The number of individuals also increased over time, but significantly greater numbers were collected in reference sites and those dry for 6 weeks compared to sites dewatered for longer periods. Taxa such as Tanytarsus spp., Maoridiamesa spp. ( Diptera: Chironomidae) and the riffle beetle Elmidae ( Coleoptera) were abundant early, while Deleatidium spp. ( Ephemeroptera: Leptophlebiidae) dominated communities later. The greater abundance of Deleatidium in sites dewatered for a short time may be because desiccation was less significant at determining taxa present at these sites, or that reference sites provided a closer source of colonists and periphyton propagules to allow a more rapid numerical recovery of this taxon. In contrast, Elmidae dominated communities at sites dewatered for longer periods because Elmidae may be more resilient to desiccation than Deleatidium. These data suggest that the duration of dewatering may regulate taxonomic composition in the short term by subjecting taxa to desiccation and/or food resource restrictions, but not in the long term because food resources recovered over time and sites with similar physicochemical conditions should have similar communities.