Recolonization and alterations of aquatic invertebrate communities from perennial and intermittent stream sites following two extreme droughts within 7 years

Background/Question/Methods

Extreme droughts are known to eliminate aquatic biota and alter community structure. Following a supra-seasonal drought in North-central Texas, benthic macroinvertebrate communities from a perennial stream and a downstream intermittent site were studied over 18 months. The study stream was the only stream in the region to maintain flow throughout the 2005/2006 drought. Centrally located in the southern United States there are no comparable studies at this latitude that have studied benthic community recolonization following a supra-seasonal drought at an intermittent site downstream from a perennially-flowing stream reach in a groundwater-dependent stream system. The objectives of this study were to 1) determine the recolonization rate of benthic macroinvertebrates following a supra-seasonal drought that extended over two winters in a groundwater-dependent ecosystem and 2) determine if the perennial headwaters’ flow permanence supports a stable benthic community as indicated by trophic structure. We hypothesized that sites with permanent flow throughout the drought would support higher taxa richness and densities of benthic macroinvertebrates throughout the study period, and that due to effects of extreme drought, recolonization and recovery of benthic macroinvertebrates at an intermittent site would take longer than periods reported for other intermittent streams.

Results/Conclusions

Following 2006 drought, perennial headwaters supported higher mean taxa richness, densities and Shannon’s Diversity than the intermittent site over the duration of the study.  However, taxa richness at the perennial headwaters declined from August 2006 to April 2008 which may be related to multiple spates spring 2007 and may also reflect lag effects of the drought on processing organic matter in the hyporheic zone. Recovery in shredder densities, Marilia sp. (Trichoptera: Odontoceridae) and Tipula sp. (Diptera: Tipulidae), post-drought took over a year at both sites. Recovery of taxa richness at the intermittent site took 9 months compared to 1 to 2 months reported in arid and semi-arid streams in the United States recovering from seasonal drying and floods. After a second extreme drought in 2011, stream sites were sampled spring 2012, and these results will be presented. Groundwater-dependent headwaters are imperiled ecosystems that provide a range of ecosystem services. Stressors of predicted climate change and unsustainable groundwater abstraction places these systems at high risk for passing a critical threshold and changing to a state with a diminished capacity to deliver ecosystem services and recover from disturbances.