Ecology and recolonization of benthic macroinvertebrates in a groundwater-dependent stream in North Central Texas during a supra-seasonal drought

Extreme climatic events such as droughts are known to eliminate aquatic biota and alter community structure and function. Perennial headwater springs provide important drought refugia to benthic macroinvertebrates and an important source of colonists via drift or aerial adults to intermittent streams post-drought. During a supra-seasonal drought in North-central Texas summer and fall 2006, benthic macroinvertebrates from persistent groundwater-dependent macrohabitats of varying hydrological connectivity and riparian shading were studied: perennial riffles, connected pools, shaded disconnected pools, and full sun disconnected pools. Riffles were a distinct habitat with significantly higher taxa richness, proportion of lotic taxa, diversity and evenness than other macrohabitats. Macrohabitats were found to be important refugia for 106 benthic macroinvertebrates and 4 microcrustacean taxa. Throughout the extreme drought, perennially flowing habitats were refugia to 19 taxa (17.9% total taxa) not collected in disconnected pools. Shaded disconnected pools contained lotic taxa not previously known to be able to complete their lifecycles in lentic habitats, emphasizing the importance of groundwater effluent and shading. With the resumption of flow at a downstream intermittent site of Ash Creek in mid-October 2006, an annual recolonization study was conducted comparing the perennial headwaters' benthic macroinvertebrate taxa richness, densities and community ecology with the downstream intermittent site. The headwaters supported higher mean taxa richness than the intermittent site over the duration of the study (ANOVA P < 0.001). However, the unexpected result of overall decreasing taxa richness at the perennial headwater site from August 2006 to April 2008 appears to reflect lag effects of the supra-seasonal drought combined with effects of multiple spates of 2007, which are factors confounding the point of recovery for taxa richness. Recovery of taxa richness at the intermittent site took 9 months compared to 1 to 2 months reported in other arid and semi-arid streams in the United States recovering from seasonal drying and floods. Sustainable use of groundwater resources and conservation of riparian corridors is vital to protecting groundwater-dependent ecosystems that play a vital role in maintaining regional biodiversity by serving as biotic refugia during catastrophic disturbance.