Macroinvertebrate recolonization dynamics in response to drought and flood in three Austin, Texas streams: effects of urbanization.

B.L. Holmes and T.L. Arsuffi. Department of Biology-Aquatic Station, Southwest Texas State University, San Marcos, TX 78666

Impervious cover of urbanized areas exaggerates the hydrologic disturbance (intensity of spates and duration of dry periods) common in central Texas. The objective of this study was to determine how benthic macroinvertebrate community composition, diversity, resilience, and recolonization in three Austin, TX streams that vary (3%, 16%, 55%) in degree of impervious cover are affected by such conditions. Benthic macroinvertebrates were quantified in 3 riffles in each of the 3 streams. Recovery from drought and flood were determined by: 1) 2 bi-weekly samples after flow resumed in September 2001, following the summer dry period and monthly sampling until flood disturbance; 2) 2 bi-weekly samples after flooding in November 2001, and monthly sampling thereafter for three months. Hydrologic disturbance had a larger effect in the most urbanized watershed, where taxa richness and abundance was lowest and chironomids dominated. Greatest species richness occurred at the moderately disturbed stream where Baetidae, Elmidae and Chironomidae were dominant. Results indicated that rate of recolonization following disturbance was inversely related to degree of impervious cover. Impervious cover appears to interact with natural hydrologic disturbances in determining structure and function of the benthic community in urbanized streams.