Benthic macroinvertebrate population dynamics in an intermittent north Texas prairie stream: What happens during drought?

Background/Question/Methods

Benthic macroinvertebrate population dynamics were studied over a year long wet - drought cycle in Clear Creek, an intermittent stream (watershed ~ 236,000 acres²) located in the Cross Timbers Ecoregion of North Central Texas. Drought cycles in this stream, a tributary of the Elm Fork of the Trinity River, are unpredictable.  The benthic macroinvertebrate community in Clear Creek during normal flow has been studied for a number of years, and provides a good historical record.  During flow years the riffles are populated with Chironomidae (Diptera), the dominant organism, followed by Plecoptera, Coleoptera, Ephemeroptera, Trichoptera and other Diptera.  Flow began to diminish during the fall and early winter (September – February) of 2005 and the summer and fall (June – November) of 2006, when mean monthly discharge decreased from 98 to 0 cfs.  During the drought the streambed completely dried.  The main objective of this study was to describe recolonization dynamics of the aquatic invertebrates within Clear Creek.  Sampling began in February 2005 and continued through March 2007, during which eleven collections were made over the sampling period at six different sites, encompassing approximately 67 km of the 76 km stream.  Various benthic sampling techniques were employed to ensure an adequate representation of the recolonizing organisms. These included: D-frame kicknets, drift nets, floating emergence traps, Malaise traps and Hess samplers. Additionally, floating emergence traps were placed in remaining pools and sediment samples were collected in order to rear drought resistant life stages of invertebrates. 
Results/Conclusions

Results suggest that populations of Ephemeroptera, Coleoptera, Diptera and Odonata decreased during drying periods and the remaining taxa (Diptera: Chironomidae) sought refuge in pool and hyporheic habitats.  Monitoring recolonization of invertebrates began in February, approximately one week after flow returned using drift nets, directional malaise traps and D- frame kicknet samples.  Preliminary data from drift net samples indicate (arranged in order of abundance, high to low) Amphipoda and Diptera (Chironomidae), Collembola, Coleoptera (Haliplidae), and Ephemeroptera (Baetidae) were dispersing from upstream pools and the hyporheic zone which acted as a refugium during the drought.  Post-drought, riffle-inhabiting macroinvertebrate counts were highly variable.  During this period Diptera (Chironomidae) and Plecoptera (Perlidade) were the dominant taxa.  Over time, community structure began to resemble pre-drought assemblages and variability in population counts of the dominant organisms was reduced.  Understanding the dynamics of these systems is important to the development of effective bioassessment techniques and conservation practices in the future.