Macroinvertebrate Structure and Drift in the Blanco River: A Karst Texas Stream Subject to Hydrologic Variability
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Patterns in benthic macroinvertebrate communities were assessed within the Blanco River drainage of central Texas to determine structuring mechanisms related to abiotic and biotic influences of the watershed. The Blanco River is a flashy karst system and naturally fragmented because of water loss to the Trinity and Balcones Fault Zone Edwards aquifers and because of unique geological formations in the middle reaches. Seven mainstem sites, two tributary sites, and two seep sites were sampled qualitatively and quantitatively within the Blanco River drainage seasonally from October 2003 through July 2005. In addition, 24-h drift samples were taken in spring, summer and fall from three sites. Ephemeroptera, Trichoptera and Diptera were the most abundant taxa in benthic and drift samples, although riffle beetles (Coleoptera) and Argia (Odonata) were among the most ubiquitous. Habitat, site, and season explained 33% of community variation in the drainage (F = 1.35, P < 0.01). Current velocity and substrate were primary habitat factors associated with taxa occurrence and distribution. Adjacent communities differed (analysis of similarity: R = 0.086, P = 0.02) among mainstem sites suggesting local habitat factors were more important than spatial factors in determining community structure. Tributary and seep communities were similar through time and provided habitat for several taxa not found in the mainstem and habitat for source populations for many mainstem taxa. Biodiversity of the Blanco River watershed is dependent upon the diversity and persistence of mainstem, tributary, and seep habitats. However, structure and function of the macroinvertebrate communities and their habitats are susceptible to degradation by increasing surface and groundwater withdrawals, urban development, and water detention devices such as low-head dams.