Responses of Aquatic Benthic Macroinvertebrate Communities to a Large Flow Pulse in the Guadalupe, San Antonio and Brazos River Basins, Texas
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Riverine benthic macroinvertebrates (BMI) communities are regulated, in part, by the dynamic character of the river’s flow regime. Purpose of this study was to assess the influence of a flow regime component (i.e., large flow pulse) on BMI riffle communities, specifically that large flow pulses shifted structure (i.e., richness and density), and therefore maintain the biotic integrity of riverine riffle BMI communities. Predictions were that BMI richness and density would decrease and recover with large flow pulses, ranging between 1 in 2 year events (340 m3/s) to 1 in 5 year events (331 to 886 m3/s), but that density reductions and recovery would differ among taxa categorized as swift, moderate, and slack forms. BMI communities were monitored at 11 sites located in three river basins and distributed among upper and lower reaches of major rivers and tributary streams. A total of 93,400 aquatic macroinvertebrates were identified to family and used to estimate BMI richness and BMI density among 102 riffles (61 riffles pre-flood and 41 riffles post-flood) between 2014 and 2017. Physical and chemical aspects of riffle habitats were similar between pre-flood and post-flood, except that post-flood riffles had less sand and gravel than pre-flood. BMI communities were similar among river basins and were segregated along environmental gradients related to physical and chemical (16%), season (6%), and flood (2%) effects. Only a few sites differed in BMI richness and density between pre-flood and post-flood, indicating that BMI communities among seven of the 11 sites likely recovered before the post-flood sampling events. Increased densities or relative abundances were detected at four sites for swift BMI, at one site for moderate BMI, and at one site for slack BMI. Decreased densities or relative abundances were detected at one site for moderate BMI and at two sites for slack BMI. Among taxa, relative abundances of seven BMI families, which were categorized as swift or moderate, generally increased among the 11 sites, whereas relative abundances of five BMI families, which were categorized as moderate or slack, generally decreased among the 11 sites. Although increasing or decreasing trends in BMI categories and families between pre-flood and post-flood periods were not consistent among all sites, study results suggest that density and relative abundance of some BMI taxa responded positively (e.g., Baetidae, Hydropsychidae, Isonychiidae) and negatively (e.g., Elmidae, Leptophlebiidae, Chironomidae) to high flow pulses. Therefore, flow-responsive BMI taxa found in this study provide potential indicator species for environmental flow standards assessments, although the ubiquitous use of these indicator species across and within drainages is limited.