Potential Influence of Hydrological Flow Regimes in the Arid West on Aquatic Biological Communities – A Review

The Pima County Wastewater Management Department (PCWWM), in Tucson, Arizona has been funded by the U.S. Environmental Protection Agency (EPA) to conduct the Arid West Water Quality Research Project (WQRP) to improve the scientific base for regulation of water quality, protection of species, habitats, and uses of watercourses, and designation of appropriate treated wastewater effluent controls in ephemeral and effluent-dependent watercourses of the arid and semi-arid western states. As part of the WQRP a number of research questions or “hypotheses” have been posed. One such hypothesis postulates that hydrologic flow regimes of arid west streams (low base flows with frequent, often unpredictable flood events) are a controlling factor, which can limit the richness and abundance of aquatic communities. As part of the WQRP, we conducted an extensive literature review on the effects of flooding on the aquatic biota in lotic systems in the arid western United States. Current understanding of stream ecology suggests stream communities are structured differently by biotic and abiotic controls along some type of environmental gradient. The idea is that as one moves from a frequently disturbed, unpredictable environment to an infrequently disturbed, predictable environment, there will be a predictable change in the attributes of the biotic community. Periphyton communities appear to be the most easily disturbed from shear stress and scouring, but studies that have closely monitored recovery seldom report recovery times longer than 1 month once seasonality factors are taken into account. The literature on macroinvertebrates in unpredictable flood environments suggests that recovery in frequently flooded systems in arid regions can occur in approximately 2 months. Fish are the most limited in their ability to recolonize disturbed areas – they must either remain in place (characteristic of the southwestern fauna and their morphological adaptations to high discharge) or return from undisturbed upstream and/or downstream sources (characteristic of the small cyprinid assemblages of the western plains). EPA water quality criteria guidance assumes a 3-year recovery period is necessary (i.e., allowable exceedences are 1–3 years). This review indicates that lotic communities with high natural background disturbance frequencies are actually predisposed to recover more rapidly because only species that are able to recolonize and reproduce quickly, or perhaps to avoid disturbances, can persist there. This does not imply that they are more resistant to novel anthropogenic disturbances with which they have had no previous evolutionary experience; it only implies that they are predisposed to recover quickly once the disturbance is gone.