Flood disturbance effects on desert aquatic invertebrates with varying life histories

Authors: Laura E. McMullen, David A. Lytle

Year: 2008 (xl)

Abstract

Most desert rivers experience frequent disturbances in the form of floods and droughts. Dam operations often reduce the magnitude and frequency of these disturbance events, creating more stable flow conditions. Organisms adapted to life in desert rivers have adaptations that allow their populations to persist and even thrive despite large flow fluctuations, and the effect of practically eliminating these disturbances from the environment of native desert aquatic organisms is largely unknown. We studied the effect of three separate experimental flood releases (springs of 2006, 2007, and 2008) on aquatic invertebrate populations in the managed Bill Williams River, AZ, USA. In particular, we focused on three target desert aquatic invertebrate taxa that vary greatly in their life histories: the gray sand dragon, Progomphus borealis, a small mayfly, Fallceon quilleri, and ostracods. Effects of floods on particular aquatic species depends much on the details of their life histories. Gray sand dragon numbers were reduced immediately post floods, but numbers rebounded within weeks, indicating that larvae had been laterally displaced and recovered through rheotaxic crawling behaviors. Mayfly numbers were dramatically lowered post floods but quickly rebounded sometimes to higher levels than pre floods. This is attributed to their quick lifecycles of 7-11 days and year-round reproduction. Ostracod numbers were reduced post flood and continued to plummet in later weeks, indicating their susceptibility to flood events. These data are important to the design of environmental flow plans on desert rivers, and emphasize the importance of understanding baseline life history information about species’ in order to determine the potential effects of disturbances on their populations.