Influence of Impoundments on Mayfly Diets, Life Histories, and Production

We investigated the influence of stream regulation on the dietary composition, life histories, and production of four mayfly species (Drunella grandis, Ephemerella infrequens, Paraleptophlebia heteronea, and Baetis tricaudatus) at three sites in the Upper Colorado River. Sites (unregulated, intensely regulated, and recovery) were selected based upon degree of regulation (i.e., upstream, 0.4 km downstream, and 4.0 km downstream from a deep-release reservoir). Although levels of food abundance (diatom density and sedimentary detritus) varied between sites, the dietary compositions of B. tricaudatus (primarily diatoms) and P. heteronea (detritus) did not change. However, D. grandis and E. infrequens consumed diatoms downstream from the reservoir whereas their diet was strictly detritivorous at the upstream site. The number of degree days and mean annual water temperatures at the downstream sites were greater than at the free-flowing unregulated site. Baetis tricaudatus responded to regulated conditions by higher rates of development, altered voltinism, and extended emergence. Ephemerella infrequens and P. heteronea were univoltine at each site. However, development of E. infrequens and P. heteronea was poorly synchronized and emergence was extended at both downstream sites. Drunella grandis was univoltine and well synchronized with a short emergence period at each site. Life history flexibility, in rates of egg development, growth, maturation, and emergence, at the population level may be determined by a lack of dependence by individuals upon temperature or photoperiod cues to synchronize the developmental process. Annual production estimates at downstream sites were higher for all four species than at the upstream site. Annual production of D. grandis (15.38 g/m2) at the recovery site and of E. infrequens (15.89 g/m2) at the regulated site are the highest production estimates recorded for mayflies.