Effects of Thermal Regime on Size, Growth Rates and Emergence of Two Species of Stoneflies (Plecoptera: Taeniopterygidae, Pteronarcyidae) in the Flathead River, Montana

Abstract

The life histories of two species of stoneflies were compared in adjacent unregulated and partially regulated reaches of the Flathead River in northwestern Montana. Approximately one-third of the discharge in the partially regulated reach is from the South Fork, which is controlled by a hypolimnial-release dam. Responses of winter-emerging Taenionema pacificum to altered environmental conditions included larger nymphal sizes and altered growth rates and emergence times. Pteronarcella badia, a late-spring emerger, responded with differences in population densities and larger nymphal sizes, but not with significantly altered growth rates or emergence times. Monthly mean temperatures were positively correlated (P < 0.01) with mean specific growth rates of F badia but not of 7T pacafacum. Specific growth rates (calculated from measurements of interocular distance) ranged from -0.2 % to 2.6 % /day for T pacificum and from -0.1 % to 2.5%/day for P badia. Growth rates differed during the 2 years of the study as a result of varying discharge regimes and weather patterns. INTRODUCTION Temperature is important in the ecology of aquatic insects (Ward and Stanford, 1982), and differences in life histories of populations in regulated rivers are expected as a result of altered thermal regimes. Timing of life cycles of some species is flexible, and growth rates and emergence times are adjusted in accordance with the prevailing temperature regimes (Nebeker, 1971a, 1971b; Rupprecht, 1975; Hynes, 1976; Vannote and Sweeney, 1980). Warm water temperatures downstream of deep-release dams in winter can accelerate larval development and cause early emergence of adults of certain species (Ward and Stanford, 1979). The Flathead River in northwestern Montana encompasses sites within the same river channel that have different thermal regimes. The confluence of the South Fork (which is regulated by a hypolimnetic-release dam) with the mainstem Flathead River separated two study sites, and permitted comparison of the effects of relatively small changes in temperature on the growth of benthic insects in adjacent reaches of the river. Environmental variables in the partially regulated reach were changed less than in many fully regulated rivers below deep-release dams, such as the South Fork Flathead River. Comparisons of the adjacent unregulated and partially regulated sites facilitated observations of differences between populations which were a result of river regulation, without some of the variation that is commonly associated with between-site habitat differences in adjacent drainage basins. Studies of insect populations in regulated vs. unregulated rivers may allow testing of theoretical models for aquatic biotas (e.g., thermal equilibrium hypothesis, Vannote and Sweeney, 1980). The thermal equilibrium hypothesis predicts that subpopulations of lotic species in nonoptimal thermal habitats are characterized by low population density and small-bodied individuals (Vannote and Sweeney, 1980). The purpose of this study was to investigate the phenology of natural populations of stoneflies in adjacent river reaches differing in thermal regimes. Small environmental alterations caused by a dam provided an experimental situation under natural field con'Present address: 116 S. Park Dr., Arlington, Virginia 22204. 2Author to whom reprint requests should be sent.