Ecophysiology and Food Web Dynamics of Spring Ecotone Communities in the Edwards Aquifer, USA
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Spring orifices serve as ecotones between groundwater and surface water habitats. It is thought that organisms living in physicochemically stable spring ecotones should exhibit small tolerance ranges; however, previous experiments examining this prediction are equivocal. I examined this hypothesis by investigating effects of elevated temperature and decreased dissolved oxygen on several riffle beetle species (Coleoptera: Elmidae), including two ecotone specialists. Results indicate that ecotone-associated species exhibit stenothermal tolerance profiles when compared to surface species. I also examined resource use in invertebrate communities at two spring ecotones using stable isotopes of carbon (d13C) and nitrogen (d15N) and amino acid-specific stable isotopes (d13CAA). Results indicate that spring ecotones contain trophically complex communities with substantial niche partitioning among species. I finally examined the hypothesis that subterranean organisms in systems with ample energy resources (such as guano caves or spring ecotones) may not exhibit reduced metabolic rates. I assessed metabolic and biochemical responses of a subterranean amphipod (Stygobromus pecki) that inhabits spring ecotones and compared these responses to their epigean relative. Results indicate that S. pecki, despite its occupation of relatively resource rich spring opening ecotones, still exhibit lower metabolic rates relative to their epigean relative. Cumulatively, this body of research provides new and critical information on the ecology and evolution of spring ecotone communities, which are among the least studied and poorly understood aquatic ecosystems.