Hydrology and geology as structuring mechanisms of semi-arid fish communities
In the semi-arid regions of the Edwards Plateau in southwest USA, spring-associated fishes, including several federally and state listed species, are closely associated with spring outflows of karst aquifers. However, not all spring outflows are of sufficient volume to provide surface flows for long distances downstream. Stream disconnectivity can thus occur during dry periods, especially in systems where the surficial geology transitions from water gaining reaches (i.e., spring outflows in Cretaceous Gaining Reach) to water losing reaches (i.e., surface waters lost to recharging another karst aquifer or alluvium in either Cretaceous Losing Reach or Quaternary Losing Reach). The purpose of this study was to describe habitats and fish community distributions, with an emphasis on spring-associated fishes, along a longitudinal gradient consisting of different geologies and gaining/losing reaches between a Wet Hydrological Period (2015 – 2016) and Dry Hydrological Period (2021 – 2022) in three independent river systems. Predictions of the study were that hydrological period would have an effect on the overall fish community along a longitudinal and surficial geological gradient, with a positive relationship between relative abundances of spring-associated fishes and volume of spring outflows (Craig et al. 2016). During five collection events, fish communities consisted of 26 species (6 spring-associated fish species) and 21,297 individuals (10,317 spring associated fishes). Among 366 habitats quantified, reaches consisted primarily of riffles and runs with gravel and cobble substrates, moderately swift current velocities (mean ± 1 SE; 0.33 ± 0.29 cm/s) and shallow depths (0.38 ± 0.27 m) during the Wet Hydrological Period, although two rivers were dry or surface waters were restricted to isolated pools in the Quaternary Losing Reach. During the Dry Hydrological Period, habitats were similar in the Cretaceous Gaining reaches, isolated pools formed in the Cretaceous Losing reaches, and streambeds were completely dry in the Quaternary Losing reaches. Correspondingly, species richness, diversity, and evenness were generally greater in the Cretaceous Gaining and Losing reaches compared to the Quaternary Losing reaches during Wet and Dry Hydrological periods. Surprisingly, spring-associated relative abundances were similar in the Cretaceous Gaining and Losing reaches, despite the formation of isolated habitats in the Cretaceous Losing Reach. In the past, losing reaches of Edwards Plateau streams were interpreted as sinks for spring-associated fishes, yet this study demonstrated that losing reaches can serve as aquatic refugia for sources of spring-associated fish populations even into exceptional drought periods and therefore have conservation value for a number of the Edwards Plateau endemic fauna.
hydrology, geology, fish, Nueces River, losing reach, evolutionary refugia, ecological refuge, Karst springs
Chappell, L. (2023). Hydrology and geology as structuring mechanisms of semi-arid fish communities (Unpublished thesis). Texas State University, San Marcos, Texas.