Distribution and Occurrence of Stygobionts in Hyporheic Systems Downstream of Karst Regions

Sparks, Kenneth
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The hyporheic zone is the ecotone between surface water and groundwater in which diverse and complex biological and geochemical processes occur (Krause et al., 2011). This ecotone and the interactions that occur in it provide aquatic organisms with nutrients and habitat. Epigean taxa, and stygobionts that dwell in groundwater systems, may coexist, and interact in the hyporheic zone. Although the Edwards Aquifer is the most biodiverse and well-studied groundwater habitat known in Texas, the distribution and abundance of stygobiont and hyporheic-obligate taxa in hyporheic zones outside of the Edwards Aquifer and Edwards Plateau, are not well characterized (Hutchins et al., 2018 & 2020). Recent discoveries of stygobionts at sites distal to mapped karst regions suggests that there may be extensive non-karstic habitat for these taxa, including in alluvial systems downstream from the karst (Hutchins et al., 2020). Hutchins et al. (2020) showed that stygobiont species richness is positively correlated with the presence of karst, which suggests that karst aquifers may be source populations for emigration from the karst into adjacent non-karst regions. This also implies that the distance from karst may be related to stygobiont presence, abundance, and diversity. The goal of this research was to 1) assess the potential for off-karst alluvial systems to serve as habitat for ‘karst-adapted’ species, and 2) if stygobionts are found in non-karst alluvial sites, to determine how their diversity and abundance are related to distance from karst. I studied the abundance and diversity of stygobionts at 11 non-karst alluvial sites in the San Marcos River between 3 and 140 river km downstream from the Edwards Aquifer. I collected 55 hyporheic invertebrate samples and sorted, counted, and identified them at the lowest feasible taxonomic level. Environmental parameters (temperature, dissolved oxygen (DO), pH, specific conductivity (SC)) and hydraulic conductivity (K) were measured at each sample location. Linear mixed effects models were used to investigate relationships between response variables (abundance and biodiversity) and predictor variables: K, (DO), percent alluvium in 2-km buffer surrounding the site, and distance downstream from karst. Abundance and diversity of stygobiontic invertebrate counts per sample were significantly affected by K, DO, and distance from karst. Percent alluvium and underlying geological formations did not predict either abundance or diversity. Stygobiont abundance was exceptionally high in some samples; higher than in any other hyporheic sample collected in Texas. I also expanded the known ranges of several uncommon stygobionts and provided important information about the range of habitat conditions that these and other species may occupy. This was the first study in Texas to systematically investigate the extent to which stygobionts utilize alluvium downstream from karst as habitat and shows that these habitats have unrecognized importance for species previously assumed to primarily inhabit karst systems. Alluvial habitats downstream from karst systems should be further evaluated for presence and abundance of stygobionts. This is especially true for species that are of conservation concern, as these findings may have implications for management and conservation actions.
hyporheic, Karst, Biology
Sparks, K. (2023). Distribution and occurrence of stygobionts in hyporheic systems downstream of Karst regions (Unpublished thesis). Texas State University, San Marcos, Texas.