Environmental influences on invertebrate diversity and community composition in the hyporheic zone ecotone in Texas, USA: contrasts between co-occurring epigean taxa and stygobionts

Abstract

The hyporheic zone (HZ) of streams harbors co-occurring epigean taxa and stygobionts (including HZ specialist taxa and deeper groundwater taxa) that face the same set of local environmental factors, but are derived from spatially and functionally discrete colonist pools. We investigate whether occurrences of epigean taxa and stygobionts are differentially influenced by the same set of environmental variables in HZ samples from Texas, USA. We increase the number of records for described stygobionts in the state by nearly 12%, but 26 stygobionts (35%) were undescribed species. Stygobionts had smaller average ranges (3549 km2) than epigean taxa (24,046 km2), but occurred in similar abundances. Dissolved oxygen and temperature influenced community composition in both groups, but stygobiont community composition, richness, and abundance were influenced by association with karst aquifers and surface water-groundwater exchange, whereas epigean taxa composition was only weakly influenced by the association with karst aquifers. Contrasts between epigean taxa and stygobiont occurrence patterns are primarily explained by differences in range size, dispersal ability, and the pool of potential colonizer species. This first documentation of Texas HZ communities includes 14 stygobionts previously recorded only from karst aquifers: their discovery in the HZ has important management implications.