HYPORHEIC SYSTEMS IN KARST: ZONES OF EXCEPTIONAL BIODIVERSITY

The hyporheic zone is one of the most dynamic zones of groundwater-surface water interaction, where geochemical gradients and interfaces transform, cycle, and sequester organic and inorganic compounds. It provides habitat for diverse assemblages of epigean and groundwater-obligate invertebrates (stygobionts). As a zone of dynamic exchange between surface- and groundwaters, the importance of hyporheic geochemical, microbiological, and ecological processes is well documented. However, little work has been done to characterize its function as an ecotone in karst landscapes, where it links epigean and deeper, phreatic metazoan communities. In Texas, USA, the hyporheic zone is almost completely uninvestigated, despite exceptionally diverse phreatic karst groundwater fauna, and economically important karst aquifers spanning major biogeographic provinces.

We asked how distributions of presumably karst-obligate stygobionts in the hyporheic zone are related to regional geology and geochemistry. From 2015-2017, we used a Bou-Rouch pump to collect 133 hyporheic samples from 31 sites across an E-W transect in Texas. In each sample, we identified hyporheic invertebrates and collected physicochemical, nutrient, and isotopic data.

Macroinvertebrate stygobionts were strongly associated with hyporheic connectivity to karst aquifers, and substrate size and type. Many sites represent habitat and/or range extensions, and many vulnerable, threatened, or endangered species were found. Where they occurred, stygobionts were as abundant as epigean taxa although stygobiont sizes were, on average, an order of magnitude smaller than of hypogean taxa. In other words, stygobionts are abundant where they occur, but occur in fewer areas.

A better understanding of surface water-groundwater connections in karst regions can lead to better management of water in the hyporheic zone in water-limited regions such as the arid Trans-Pecos. The hyporheic zone may be an important dispersal corridor and refuge for epigean and groundwater-obligate species, and persistence of groundwater-obligate taxa in deeper karst aquifers may allow rapid recolonization by stygobionts after natural or anthropogenic hyporheic dewatering. Activities like in- or near-stream gravel mining are threats to both epigean and groundwater taxa.