TEMPORAL VARIATIABILITY IN SPECIES RICHNESS AND ABUNDANCE WITH HIGH-FREQUENCY SAMPLING FROM AN EDWARDS AQUIFER ARTESIAN WELL IN SAN MARCOS, TX, USA: CAN BIOTIC TRANSPORT PROVIDE INFORMATION ABOUT FLOWPATHS?

The distribution, abundance, and transport of animals in groundwater is dependent on biotic factors (life history, nutrient availability, species interactions, etc.), abiotic factors (physiochemistry, porosity, groundwater flow rates) and the interaction between biotic and abiotic factors. However, the use of biologic data to make inferences about hydraulic conditions within phreatic karst aquifers has been limited. To assess the relationship between species data and aquifer conditions, we conducted high temporal frequency sampling of the artesian well on the Texas State University campus (TX well # LR 6701828). This site is established as one of the most biodiverse groundwater sites known in the world.

Between February 13 and April 15, 2013, 36 samples were collected in a 100 μm net over the artesian outflow for sampling intervals ranging from 23 to 72-hours. Samples were sorted under a microscope and all visible metazoans were counted and identified to lowest possible level (species, in most cases) using published keys. 27 taxa were identified and several taxa include undescribed species. Biomass is dominated by one species of shrimp, and numbers of individuals are dominated by the same shrimp species and several species of unidentified copepods and ostracods. Combined, these three groups comprise 91% of the individuals. A diverse amphipod fauna (10 species) dominates the remainder.

Temporal variability in the number of individuals being discharged varied among species, with some showing random variability around a mean, while several others have distinct peaks in abundance that do not correspond temporally. These findings suggest that stygobitic aquifer species, which are often assumed to maintain relatively stable populations, have non-random abundances. Hypotheses for why total and species-specific numbers vary include: 1) piston flow during recharge events, 2) changes in discharge, 3) biological factors such as variability in population or resource availability in the aquifer, 4) changes in physicochemical parameters, and 5) variability in organism mobility. Furthermore, because changes in abundance appear to be species-specific, biota may not be useful for making inferences about hydrologic processes without additional ecological knowledge about the species present at a site.