Hydrogeologic and geochemical controls on nutrient availability and food web dynamics in the biodiverse karstic Edwards Aquifer, TX, USA

Background/Question/Methods

Phreatic karst aquifers are inaccessible and understudied habitats that can contain diverse and unusual faunal assemblages. Fractures, bedding planes, and lithology control the distribution and morphology of surface-subsurface connections (i.e. caves, sinkholes, sinking streams) and conduits within an aquifer, which in turn control the amount, distribution, and transport of photosynthetically derived allochthonous input. Surface derived organic matter is usually the primary carbon and nutrient source in aquifer ecosystems. Less commonly, they may be supported by in-situ primary production by chemolithoautotrophic microbial communities which fix inorganic carbon with energy derived from redox reactions involving inorganic molecules. The Edwards Aquifer of south-central Texas is a hydrologically and geochemically complex karst aquifer containing a globally diverse assemblage of subterranean, aquatic organisms (stygobionts). Hydrologic, geochemical, stable isotope, and microbiological analyses were used to characterize trophic structure in the Edwards Aquifer and the effects of environmental heterogeneity on resource availability. A database maintained by the Edwards Aquifer Authority was analyzed for spatial trends in physicochemistry. 25,800 physicochemical records were used to generate isopleth maps in ArcGIS. Several geochemically diverse wells, springs, and caves were sampled to characterize microbial diversity, organic matter fluorescence, and stygobiont stable isotopic composition. 

Results/Conclusions

Isopleth maps illustrate 1) the geochemical influence of surface inputs, primarily along recharging streams, 2) the presence of a freshwater-saline water interface characterized by a redox gradient and rapid changes in temperature, dissolved oxygen, and SO₄, and 3) geochemical gradients across fault controlled flowpaths. These data corroborate current hydrologic models for the aquifer and control microbial assemblage structure. Animals collected from sites hydrologically distant from recharge features and proximal to the freshwater-saline water interface had isotopic ratios (δ¹³C = -27 to -37‰; δ¹⁵N = 2 to 16‰) suggesting carbon sources that are, at least in part, of chemolithoautotrophic origin (terrestrial sources, δ¹³C ≈ -20‰).  The fauna of San Marcos Springs, the second largest spring complex in Texas are representative of this deep aquifer assemblage. Isotopes in microbial mats from sulfidic waters (δ¹³C = -36 to -42‰; δ¹⁵N = -3 to 4‰), the presence of microbial taxa containing putative chemolithoautotrophs (including Nitrospira, Thothrix, ε-Proteobacteria), and dissolved organic matter of  predominately microbial origin also suggest a chemolithoautotrophic nutrient base for this assemblage. Conversely, Comal Springs, Texas’ largest spring complex contains a distinct, “spring assemblage” that is likely more dependent on roots and terrestrial inputs. This should be reflected in isotopic composition.