Variability in Terrestrial and Microbial Contributions to Dissolved Organic Matter Fluorescence in the Edwards Aquifer, Central Texas

Most cave and karst ecosystems are believed to be dependent on an influx of allochthonous organic carbon. Although microbes are largely responsible for the fate of dissolved organic matter (DOM) in karst, the role of microbes in chemosynthetic (autochthonous) production and processing of DOM has received limited attention. Chromophoric dissolved organic matter (CDOM) is the fraction of DOM that absorbs ultraviolet and visible light, and differences in the fluorescence spectral characteristics of humic-like (terrigenous) and protein-like (microbially-derived) CDOM allow for tracing the relative contributions of allochthonous or autochthonous carbon sources, respectively, in water. We investigated CDOM in karst-aquifer well and spring waters along the freshto saline-water transition zone of the Edwards Aquifer, Central Texas, over a four year period. The groundwater fluorescence spectral characteristics were distinct from those generally observed in surface waters and soil porewaters. The dominant source of organic carbon in the aquifer waters may be a product of chemolithoautotrophic primary production occurring in situ. It is possible that the absence of a strong terrestrial CDOM signature may be due to filtering effects in the epikarst or rapid utilization by heterotrophs in the aquifer. Our results indicate that intense recharge following periods of drought may influence the intensity of microbial activity, either due to an influx of DOM or nutrients from the surface that was not quantified by our analyses or because of increased in situ autotrophic activity, or both. The variable contributions of allochthonous and autochthonous DOM during and after recharge events call into question whether karst aquifer ecosystems are necessarily dependent on allochthonous organic matter.