VARIABILITY IN TERRESTRIAL AND MICROBIAL CONTRIBUTIONS TO DISSOLVED ORGANIC MATTER FLUORESCENCE IN THE EDWARDS AQUIFER,CENTRAL TEXAS

摘要

Most cave and karst ecosystems are believed to be dependent on an influx ofallochthonous organic carbon. Although microbes are largely responsible for the fate ofdissolved 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 absorbsultraviolet and visible light, and differences in the fluorescence spectral characteristics ofhumic-like (terrigenous) and protein-like (microbially-derived) CDOM allow for tracingthe relative contributions of allochthonous or autochthonous carbon sources,respectively, in water. We investigated CDOM in karst-aquifer well and spring watersalong the fresh- to saline-water transition zone of the Edwards Aquifer, Central Texas,over a four year period. The groundwater fluorescence spectral characteristics weredistinct from those generally observed in surface waters and soil porewaters. Thedominant source of organic carbon in the aquifer waters may be a product ofchemolithoautotrophic primary production occurring in situ. It is possible that theabsence of a strong terrestrial CDOM signature may be due to filtering effects in theepikarst or rapid utilization by heterotrophs in the aquifer. Our results indicate thatintense recharge following periods of drought may influence the intensity of microbialactivity, either due to an influx of DOM or nutrients from the surface that was notquantified by our analyses or because of increased in situ autotrophic activity, or both.The variable contributions of allochthonous and autochthonous DOM during and afterrecharge events call into question whether karst aquifer ecosystems are necessarilydependent on allochthonous organic matter.