Chemolithoautotrophic contributions to stygofaunal food webs in the Barton Springs Segment of the Edwards Aquifer, Texas

Karst groundwater ecosystems can rely on organic matter (OM) sourced from the surface, but dependance on this can be unreliable depending on how surface water recharges the subsurface. Instead, autochthonous, chemolithoautotrophically-produced OM can also support karst ecosystems. The reduced compounds fueling chemolithoautotrophy can be derived from both OM decomposition and mineral dissolution. The relative contributions of allochthonous versus autochthonous OM to stygofaunal food webs remain unassessed in most settings. Here, we examined OM contributions to metazoan communities in the Barton Springs segment of the Edwards Aquifer (Texas, USA). Geochemical gradients corresponded to variations in microbial community compositions, and biomass-weighted metabolic profiling provided evidence for chemolithoautotrophic OM production, particularly in sulfidic wells where heterotrophic contributions to fine particulate OM (FPOM) were the lowest. Chemolithoautotrophic contributions to OM were calculated using Bayesian isotope mixing models of bulk stable carbon and nitrogen isotope ratios of FPOM, with chemolithoautotrophic contributions as high as 44.8% in one groundwater well. However, bulk δ ¹³ C of metazoan assemblages were not strongly depleted (< -35‰), and Bayesian isotope mixing models indicated diets primarily supported by allochthonous OM instead of chemolithoautotrophic OM. The dampened isotopic evidence for chemolithoautotrophy may be due to the hydrologic dynamics of the Barton Springs segment, where rapid recharge and relatively short groundwater residence times likely promote allochthonous OM use by stygofauna. These findings highlight the importance of hydrologic context in determining energy pathways in subterranean food webs and have implications for understanding the vulnerability of stygobiontic communities to changes in land use, including urban development.