DRIP WATER CHEMISTRY FROM THE CASCADE CAVE SYSTEM, KENTUCKY, AND IMPLICATIONS FOR EPIKARST-DERIVED MICROBIAL COMMUNITIES

Water movement through the epikarst influences subsurface water chemistry and microbial diversity, and plays a role in the overall input of elements and nutrients into karst systems. However, little is known about the microbial community composition or the geochemistry of karst drip waters. Previous work in Cascade Cave system indicates dramatic seasonal variation in microbial community composition and geochemistry of water underlying drip sites in this study. We hypothesized that drip water composition plays a role in the evolution of microbial community composition, which in turn affects the water chemistry of the entire hydrologic system in the cave. Six different drip water locations were sampled above stream water flow during the summer. Drip waters were oversaturated with respect to calcite, compared to the cave stream water that was undersaturated with respect to calcite. Drip water Na (8.16-61.70 mg/L), K (0.27-4.48 mg/L), and Mg (1.19-6.78 mg/L) was more variable than the stream water Na (1.54-12.88 mg/L), K (1.55-3.57 mg/L), and Mg (5.94-10.70 mg/L) concentrations. Ammonium was comparable between the drip and stream waters (0.03-0.05 mg/L). Calcium concentrations were higher in the stream (29.80-44.92 mg/L) compared to drip water (1.19-6.78 mg/L). Drip water sulfate concentrations (6.20-90.83 mg/L), Cl (28.58-289.50 mg/L), and nitrate (2.07-4.43 mg/L) were higher than stream water, sulfate (8.40-31.06 mg/L), Cl (9.27-34.28 mg/L), and nitrate (0.30-1.72 mg/L) concentrations. Total organic carbon ranged from 0.59-1.85 mg/L in drip water, with dissolved inorganic carbon ranging from 48.11 to 120.7 mg/L. Based on fluorescence spectroscopy, the drip water chromophoric DOM was less humified than the cave stream water, and the slowest drip rates correlated to a decrease in the fulvic acid UVA peak. The distinct geochemical differences between drip and stream water indicate that microbial community structure in drip water should also differ from the cave stream water, but the relationships between drip and stream water will need to be assessed over time in order to understand whether seasonal variations in stream water diversity are similar to those from drip water communities.