DISSOLVED ORGANIC MATTER TRANSFORMATIONS IN KARST AQUIFERS: A CASE STUDY FROM THE CASCADE CAVE SYSTEM, KENTUCKY

Microbial processes affect aquifer ecosystem energetics, but because access to most aquifers is limited, understanding nutrient types and sources for microbes can be difficult. Karst systems allow for direct sampling of groundwater, where the flux of dissolved organic matter (DOM) used by heterotrophs at the base of ecosystems can be tracked. Characterization of the nature and behavior of DOM in karst aquifers, or even single caves, has been limited. DOM sources can include allochthonous surface input (soil, plant matter), autochthonous microbial processes (chemolithoautotrophy), and even continued degradation of allochthonous DOM into new biomass. This would cause the allochthonous DOM to appear like autochthonous DOM through time, thereby erasing the original DOM source. This study evaluated the DOM, water chemistry, hydrology, and microbial communities from the headwaters to the resurgence of the Cascade Cave System, Carter Caves State Resort Park, Kentucky. Discerning the allochthonous DOM (humics) from more biologically labile, autochthonous DOM (amino acids, proteins, peptides) was done from excitation-emission matrices (EEM) constructed from fluorescence spectroscopy approaches, such as the Synch-75 index (SI₇₅). The SI₇₅ method is based on the ratio of emission intensities of the fluorescent peak regions T and M (microbial signatures) versus peak C (humic signature). SI₇₅ values were low from the 2 km long cave system flowpath, indicating low fluorophore intensities overall. Minor increases in T+C regions were from areas having stagnant water, and the higher SI₇₅ values were from water exposed to sunlight (karst windows, entrances). Changes in fluorophore types and intensities due to water chemistry were unlikely, as the slight increase in total dissolved solids at the end of the flowpath was attributed to enhanced carbonate dissolution. No other geochemical or redox changes were noted. Based on the minor DOM changes along the flowpath, the lack of DOM alteration may indicate that microbes are not able to adequately utilize DOM as a nutrient source at higher flow rates in the Cascade Cave System. These results have broad implications to aquifer DOM turnover rates and geological processes linked to carbon-based metabolism, like carbonate precipitation and/or dissolution.