THE RESPONSE OF A GROUNDWATER MICROBIAL COMMUNITY TO VARIATIONS IN THE BIOGEOCHEMISTRY OF RECHARGE WATERS: INSIGHTS FROM AN EOGENEIC CARBONATE AQUIFER
This study investigates the response of the natural microbial community within a shallow eogenetic carbonate aquifer on North Andros Island, Bahamas, to inputs of differing concentrations and compositions of dissolved organic carbon (DOC) sourced from various recharge pathways. During an individual storm, geochemical modification of rainfall was evaluated within throughflow, stemflow, overland flow and infiltration through the epikarst. The total concentration of DOC was measured and its composition was characterised using fluorescence spectroscopy. The biological oxygen demand (BOD) of subsamples of the shallow groundwater inoculated with different recharge waters was measured to determine the bioavailability fraction of the DOC.
There were distinct geochemical differences between the types of recharge waters, driven mostly by interaction with organic acids, with stemflow having a very low pH when compared to the rainfall. The majority of recharge waters stimulated significant microbial respiration relative to the control, with this response differing between the samples. However, the BOD does not correlate directly with the total concentration of DOC in the samples. Fluorescence spectroscopy highlighted differences between the characteristics of the DOC, with the degree of interaction with vegetation being a dominant factor controlling the composition. There was a strong positive correlation between the ratio of C:A peaks, the two peaks relating to humic-like substances, and the BOD. The C peak has previously been noted to represent microbially reworked aromatic and aliphatic fulvic-like substance. This suggests that a by-product of the respiration of the microbial community at the shallow water table could be an increase in C peak abundance, which could be used as a proxy for microbial response within the aquifer.