Effects of surface water interactions with karst groundwater on microbial biomass, metabolism, and production

Abstract. Unearthing the effects of surface water and groundwater interactions on subsurface biogeochemical reactions is crucial for developing a more mechanistic understanding of carbon and energy flow in aquifer ecosystems. To examine physiological characteristics across groundwater microbial communities that experience varying degrees of interaction with surface waters, we investigated ten springs and a river sink and rise system in North Central Florida that discharge from 10 and/or mix with the karstic Upper Floridan Aquifer (UFA). Groundwater with longer residence times in the aquifer had lower concentrations of dissolved oxygen, dissolved and particulate organic carbon, and microbial biomass, as well as the lowest rates of respiration (0.102 to 0.189 mg O₂ L^-1 d^-1) and heterotrophic production (198 to 576 μg C L^-1 d^-1). Despite these features, oligotrophic UFA groundwater (< 0.5 mg C L^-1) contained bioavailable organic matter that supported doubling times (14 to 62 h) and cell specific production rates (0.0485 to 0.261 pmol C cell^-1 h^-1) comparable to those 15 observed for surface waters (17 to 20 h; 0.105 to 0.124 pmol C cell^-1 h^-1). The relatively high specific rates of dissimilatory and assimilatory metabolism indicate a subsurface source of labile carbon to the groundwater (e.g., secondary production and/or chemoautotrophy). Our results link variations in UFA hydrobiogeochemistry to the physiology of its groundwater communities, providing a basis to develop new hypotheses related to microbial carbon cycling, trophic hierarchy, and processes generating bioavailable organic matter in karstic aquifer ecosystems.