DISSOLVED ORGANIC CARBON IMPACTS ON BIOLOGICALLY MEDIATED CARBONATE MINERAL DIAGENESIS IN KARST RIVER SYSTEMS

Both gross primary production (GPP) and carbonate mineral diagenesis can be sources and sinks for dissolved inorganic carbon (DIC) in streams flowing through carbonate terrains and thus may impact C cycling. In this study, the link between GPP and carbonate dissolution were assessed in the Santa Fe River (SFR) in north-central Florida. River water was sampled hourly during a 48-hr period to measure how light variations control GPP and its effects on carbonate mineral saturation states. The Santa Fe River alternates between two phases: one at high flow with elevated dissolved organic carbon (DOC) concentrations, and thus dark water, and the other at low flow with low DOC concentrations. Initial sampling occurred at historically low flow conditions and showed strong diel cycles in dissolved oxygen, NO₃^-, Ca²⁺, and DIC concentrations, as well as pH, δ¹³C_DIC, and calcite saturation state. These data were compared to previous results collected from the Ichetucknee River (IR; de Montety et al., 2011), a continuously clear, spring-fed river. The Santa Fe River had similar diel variations in Ca²⁺ (SFR: 1.70 to 1.76; IR: 1.34 to 1.40), and DIC concentrations (SFR: 3.5 to 3.7 mM; IR: 3.0 to 3.2 mM), δ¹³C_DIC values (-11.5 to -10‰), and average GPP (8.7 g O₂/m²/d) to that of the Ichetucknee River. In contrast, the Santa Fe River had smaller changes in diel variations of NO^-₃ concentrations (SFR: 0.057 to 0.062 mM; IR: 0.027 to 0.034 mM), pH (SFR: 7.4 to 7.7; IR: 7.5 to 8.1), and calcite saturation state (SFR: 0.2 to 0.4; IR: 0.0 to 0.4) to that of the Ichetucknee River. These data indicate the two rivers have similar GPP magnitudes of biologically mediated carbonate mineral diagenesis, at least during the clear-water phase on the Santa Fe River. The differences in absolute Ca²⁺ concentrations in the two rivers could result from a variety of factors including the magnitude of discharge relative to that of wetted surfaces, organic carbon coating the minerals, prior weathering of the carbonate minerals due to the low pH during high flow, or bedload material may contain less carbonate minerals than the Ichetucknee River. These hypotheses will be tested in the future through additional time-series sampling during the dark-water phase.