INFLUENCE OF PHOTOSYNTHESIS ON CARBONATE EQUILIBRIUM IN A KARST RIVER

Variations of temperature, photosynthesis, and respiration force diel variations in pH and dissolved CO₂ content of surface streams, which affect carbonate equilibrium. Photosynthesis and respiration should thus influence speleogenesis of carbonate terrains. This work assesses how biogeochemical processes affect saturation state of carbonate minerals in the Ichetucknee River, a karst river sourced entirely by springs of the Floridan aquifer in north central Florida. Diel variations in water chemistry was measured using automated logging sensors (temperature, pH, dissolved oxygen (DO) and NO₃ concentrations) at 15-minutes or one hour intervals, and in grab samples for major ion concentrations and dissolved inorganic carbon concentration and stable isotopic composition (DIC and δ¹³C_DIC). Grab samples were collected over two one-week periods at a 4-h sampling interval and over a single 24-hour period at a 1 h sampling period. Diel variations of calcium (0.06 mM) and alkalinity (0.07 mM) cause changes in specific conductivity (9 µS/cm) of the stream. These diel variations are controlled by photosynthesis as reflected in increases in DO (0.14 mM) and decreases on NO₃ (0.06 mM) concentrations at a 24-hour periodicity. Photosynthetic control of δ¹³C_DIC values is reflected in a 1‰ enrichment during the day simultaneous with the decrease in specific conductivity, alkalinity and Ca concentrations. Stream water was never undersaturated relative to calcite through the diel cycle (SI ranges from 0 to 0.5). Calcite and dolomite were most supersaturated during the day because of elevated temperature and loss of CO₂ by photosynthesis. Calcite remains saturated at night, indicating CO₂ production by plant respiration is insufficient to induce carbonate dissolution. These saturation values indicate that δ¹³C_DIC enrichment reflects fractionation of the DIC by plants during photosynthesis rather than carbonate dissolution. Changes in carbonate equilibrium, linked to in-stream biological processes, have important implication for Ca cycling and development of channel geomorphology. Specifically, this study suggests that aquatic plant respiration is insufficient to drive dissolution in karst rivers.