Daytime deposition and nighttime dissolution of calcium carbonate controlled by submerged plants in a karst spring-fed pool: insights from high time-resolution monitoring of physico-chemistry of water

Water temperature, dissolved oxygen (DO), pH, and specific conductivity (spc) were measured in a time interval of 15 min in a karst spring and the spring-fed pool with flourishing submerged plants in Guilin, SW China under dry weather for periods of 2 days. Measurements allowed calculation of calcium and bicarbonate concentrations (Ca²⁺] and HCO₃ ⁻]), and thus CO₂ partial pressure ( ) and saturation index of calcite (SIc). Results show that there were not any diurnal variations in the physico-chemical parameters of the water for the spring. However, during daytime periods, pool water decreased to far less than the spring water in a few hours, pH and SIc increased to greater than the spring, and Ca²⁺] and HCO₃ ⁻] decreased to less than the spring. During nighttime periods, pool water returned to or even increased to greater than the spring, pH and SIc decreased to less than the spring, and Ca²⁺] and HCO₃ ⁻] increased to greater than the spring. The decrease in Ca²⁺] and HCO₃ ⁻] to less than the spring during daytime periods implies daytime deposition of calcium carbonate, while the increase in Ca²⁺] and HCO₃ ⁻] to greater than the spring during nighttime periods implies nighttime dissolution of calcium carbonate. The direction of the observed changes depended essentially on the illumination, indicating that daytime photosynthetic and nighttime respiratory activities in the pool aquatic plant ecosystem, which were further evidenced by the increase and decrease in DO during daytime and nighttime periods respectively, were the main processes involved. The large variations of the components of the carbonate system imply considerable changes of the capacities of CO₂ and O₂ in water. The finding has implications for water sampling strategy in slow-flowing karst streams and other similar environments with stagnant water bodies such as estuaries, lakes, reservoirs, and wetlands, where aquatic plant ecosystem may flourish.