Carbon sequestration and decreased CO2 emission caused by terrestrial aquatic photosynthesis: Insights from diel hydrochemical variations in an epikarst spring and two spring-fed ponds in different seasons

Whether carbonate weathering could produce a stable carbon sink depends primarily on the utilization of dissolved inorganic carbon (DIC) by aquatic phototrophs (the so-called Biological Carbon Pump-BCP effect). On this basis, water temperature (T), pH, electrical conductivity (EC) and dissolved oxygen (DO) were synchronously monitored at 15-min resolution for one and two days respectively in January and October 2013 in Maolan Spring and the spring-fed midstream and downstream ponds in Maolan Nature Reserve, China. A thermodynamic model was used to link the continuous data to allow calculation of CO2 partial pressures (pCO2) and calcite saturation indexes (SIC). A floating static chamber was placed on the water surface successively at all sites to quantify CO2 exchange flux between atmosphere and water so as to evaluate the BCP effect. Results show that, in both winter and autumn, remarkable diel variations of hydrochemical parameters were present in the midstream pond where DO, pH, and SIC increased in the day and decreased during the night while EC, [HCO3−], [Ca2+] and pCO2 showed inverse changes mainly due to the metabolic processes of the flourishing submerged plants, with photosynthesis dominating in the day and respiration dominating at night. However, hydrochemical parameters in the spring and downstream pond show less change since few submerged plants developed there. It was determined that the BCP effect in the midstream pond was 285 ± 193 t C km−2 a−1 in winter and 892 ± 300 t C km−2 a−1 in autumn, indicating a potential significant role of terrestrial aquatic photosynthesis in stabilizing the carbonate weathering-related carbon sink.