Biogeochemical controls on daily cycling of hydrochemistry and δ13C of dissolved inorganic carbon in a karst spring-fed pool

SummaryVariations in temperature, photosynthesis and respiration can force daily variations in pH, DO and DIC in surface water, potentially driving calcite precipitation or dissolution of calcium carbonate. Diel cycles of hydrochemistry and δ13CDIC were measured at high time-resolution (1 h) to assess the relative magnitudes of biological and geochemical controls on carbonate chemistry and carbon cycling in a spring-fed pool with flourishing submerged plants in Chongqing, SW China under sunny weather. Results show that there were no diurnal variations in the physical and chemical parameters of the Shuifang spring water. However, during the daytime periods, SC, Ca2+, alkalinity, NO3- and pCO2 in the pool water decreased to less than those in the spring water, while pH, DO and δ13CDIC in the pool water became greater than those in the spring water. Conversely, during nighttime periods, pool water SC, Ca2+, alkalinity, NO3- and pCO2 returned to or even became greater than the spring water, while pH, DO and δ13CDIC decreased to less than the spring water. This work shows that photosynthesis and respiration of subaquatic communities are the dominant processes influencing the observed diel variations of hydrochemistry in karst spring-fed pool water. During the daytime, a simultaneous increase of δ13CDIC and DO, and decrease in DIC indicates that photosynthesis was the primary control on hydrochemistry of the pool water. Conversely, the water remained saturated with respect to calcite (SIc ranging from 0.04 to 0.15) and δ13CDIC values decreased at nighttime, indicating that respiration of the subaquatic community had a dominant influence over calcite dissolution and outgassing in the pool water. The total amount of DIC loss was estimated to be about 110,785 mmol/day which represented about 1.33 kg C/day. More specifically, the amount of DIC loss through carbonate precipitation was about 38,775 mmol/day (0.47 kg C/day), whereas photosynthetic uptake was about 60,700 mmol/day (0.86 kg C/day) in the pool water. On one hand, the loss of C through consumption of CO2 during carbonate precipitation was balanced by carbonate dissolution. On the other hand, the loss of C by photosynthetic uptake was diverted into organic C and constitutes a relatively long-term natural carbon sink in the karst system. In addition, the diurnal variations of NO3- and pH in the pool water, caused by photosynthesis and respiration, indicated that biogeochemical processes impact water quality on a daily timescale.

► Diel variation of hydrochemistry and δ13CDIC occur in a karst spring-fed pool.
► Diel variations of hydrochemistry are dominantly controlled by biological processes.
► The DIC loss caused by photosynthetic uptake constitutes a natural sink of carbon.
► The DIC loss caused by calcite precipitation accounted for 35%.
► Biogeochemical processes of stream impact on the water quality and carbon cycling.