Biogeochemical and physical controls on the evolution of dissolved inorganic carbon (DIC) and δ13CDIC in karst spring-waters exposed to atmospheric CO2(g): Insights from laboratory experiments

Karst waters (spring-fed streams, lakes and reservoirs) characterized by relatively high concentrations of dissolved inorganic carbon (DIC) and pCO(2) significantly impact regional and global carbon cycles by releasing carbon dioxide to the atmosphere. Investigating the transfer of DIC from karst waters to the atmosphere is important to further our understanding of carbon cycling in karst environments. There is still considerable uncertainty about the controls of DIC transfer in karst waters because of challenges associated with investigations that aim to mimic the continuum of changes in DIC concentrations to equilibrium with atmospheric CO2 in natural settings. Laboratory simulations can create controlled conditions that allow targeted investigations. In this study, four tanks were taken to investigate the variations of pCO(2), DIC and delta C-13(DIC) when karst spring-waters were exposed to the atmosphere from 40 to 360 h as: (1) agitated water containing Hydrilla verticillata; (2) static water containing Hydrilla verticillata; (3) agitated water without Hydrilla verticillata; and (4) static water without Hydrilla verticillata. The rates of photosynthesis/respiration of submerged plants, CO2 outgassing and carbonate precipitation/dissolution were quantified by a time-stepping chemical/isotopic mass balance model. This experiment was designed to create ideal conditions to estimate the temporal evolution of DIC and delta C-13(DIC), and investigate mechanisms that control their evolution when karst spring-waters interact with atmospheric CO2. Results show: (1) generally a steep decrease in DIC concentrations and delta C-13(DIC) enrichment; (2) DIC loss and delta(CDIC)-C-13 enrichment are faster in the agitated waters with submerged plants; (3) DIC evolution is mainly controlled by the metabolism of aquatic plants; (4) carbonate precipitation/dissolution and CO2 outgassing has a lower effect on the DIC evolution in waters with submerged plants; (5) delta C-13(DIC) evolution is mainly controlled by the metabolism of submerged plants; (6) CO2 evasion, photosynthesis and delta C-13(DIC) enrichment are accelerated by the agitation of waters. Our analyses show that more than 40% of the total DIC resulting from carbonate weathering was used for photosynthesis by submerged aquatic plants thereby transforming the DIC into organic carbon (OC), suggesting that intense aquatic photosynthetic activities in continental surface water systems could play an important role as natural carbon sinks.