Tracing source, mixing and uptaking processes of carbon in an epikarst spring-pond system in southeastern Guizhou of China by carbon isotopes (13C-14C)

Abstract:

^δ13C and Δ¹⁴C of dissolved inorganic carbon (DIC), particulate organic carbon (POC) and aquatic plants from a karst spring and two spring-fed ponds in Laqiao, Maolan County, Guizhou Province in January, July and October of 2013 were measured to understand the roles of aquatic photosynthesis through DIC uptake in karst surface waters. The mean Δ¹⁴C and δ¹³C values of DIC for the spring pool, midstream and downstream ponds are -60.6±26.3‰ and -13.53±1.97‰, -62.8±62.9‰ and -11.72±2.72‰, and -54.2±56.5‰ and -9.40±2.03‰, respectively. Both Δ¹⁴C and δ¹³C show seasonal variations, with lower Δ¹⁴C values but heavier δ¹³C values in dry season and vice versa in summer rainy season. This observation indicates that (1) the main carbon source of the spring DIC is from limestone bedrock dissolution and soil CO₂ with higher contribution in summer due to higher productivity; and (2) ¹³C and ¹⁴C have different behaviors during DIC uptake by aquatic plants and during CO₂ exchange between DIC and the atmospheric CO₂. Biological uptake of CO₂ will not affect the Δ¹⁴C of DIC, but lead to δ¹³C_DIC enrichment. CO₂ exchange between DIC and the atmospheric CO₂ should elevate both the Δ¹⁴C and δ¹³C of DIC. In Laqiao spring-pond system, it seems that the effect of biological uptake on the Δ¹⁴C and δ¹³C of DIC is much stronger than that of CO₂ exchange with the atmosphere. The mean Δ¹⁴C values of POC from the spring pool, midstream and downstream ponds are -308.1 ±64.3‰, -164.4±84.4‰ and -195.1±108.5‰, respectively, indicating mixture of aquatic algae and detrital particle (clay and dust). More aquatic algae were formed in the stream ponds especially in the summer. SEM results of the POC samples support this conclusion. Furthermore, the Δ¹⁴C values of the submerged aquatic plants range from -200.0‰ to -51.3 ‰ and were similar to those of the DIC, indicating that the aquatic plants used DIC for photosynthesis. The Δ¹⁴C value of an emergent plant is -8.0±0.3‰, showing higher contribution of the atmospheric CO₂ during photosynthesis.