Dynamics and internal links of dissolved carbon in a karst river system: Implications for composition, origin and fate

Dissolved carbon (DC) deciphers biotic and abiotic processes in aquatic ecosystems, representing a critical component of global carbon cycling. However, underlying drivers of riverine DC dynamics and internal links have yet to be studied. Here, we investigated fluvial physicochemical characteristics, dissolved inorganic carbon (DIC) species, carbon dioxide (CO2) exchange, dissolved organic carbon (DOC) compositions and properties in a karst river system Qijiang, Southwest China. Carbonate dissolution combined with photosynthetic uptake could explain dynamics of DIC species. Carbon sequestration caused low-magnitude of partial pressure of aqueous CO2 (pCO2, 620.3 ± 1028.7 μatm) and water-air CO2 flux (F, 154.3 ± 772.6 mmol/m2/d), yielding an annual CO2 emission of 0.079 Tg CO2/y. Relatively high biological index (BIX, 0.77–0.96 on average) but low humification index (HIX, 0.67–0.78 on average) indicated notable autochthonous processes. Humic-like component was the predominant DOC, accounting for 39.0%-75.2% with a mean of 57.2% ± 6.17%. Meanwhile, tryptophan-like component (5.84% ± 2.31%) was also identified as collective DOC by parallel factor analysis (PARAFAC) across samples. Biological metabolism established internal linkages between DIC and DOC in the karst river system. Our findings highlighted biological process as a determinant for DC cycling in karst aquatic ecosystems.