Systematical δ13C investigations of TOC in aquatic plants, DIC and dissolved CO2 in lake water from three Tibetan Plateau lakes

[Display omitted] •δ13C of CO2 in lake water was in situ measured from three Tibetan Plateau lakes.•δ13C of CO2 in lake water at different water depths show small variations.•δ13C of CO2/HCO3– cannot fully explain δ13C difference of measured aquatic plants. The carbon isotope values (δ13C) of aquatic plants are of great significance for recognizing carbon cycles and tracing environmental changes in lakes. Yet, the influence of δ13C values of their utilizing carbon sources (especially in the case of CO2 separated from the HCO3– in water) on their δ13C differences have never been evaluated, which is mainly due to the lack of CO2 δ13C investigations in lake water. In this study, we used a newly developed fast-response automated gas equilibrator for real-time concentration and δ13C determination of dissolved CO2 in the surface and bottom lake water of Lake Tuosu, Lake Qinghai and Lake Keluke, China. Moreover, we determined the corresponding δ13C values of total organic carbon (TOC) of a submerged plant (Potamogeton) and an alga (Cladophora), as well as the δ13C values of dissolved inorganic carbon (DIC) in bottom lake water. The results showed that the CO2 δ13C values of bottom and surface water in these three lakes did not show significant differences and the δ13C values of CO2 at different water depths also did not show significant differences. However, Potamogeton had higher TOC δ13C values (average 14–16‰) than Cladophora in these lakes. Although Potamogeton and Cladophora could utilize HCO3– as an additional carbon source whose δ13C values were more positive (ca. 7–8‰) than CO2, this effect on their δ13C values was no >4‰ after evaluation. Thus, our results show that the δ13C values of carbon sources (including CO2 and HCO3–) are not the main factors in determining the TOC δ13C difference between Potamogeton and Cladophora, which could be primarily related to the biosynthetic fractionation for different aquatic plant species. Our results benefit further understanding and interpretations of δ13C variations in carbon cycles and paleoenvironment reconstructions for lakes.