Systematical d???????13C investigations of TOC in aquatic plants, DIC and dissolved CO2 in lake water from three Tibetan Plateau lakes

The carbon isotope values (C-13) of aquatic plants are of great significance for recognizing carbon cycles and tracing environmental changes in lakes. Yet, the influence of C-13 values of their utilizing carbon sources (especially in the case of CO2 separated from the HCO3- in water) on their C-13 differences have never been evaluated, which is mainly due to the lack of CO2 C-13 investigations in lake water. In this study, we used a newly developed fast-response automated gas equilibrator for real-time concentration and C-13 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 C-13 values of total organic carbon (TOC) of a submerged plant (Potamogeton) and an alga (Cladophora), as well as the C-13 values of dissolved inorganic carbon (DIC) in bottom lake water. The results showed that the CO2 C-13 values of bottom and surface water in these three lakes did not show significant differences and the C-13 values of CO2 at different water depths also did not show significant differences. However, Potamogeton had higher TOC C-13 values (average 14-16%o) than Cladophora in these lakes. Although Potamogeton and Cladophora could utilize HCO3- as an additional carbon source whose delta 13C values were more positive (ca. 7-8%o) than CO2, this effect on their C-13 values was no > 4%o after evaluation. Thus, our results show that the C-13 values of carbon sources (including CO2 and HCO3-) are not the main factors in determining the TOC C-13 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 C-13 variations in carbon cycles and paleoenvironment reconstructions for lakes.

Automated summary

The C-13 values of aquatic plants in lakes are important for understanding carbon cycles and environmental changes. This study evaluated the influence of carbon sources' C-13 values on their C-13 differences, revealing that it is not the main factor determining the C-13 difference between different aquatic plant species. This finding contributes to the understanding of C-13 variations in carbon cycles and paleoenvironment reconstructions for lakes.