1. INTRODUCTION
Carbon cycles have become one of the most extensively researched topics with regard to global climate change (Yang et al., 2021). Lakes, accounting for 3.7% of the global land area, are an important component of the inland water system, which regulate the carbon cycle by storing, transporting, and transforming carbon (Tranvik et al., 2009; Holgerson and Raymond, 2016; Cole et al., 1994; Holgerson & Raymond, 2016; Ran et al., 2017; Yan et al., 2018). For example, lake sediments can contain 0.03–0.07 Pg C a–1 (Molot & Dillon, 1996; Dean & Gorham 1998; Kortelainen et al., 2006), which is roughly equal to or higher than the carbon buried in marine sediments (Cole et al., 2007). The annual CO2 emissions from lakes has been estimated at 0.11–0.57 Pg C a–1 (Sobek et al., 2005; Holgerson and Raymond, 2016). Additionally, the CO2 and CH4 released by organic matter mineralization play a major role in and significantly affect the terrestrial carbon cycle (Kortelainen et al., 2006; Tranvik et al., 2009; Holgerson & Raymond, 2016; Martinsen et al., 2020). Knowledge of the lake carbon cycle can therefore contribute to a comprehensive understanding of the terrestrial carbon cycle (Hanson et al., 2004; Balmer & Downing, 2011).
Dissolved inorganic carbon (DIC) occurs in water in the form of CO2, CO32–, HCO3, and H2CO3. The primary sources of DIC in lake water are external inflowing water, atmospheric CO2, organic matter decomposition, and CO2 produced during metabolism of aquatic organisms. Carbon isotopes (δ13C) can record the cycling of carbon cycling during each of these links, which involve equilibrium and kinetic fractionation (Zhang et al., 1995; Myrbo & Shapely, 2006). Thus, δ13C analyses of the DIC (δ13CDIC) provide a powerful tool for tracing the lake carbon cycle and elucidating carbon fluxes (Quay et al., 1986; Herczeg, 1987; Stiller and Nissenbaum, 1999; Lei et al., 2012; Mu et al., 2016; Han et al., 2018; Shtangeeva et al., 2019). For instance, Striegl et al. (2001) compared the δ13CDIC values of 142 lakes during ice cover, suggesting that lakes with a higher CO2(p CO2) partial pressure had lower δ13CDIC values owing to the dominance of respiration of terrestrial organic material.
Lake ecosystems on the Qinghai-Tibetan Plateau are fragile and sensitive to changes in climate and environment (Fang et al., 2016). Lakes on the Qinghai-Tibetan Plateau account for approximately 50 % of the total lake area in China (Guan et al., 1984). Most studies on the δ13CDIC in lakes on the Tibetan Plateau have focused on the climatic and environmental implications of the carbon isotope compositions of lake sediments. Variations in the δ13CDIC of various lake types may have different responses to carbon cycle processes. For example, Lei et al. (2012) analyse the characteristics of the δ13CDIC of 24 lakes (mainly closed lakes) across the Qiangtang Plateau, finding that the high δ13CDIC values of closed lakes could be mainly attributed to significant catchment-scale contributions from carbonate weathering and the evasion of dissolved CO2induced by enhanced lake water evaporation. Therefore, performing a more in-depth study on the changes in the δ13CDIC values of lakes on the Qinghai-Tibetan Plateau is necessary.
This study explored the relationships among the δ13CDIC values of the Genggahai Lake, the lake environment, and the watershed climate based on the observed water physicochemical parameters in areas with different types of submerged macrophyte communities, as well as the changes in the temperature and precipitation during the same period. Our objective was to provide a theoretical basis to elucidate the response mechanisms of δ13CDIC to the lake carbon cycle.