6. IMPLICATIONS OF LAKE WATER DIC ISOTOPIC COMPOSITION ON CARBON CYCLE
Lake ecosystems are active components of the global carbon cycle as they continually fix and release carbon through various biological processes, including photosynthesis, food web activity, and bacterial degradation (Tranvik et al., 2009; Holgerson & Raymond, 2016). The lake carbon cycle is mainly composed of carbon input via runoff and atmospheric CO2 diffusion, different forms of carbon transformation in the lake (including organic matter decomposition, photosynthesis/respiration of aquatic plants, and carbonate precipitation), and carbon output via runoff and gaseous CO2 emissions through the water surface (Hu et al., 2014).
The DIC and its isotopes are an important tool to elucidate the carbon cycle of lake ecosystems (McKenzie, 1985; Quay et al., 1986; Herczeg & Fairbanks, 1987; Wachniew & Różański, 1997; Herczeg et al., 2003; Lei et al., 2012; Mu et al., 2016). Striegl et al. (2001) found that during the ice melting period, the average δ13CDIC-L value in 132 freshwater lakes in temperate and cold regions was –14 ‰. Similarly, Bade et al. (2004) reported that the average δ13CDIC-L value in 108 freshwater lakes in different regions was –15 ‰. In freshwater lakes, the photosynthesis of aquatic plants and organic matter decomposition in sediments are active components of the carbon cycle (Lei et al., 2012). In freshwater lakes on the Qiangtang Plateau, variations in δ13CDIC-L also showed that organic matter decomposition significantly contributed to the carbon cycle of the lake ecosystems (Lei et al., 2012). However, Lei et al. (2012) found that the δ13CDIC-L values of endorheic lakes on the Qiangtang Plateau were relatively high, approaching the δ13CDIC-L values of equilibrated CO2 in water to atmospheric exchange.Therefore, water surface to atmospheric CO2 exchange drives the carbon cycle in endorheic lakes on the Qiangtang Plateau (Lei et al., 2012).
The δ13CDIC-L values of the Genggahai Lake significantly exceeded the δ13CDIC values of freshwater lakes (> 8 ‰), but were more negative than those of lakes on the Qiangtang Plateau (> –5.71 ‰) (Lei et al., 2012). This indicates that the carbon cycle of the Genggahai Lake significantly differs from those of freshwater lakes and lakes on the Qiangtang Plateau. Variations in the δ13CDIC-Lvalues of the Genggahai Lake indicate that organic matter decomposition and water to atmospheric CO2 exchange are not likely the main components of its carbon cycle (section 5.1). Carbon input from inflowing groundwater and the photosynthesis of aquatic plants may be the main components of its carbon cycle.
Recent vegetation surveys have shown that the main terrestrial plants in the watershed of the Genggahai Lake are Artemisia desertorum ,Oxytropis aciphylla , Achnatherum splendens , Orinus kokonorica , and Agropyron cristatum . This is identical to that in the Qinghai Lake watershed (Duan & Xu, 2012; Liu et al., 2013). The δ13Corg values of 3C plants in the Qinghai Lake watershed varied from –27.7 to –24.5 ‰. The δ13Corg values of soil in the Qinghai Lake watershed varied from –26.9 to –24.8 ‰ (Liu et al., 2013). When the DIC in groundwater only originates from soil respiration and organic matter decomposition, if we neglect the effect of carbon isotope fractionation due to organic matter decomposition, the δ13CDIC of groundwater ranges from –27.7 to –24.5 ‰. Considering the isotope fractionation between CO2 (aq) and HCO3(approximately –10 ‰) (Zhang et al., 1995), the δ13CDIC of groundwater ranges from –17.7 to –14.5 ‰. As previously mentioned, most δ13CDIC values of groundwater in the Genggahai Lake watershed were more positive than –14.5 ‰. This indicates that soil respiration and organic matter decomposition may not be the only carbon source for the groundwater DIC in the Genggahai watershed.
Significantly higher δ13CDIC(approximately –3 to +3 ‰) values in groundwater can occur in karstic regions where a proportion of the carbon atoms derive from the dissolution of catchment limestones (Andrews et al., 1997). For example, the δ13CDIC of groundwater from the Donggi Cona catchment on the north-eastern Qinghai-Tibetan Plateau varies from 0.9 to 2.0 ‰ (Weynell et al., 2016). Paleolake sediments that formed in the early and middle Pleistocene are widely distributed throughout the Gonghe Basin (Dong, 1993). Here, HCO3 originates from the weathering of paleolake sediments and subsequent mineral dissolution, which is enriched in 13C. Water and groundwater flows transport the HCO3, thus yielding more positive δ13CDIC values. We suggest that HCO3 originates from paleolake sediment weathering, which affects the groundwater DIC pool in the Genggahai watershed and yields relatively positive groundwater δ13CDIC values. Thus, variations in the δ13CDIC values of groundwater may reflect the relative contributions of two carbon sources to the groundwater DIC pool. One carbon source is CO2 that derives from soil respiration and organic matter decomposition while the other is HCO3 from paleolake carbonate sediments. Besides, we elucidated that carbonate weathering and soil respiration related to vegetation succession control the variations in δ13CDIC-I. Therefore, variations in the δ13CDIC-L values of the Genggahai Lake may reflect the lake productivity and carbon cycle of the watershed. Clearly, more data are required to constrain the environmental significance of the δ13CDIC-L values of the Genggahai Lake.