Variations in δ13CDIC and Influencing Factors in a Shallow Macrophytic Lake on the Qinghai–Tibetan Plateau: Implications for the Regional Carbon Cycle and Sustainable Development
Abstract
:1. Introduction
2. Study Area
3. Materials and Methods
4. Results
4.1. Physical and Chemical Parameters of Lake Water
4.2. Spatial Variations in DIC Isotopic Composition in the GGH Basin Waterbodies
4.3. Temporal Variations in DIC Isotopic Compositions in the GGH Basin Waterbodies
5. Discussion
5.1. Factors Affecting δ13CDIC in the GGH Basin Waterbodies
- (1)
- Lake inflow carbon isotope composition. The isotopic composition of the lake inflow directly affects the isotopic composition of the lake. This is especially significant in exorheic lakes or lakes with a short water retention time. In the Chara spp. community and P. pectinatus growth area, the δ13CDIC-L and δ13CDIC-I values were positively correlated, whereas in the M. spicatum community growth area, these values showed no correlation (Figure 5). This is because the M. spicatum community was located far from the small spring-water streams, where weak exchange with spring water took place (Figure 1b). This reveals that the δ13CDIC-I values were key factors influencing the δ13CDIC-L values.
- (2)
- Exchange with atmospheric CO2. The DIC pool in the lake tends to be in isotopic equilibrium with the atmosphere via CO2 exchange. During this process, 12C-rich CO2 is preferentially released from the lake surface into the atmosphere, resulting in a DIC pool that is enriched in 13C. This exchange process is slow; therefore, its effect on the δ13CDIC-L values is more notable in endorheic lakes with a long retention time. However, it is not easily observable in lakes with short retention times or rapid circulation [23]. When the exchange between lake water and atmospheric CO2 reaches an equilibrium, the δ13CDIC-L values range from 1 to 3‰ [24,25].
- (3)
- Organic matter decomposition in lake sediments. Sedimentary organic matter in lakes includes native aquatic plants and terrestrial organic debris transported into the lake from the surrounding watershed. Once degraded, this organic matter increases the 12C-enriched DIC composition of lake water [9]. Organic matter decomposition in Qingmuke Lake (a freshwater lake located on the Qiangtang Plateau) resulted in a DIC isotope value equal to, or even lower than, that of river water [10]. In contrast, the δ13CDIC-L values in the GGH Lake were significantly more positive than that of the Shazhuyu River, indicating that organic matter decomposition may have had a relatively small effect on the DIC lake composition. Additionally, methane produced by organic matter decomposition resulted in a more negative δ13CDIC value. Organic matter decomposition can cause a decrease in the δ13CDIC values to −50‰ [28]. This value is significantly lower than the mean δ13CDIC-L value of the GGH Lake, which indicates that the CO2 or methane produced via decomposition did not have a significant effect on seasonal or interannual changes in the δ13CDIC-L values.
- (4)
- Lake photosynthetic activity. In highly productive lakes, photosynthesis is a key factor that affects the δ13CDIC lake water values [29] (pp. 197–207), [30] (pp. 99–118). During photosynthesis, plants preferentially uptake 12C, which yields more negative δ13C values for plants and the δ13CDIC of the water body becomes more positive [31]. Charaphytes are an important submerged aquatic macrophyte. Compared with vascular plants, charaphytes have a higher photosynthetic rate and lower respiration rate. The preferential uptake of 12CO2 for photosynthetic purposes could have led to the 13C-enrichment of DIC in the lake water [32]. During intense photosynthesis, dissolved CO2 in lake water is limited [33]. When this occurs, charaphytes use HCO3 for photosynthetic activity. Compared with vascular plants, charaphytes can use HCO3− for photosynthetic activity more effectively [21]. According to Equations (1)–(3), the δ13C values of HCO3− were more positive than those of H2CO3 and CO32− in the lake water. In contrast, the photosynthetic activity of charaphytes results in carbonate precipitation in the surrounding waters, forming thick CaCO3 encrustations [33]. This also leads to 13C-enriched water in the charaphyte growth area.
- (5)
- Water retention time. In arid regions, with extended lake water residence times, strong evaporation leads to the preferential loss of the light 12CO2 and 16O2 isotopes, yielding more positive δ13CDIC-L and oxygen (δ18O) isotopic lake water compositions; furthermore, a significant positive correlation was observed between δ13CDIC-L and δ18OL [35]. Monitoring results revealed that the δ18OL values of the GGH Lake significantly deviated from the global meteoric water line but were consistent with the local evaporation line, indicating that evaporation affected the δ18OL lake water composition [19,36]. However, this study found that the δ13CDIC-L and δ18OL values of the GGH Lake were not correlated (Figure 6), indicating that evaporation may have had only a minimal effect on the δ13CDIC-L value of the lake.
5.2. Isotopic Composition of DIC in the GGH Basin Groundwater and Shazhuyu River
5.3. Implications of Lake Water DIC Isotopic Composition on the Carbon Cycle
6. Conclusions
- (1)
- For the overall DIC isotopic composition in the GGH Basin, we found that δ13CDIC-I was the most negative, followed by δ13CDIC-R; δ13CDIC-L was the most positive. This was caused by isotope fractionation resulting from the photosynthesis of aquatic plants after spring water inflow into the lake.
- (2)
- Owing to variations in the photosynthetic activity intensity of different aquatic plants, there were also significant variations in the δ13CDIC-L values in areas with different aquatic plants. This likely occurred because Chara spp. plants have a higher photosynthetic rate and are more capable of using CO2 for photosynthetic activity, converting them into plant organisms.
- (3)
- Variations in the δ13CDIC-L were primarily affected by the δ13CDIC-I and aquatic plant photosynthesis. The change in δ13CDIC-I to a more positive value resulted from carbon isotope equilibration between 13C from carbonate weathering in the watershed and 12CO2 from soil respiration.
- (4)
- The changes in the δ13CDIC-L composition of the GGH Lake indicated that the DIC from lake inflow and the photosynthesis of aquatic plants were the key components in the carbon cycle of the lake. This provides more supportive evidence to estimate the regional carbon budget and sustainable development.
Author Contributions
Funding
Data Availability Statement
Conflicts of Interest
References
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Water Sample Types | Measured Parameters |
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Lake water Groundwater River water | Water temperature pH DO carbon isotopes oxygen isotopes |
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Jin, Y.; Jin, X. Variations in δ13CDIC and Influencing Factors in a Shallow Macrophytic Lake on the Qinghai–Tibetan Plateau: Implications for the Regional Carbon Cycle and Sustainable Development. Sustainability 2024, 16, 3350. https://doi.org/10.3390/su16083350
Jin Y, Jin X. Variations in δ13CDIC and Influencing Factors in a Shallow Macrophytic Lake on the Qinghai–Tibetan Plateau: Implications for the Regional Carbon Cycle and Sustainable Development. Sustainability. 2024; 16(8):3350. https://doi.org/10.3390/su16083350
Chicago/Turabian StyleJin, Yanxiang, and Xin Jin. 2024. "Variations in δ13CDIC and Influencing Factors in a Shallow Macrophytic Lake on the Qinghai–Tibetan Plateau: Implications for the Regional Carbon Cycle and Sustainable Development" Sustainability 16, no. 8: 3350. https://doi.org/10.3390/su16083350