Spatial distribution and environmental significance of modern organic carbon and nitrogen stable isotopes in the source area of the Yellow River: A case study of Gyaring Lake and Ngoring Lake

Abstract

Organic carbon and total nitrogen isotopes (δ¹³C_org and δ¹⁵N) are useful for identifying sources of organic matter and for reconstructing environmental conditions in paleoecology and paleoclimatology. Gyaring Lake and Ngoring Lake are the two largest freshwater lakes in the source area of the Yellow River (SAYR), which lies in the northeastern the Qinghai–Tibet Plateau. These bodies of water were chosen as our research objects, and we analyzed the carbon and nitrogen isotopes in the surface sediments of the two lakes and in soil samples from around the lakes. The results indicate the consistency and endogenous characteristics of the organic matter sources of the distribution of δ¹³C_org and δ¹⁵N in the surface sediments of Gyaring Lake and Ngoring Lake. The δ¹³C_org values of the two lakes were significantly positive in areas near the inflow and outflow of the Yellow River. The distribution of δ¹⁵N in Gyaring Lake was similar to that of total organic carbon (TOC) and total nitrogen (TN), and its values were relatively higher than those affected by the joint influence of the Yellow River and some small tributaries, while δ¹⁵N was more positive in the deep-water area of Ngoring Lake. This study provides reliable and modern data on δ¹³C_org and δ¹⁵N for the long-term paleoenvironment and paleoecological reconstruction of the SAYR in the future.

Keywords

Gyaring Lake Ngoring Lake organic matter sources Qinghai–Tibet Plateau the source area of the Yellow River

Introduction

The Qinghai–Tibet Plateau, which is characterized by fragile ecosystems, has an important impact on changes in the ecological and climatic environment of China and even the world (Li et al., 2001; Pu et al., 2020; Zhu et al., 2008). The source area of the Yellow River (SAYR) is located in the hinterland of the Qinghai–Tibet Plateau, a region where the transitional zone between the Asian monsoon and the westerlies occurs (An et al., 2012; Chen et al., 2008). There are more than 4000 lakes in the SAYR, including 48 lakes with areas greater than 0.50 km² and a total water surface area of 1664.6 km² (Wang and Dou, 1998). These lakes are important freshwater resources in the region and have a major influence on its social and ecological systems. However, in recent decades, owing to global warming, the environment of the Qinghai–Tibet Plateau has experienced a series of changes, including rising snow lines, melting glaciers, shrinking wetlands, overloaded grasslands, decreased forests, and vegetation degradation (Shao et al., 2008). In addition, the drought-related decreased flow of the lower Yellow River that first appeared in 1972 has expanded to the SAYR since the 1990s (Dai et al., 2006; Wu et al., 1998). Researchers have successively reconstructed the climatological and environmental history of some small lakes around the SAYR, such as Donggi Cona Lake and Kuhai Lake (Li et al., 2017; Zhang et al., 2019). However, this reconstruction does not fully reflect the climate and ecological environment changes in the SAYR due to the high topographic variation in the Qinghai–Tibet Plateau, which has obvious regional limitations in the climate histories of these small lakes. Therefore, studies of more representative lakes are needed to assess the ecological and environmental conditions of the SAYR.

From the perspective of such research, lake sediments present advantages such as their continuous deposition, reliable ages, long-term record time series, and large number of proxy indicators storing biological and geochemical information, which can be used to faithfully record the environmental history over the past 100 years or longer (Last and Smol, 2001; Smol et al., 2005; Wang and Zhang, 1999). The carbon and nitrogen contents of lake sediments provide information that can faithfully record their environmental history (Zhuo and Zeng, 2020). In addition, research on the distribution and sources of nutrient elements such as carbon and nitrogen in lake sediments is helpful for assessing the characteristics of lake organic matter and systemic pollution sources (Cheng et al., 2020; Hu et al., 2006). Both δ¹³C_org and δ¹⁵N isotopes have been shown to be powerful proxies for use in identifying whether organic matter and nitrogen originated from natural processes or human activities in aquatic environments (Costanzo et al., 2001; McClelland et al., 1997; Naeher et al., 2016; Vander Zanden et al., 2005) and in reconstructing the paleoenvironment with palaeolimnological and paleoecological data (Herczeg et al., 2001; Pu et al., 2020). However, such reconstructions based on δ¹³C_org and δ¹⁵N data can still be uncertain and can lead to multiple explanations. In addition, previous studies have conducted mostly qualitative comparative research or semiquantitative estimations based on the range of variation in δ¹³C_org, δ¹⁵N, and C/N (Herczeg et al., 2001; Liu et al., 2013; Lücke et al., 2003; Pu et al., 2020). Accordingly, at present, a clear analysis based on δ¹³C_org and δ¹⁵N data corresponding to modern sedimentary processes is still needed to further clarify the source characteristics of organic matter in modern sediments and thus enable the interpretation of δ¹³C_org and δ¹⁵N data more accurately in the reconstruction of paleoclimatological and paleoecological environments.

Gyaring Lake and Ngoring Lake, the two largest high-altitude freshwater lakes in the SAYR, each with an area greater than 500 km², can comprehensively represent the ecological and climatic changes in the SAYR. However, research on the reconstruction of the climatological and ecological environments of the two great lakes remains incomplete, and most relevant studies have focused on paleoclimatological reconstruction (Pu et al., 2020; Zhao et al., 2021). Therefore, we selected Gyaring Lake and Ngoring Lake as research subjects in the present study. Our aims were to analyze the spatial distribution characteristics and environmental significance of modern organic carbon and nitrogen stable isotopes in the SAYR and to provide a more accurate scientific characterization of the study site and modern data-based analysis for further studies in the future, especially regarding the application of δ¹³C_org and δ¹⁵N isotopes in the reconstruction of paleoclimatological and paleoecological environments.

Materials and methods

Study area

The tectonic lakes Gyaring Lake and Ngoring Lake are located in the northeastern Qinghai–Tibet Plateau in Maduo County, Qinghai Province, China. The two lakes are also called “Twin Lakes” or “Sister Lakes” in the SAYR owing to their proximity of approximately 20 km (Figure 1). The average altitude of the two lakes is more than 4200 m; additionally, they are more than 190 km from Kariqu, the source of the Yellow River to the east, and more than 60 km from Maduo County to the west. The Yellow River originates from the Kariqu River and the Yueguzongliequ River at the north foot of the Bayan Har Mountains, first flowing into Gyaring Lake through Xingxiuhai and the Maqu River and then flowing into Ngoring Lake from the southwest (Wang and Dou, 1998; Zhang et al., 2010). The surroundings of the two lakes have an alpine continental climate with an average annual temperature of −3.6°C, and they receive an average annual precipitation of 326.2 mm according to meteorological observation data from the Maduo County meteorological station from 1953 to 2019 (data from China’s National Climate Center). The two lakes start to freeze in late November and remain frozen for an average of 157 days. The main water supply sources of the two lakes are rivers, atmospheric precipitation, and ice melt water. There are many small lakes around the two lakes, including 32 sub lakes around Gyaring Lake and more than 40 sub lakes around Ngoring Lake. The regional vegetation is dominated by typical alpine meadow species such as Kobresia spp. (Zhao et al., 2021).

Figure 1.

Study area, lake location, and sample sites in Gyaring Lake and Ngoring Lake. The yellow circle represents surface sample sites in Gyaring Lake; the red circle represents surface sample sites in Ngoring Lake, the green circle represents soil surface sample sites and the black circle represents aquatic plant sample sites.

Gyaring Lake (34°48′–35°01′ N, 97°02′–97°30′ E) is the second largest freshwater lake in the Yellow River basin and is relatively deep, with a maximum depth of 14.2 m, an average depth of 8.9 m, a length of 35.0 km, a maximum width of 21.6 km, and an area of 526.0 km² (Shao et al., 2008; Zhao et al., 2021). The water of Gyaring Lake is gray to white with a transparency of approximately 1.0–3.0 m and a mineralization of approximately 0.55 g/L (Shang et al., 2006). Many rivers flow into Gyaring Lake, including the Yellow River and Kariqu River, and the rivers outflowing from the lake are located in the wide valley of the Yellow River in the southeast.

Ngoring Lake (34°45′–35°05′ N, 97°31′–97°55′ E) is the largest freshwater lake in the Yellow River basin and is deep in the north and shallow in the south, with an average depth of 17.6 m, a maximum depth of 34.7 m, a length of 32.3 km, a maximum width of 31.6 km, an average width of 18.9 km, and an area of 610 km² (Pu et al., 2020; Shang et al., 2006). Except for the Yellow River, there are only two small tributaries on the northern and southern sides of Ngoring Lake (Shang et al., 2006). The water of Ngoring Lake is blue, with a transparency of 2.0–5.0 m, a mineralization of 0.38 g/L, a total hardness of 167.23 mg/L, and a pH value of 8.49.

Sample collection and proxy analysis

In August 2015, a total of 53 surface sediment samples were collected using a gravity corer from Gyaring Lake (22 samples at water depths of 4.10–13.7 m) and Ngoring Lake (31 samples at water depths of 2.10–33.3 m). All surface samples were of the top 0–1 cm and were stored at 4°C in the laboratory. A total of 17 soil surface samples were collected around the two lakes. In addition, 16 aquatic plant samples (floating plants, floating-leaved plants, and submerged plants) from some of the same sites as the surface samples in Gyaring Lake and Ngoring Lake were collected in August 2015 (Figure 1) (Supplemental Table S1, available online).

The total organic carbon (TOC) content, total nitrogen (TN) content, and δ¹³C_org and δ¹⁵N values of the lake surface samples, soil surface samples, and aquatic plants were analyzed. TOC and TN were measured with an EA3000 elemental analyzer (Eurovector, Pavia, Italy) after the removal of carbonates from the samples. The calculation of the TOC/TN ratio was conducted on an atomic basis. The δ¹³C_org and δ¹⁵N values were measured using a Thermo Fisher Delta V Advantage mass spectrometer (Thermo Fisher, Waltham, MA, USA). The analytical errors for the δ¹³C_org and δ¹⁵N isotopic values were less than ±0.2‰ (refer to V-PDB Standards) and ±1.0% (refer to AIR Standards), respectively.

Fifty-three lake surface samples from Gyaring Lake and Ngoring Lake were analyzed for grain size using a laser size distribution apparatus with a measurement range of 0.02–2000 μm (Malvern Mastersizer 2000; Malvern Instruments Ltd., Malvern, UK). Samples were pretreated with 10–20 ml of 30% H₂O₂ to remove organic matter and then with 10 ml of 10% HCl to remove carbonates. Approximately 24 h later, deionized water was added to the sample solution to rinse away acidic ions. The sample residue was finally treated with 10 ml of 0.05M (NaPO₃)₆ on an ultrasonic vibrator for 10 min to facilitate dispersion before grain-size analysis (Peng et al., 2005). The analytical error was less than ±2.0%.

Data analysis

To obtain information more intuitively about the spatial distribution characteristics of the TOC, TN, and δ¹³C_org and δ¹⁵N isotope values of the two lakes, the kriging method (ArcMap 10.2; Esri, Redlands, CA, USA) and Pearson correlation analysis method (SPSS 19.0; IBM Corp., Armonk, NY, USA) were used to further analyze the data.

Results

Carbon, nitrogen and stable isotope composition characteristics and grain sizes of lake surface sediments

Variations in the TOC, TN, δ¹³C_org, δ¹⁵N and grain sizes of surface sediments in Gyaring Lake and Ngoring Lake are determined and are presented in Figures 2 –4. The TOC and TN concentrations from Gyaring Lake ranged from 1.0% to 6.4%, with a mean value of 2.7%, and from 0.1% to 1.0% with a mean value of 0.4%, respectively, and were higher than those of Ngoring Lake, where the TOC and TN concentrations varied from 0.5% to 4.8%, with a mean value of 2.5%, and from 0.1% to 0.7%, with a mean value of 0.3%, respectively (Figure 2(a) and (b)). The δ¹³C_org values of the sediment samples from Gyaring Lake varied within the range from −34.1‰ to −22.0‰, with a mean value of −26.5‰, while in Ngoring Lake, they varied from −30.7‰ to −20.3‰, with a mean value of −27.3‰ (Figure 2(c)). However, the δ¹⁵N values followed a spatial distribution pattern opposite to that of the δ¹³C_org values, ranging from 4.5‰ to 21.5‰, with a mean value of 9.8‰, in Gyaring Lake and from 3.9‰ to 20.4‰, with a mean value of 10.2‰, in Ngoring Lake (Figure 2(d)).

Figure 2.

Spatial distributions of the values of TOC (a), TN (b), δ¹³C_org (c), and δ¹⁵N (d) from the surface sediments in Gyaring Lake and Ngoring Lake.

Figure 3.

Carbon, nitrogen and stable isotope composition characteristics, and grain sizes of lake surface sediments in Gyaring Lake.

Figure 4.

Carbon, nitrogen and stable isotope composition characteristics, and grain sizes of lake surface sediments in Ngoring Lake.

The distribution of the C/N ratios from the surface sediments in the two lakes is intuitively expressed in the form of curves in Figures 3 and 4. The C/N ratios varied between 7.1 and 11.5, with an average of 8.4, in Gyaring Lake. Moreover, the maximum values appeared at points 12 and 15 in northern Gyaring Lake (Figure 3). The C/N ratios from Ngoring Lake ranged from 7.8 to 10.5, with an average of 8.7. The sites with C/N ratios exceeding 10 were distributed among point 3 in the north and points 22 and 25 in the west of Ngoring Lake, all of which were close to the Yellow River (Figure 4).

Data on the grain-size distribution [clay (<4 μm), silt (4–63 μm), sand (>63 μm)] of surface sediments from Gyaring Lake and Ngoring Lake are presented in Figures 3 and 4. Silt comprised the highest percentage in samples from both Gyaring Lake and Ngoring Lake, with averages of 44.08% and 56.05%, respectively. The sand content of the Gyaring Lake samples varied from 12.66% to 69.21% and had an average of 32.03%, which was slightly higher than that of the Ngoring Lake samples, which ranged from 4.82% to 81.60% and had an average of 22.06%. The clay content varied between 12.70% and 34.51% and had an average of 24.07% in Gyaring Lake. In Ngoring Lake, the clay contents ranged from 5.80% to 32.84% and had an average of 22.37%. In addition, in the two lakes, the sand contents of points 12 and 15 in the north of Gyaring Lake and points 22 and 25 in the west of Ngoring Lake were abnormally higher than the silt contents at the same points (Figures 3 and 4).

Carbon, nitrogen and stable isotope composition characteristics of aquatic plants

The variations in the δ¹³C and δ¹⁵N of aquatic plants (floating plants, floating-leaved plants, and submerged plants) in Gyaring Lake and Ngoring Lake were determined and are presented in Figure 5 (Supplemental Table S1, available online). The δ¹³C and δ¹⁵N values of aquatic plants from Gyaring Lake varied within a range from −34.8‰ to −14.2‰ with a mean value of −24.1‰ and from 3.9% to 8.8% with a mean value of 6.2%, respectively, while in Ngoring Lake, the values varied from −35.2‰ to −7.7‰ with a mean value of −26.0‰ and from 3.5% to 13.9% with a mean value of 10.0%, respectively (Figure 5). The TOC, TN, and C/N concentrations of aquatic plants in Gyaring Lake and Ngoring Lake are listed in Supplemental Table S1, available online.

Figure 5.

The values of δ¹³C in ZLH (a) and ELH (b) and δ¹⁵N in ZLH (c) and ELH (d) from the aquatic plants in Gyaring Lake and Ngoring Lake.

The carbon, nitrogen and stable isotope compositions of soil surface samples around Gyaring Lake and Ngoring Lake

The values of TOC, δ¹³C_org, TN, and δ¹⁵N from the soil surface samples around Gyaring Lake and Ngoring Lake are displayed in Figure 6. In detail, the TOC concentrations varied from 0.3% to 2.6% and had an average of 1.2% (Figure 6(1)). The δ¹³C_org values of the soil surface samples varied between −27.4‰ and −22.1‰ and had an average of −25.1‰ (Figure 6(2)). The TN concentrations followed a pattern very similar to that of TOC, varying from 0.1% to 0.3%, with an average of 0.1% (Figure 6(3)). The δ¹⁵N values varied between 2.6‰ and 6.8‰, with an average of 4.6‰ (Figure 6(4)). The C/N ratios varied from 6.3 to 13.0, with an average of 9.9 (Figure 6(5)).

Figure 6.

The values of TOC (1), δ¹³C_org (2), TN (3), δ¹⁵N (4) and C/N (5) from soil surface samples around Gyaring Lake and Ngoring Lake.

Statistical analysis

Pearson correlation analysis was carried out among the carbon, nitrogen, and stable isotopes of lake surface sediments and for the measurements between lake surface sediments and aquatic plants at the same point (Supplemental Table S2, available online). Significant correlations among the carbon, nitrogen, and stable isotopes of lake surface sediments and for the measurements between lake surface sediments and aquatic plants at the same point were identified (Figures 7 and 8).

Figure 7.

Pearson’s correlation analysis among the carbon, nitrogen and stable isotopes in lake surface sediments (Gyaring Lake. (1) TN and TOC. (2) TN and δ¹⁵N; Ngoring Lake. (3) δ¹⁵N and δ¹³Corg. (4) TN and TOC. (5) TN and δ¹⁵N).

Figure 8.

Pearson’s correlation analysis among the carbon, nitrogen and stable isotopes in lake surface sediments with aquatic plants in Gyaring Lake and Ngoring Lake (Gyaring Lake (a-1): C/N, (a-2): δ¹³C_org and δ¹³C; Ngoring Lake (b-1): TN, (b-2): δ¹³C_org and δ¹³C).

The results showed positive correlations between the TN, TOC, and δ¹⁵N in the lake surface sediments in Gyaring Lake and Ngoring Lake (Figure 7). However, a negative correlation was identified between the δ¹⁵N and δ¹³C_org in the lake surface sediments in Ngoring Lake (Figure 7).

Significant positive correlations were also found between the δ¹³C in lake surface sediments with corresponding data from aquatic plant samples in Gyaring Lake and Ngoring Lake (Figure 8). In addition, the C/N in Gyaring Lake and the TN in Ngoring Lake showed significant positive correlations in the lake surface sediments with corresponding data from aquatic plant samples (Figure 8).

Discussion

There was a certain regularity and similarity in the spatial distributions of carbon, nitrogen, and their isotopes in Gyaring Lake and Ngoring Lake (Figure 2). The range characteristics of the variation in δ¹³C_org in Gyaring Lake and Ngoring Lake were from −35‰ to −20‰, basically corresponding to the trends of the other indicators (C/N, TN, TOC, and δ¹⁵N) (Figures 3 and 4). The carbon stable isotope compositions of aquatic plants are assumed to be affected mainly by the carbon source, dissolved inorganic carbon concentration and growth-rate-related fractionation during photosynthesis (Herzschuh et al., 2010; Liu, 2020). These range characteristics of the negative variation in the δ¹³C_org in Gyaring Lake and Ngoring Lake were largely caused by C3-like photosynthesis (Figures 3 and 4), which includes emergent plants, floating plants and floating-leaved plants and is related to the ability to utilize CO₂ as a carbon source (Farquhar et al., 1982; Herzschuh et al., 2010; Keeley and Sandquist, 1992). This result implies that the principal origin of the organic matter in the sediments of the two lakes is in-lake productivity. The inorganic nutrient isotopic composition and N limitation-related fractionation processes are the main control factors for the nitrogen isotopic composition of aquatic plants (Herzschuh et al., 2010). The δ¹⁵N value of the NO³⁻ dissolved in lake water and assimilated by aquatic plants are typically 5‰–10‰, with an average of 8.5‰, whereas the value of atmospheric N₂ that is used by land plants is 0‰ (Botrel et al., 2014; Meyers, 2003; Pu et al., 2020; Woodward et al., 2012). Therefore, the average value of δ¹⁵N in the two lakes (9‰–10‰) basically indicated that autochthonous origins were the major source of organic matter in Gyaring Lake and Ngoring Lake sediments. In addition, the C/N ratios (close to 10) of the surface sediments in most sampling sites of Gyaring Lake and Ngoring Lake (Figures 3 and 4) indicated the main source of organic matter and plant types (submerged and floating aquatic macrophytes or organic matter of a mixed source) in the sediments of the two lakes (Meyers, 2003). As discussed above, the total organic matter in the surface sediment in Gyaring Lake and Ngoring Lake was mainly contributed by aquatic plants, which further indicated that in-lake productivity is a major source of organic matter in the two lakes. The significantly positive relationship of δ¹³C_org in lake surface sediments with δ¹³C in aquatic plants in Gyaring Lake and Ngoring Lake further indicated the consistency and endogenous characteristics of the organic matter sources in the two lakes (Figure 7).

The distribution of carbon and nitrogen isotopes in different regions of the two lakes also had significant differences. The δ¹³C_org values were significantly positive in southeastern Gyaring Lake, where the Yellow River flows into the lake, and in southwestern and northeastern Ngoring Lake, which are basically located at the inflow and outflow, respectively, of the Yellow River (Figure 2). Around the southeastern areas of Gyaring Lake and the southwestern and northeastern areas of Ngoring Lake, which are close to the Yellow River, lacustrine swamps and large aquatic vegetation are distributed (Wang and Liang, 1981). The positive δ¹³C_org values may be due to the influence of large aquatic plants in the swamp. Modern aquatic plants also prove that the types of aquatic plants at the sampling points near the entrance and exit of the Yellow River (the water depths of points 11, 14, and 17 of Gyaring Lake and point 29 of Ngoring Lake were all less than 10 m) are mostly floating-leaved plants (Supplemental Table S1, available online) (Figure 1). The δ¹³C_org values recorded in the northern area of Gyaring Lake, where a small river flows into the lake, were significantly negative (Figure 2). Vegetation in the northern part of Gyaring Lake, where the small river is located, is rare, and there is a large distribution of exposed rocks. We inferred that the coarse exogenous particulate matter was brought into the lake from the small river, resulting in the overall proportion of organic debris to increase in this lake area (the highest value of TOC (6.35%) was recorded at point 18) (Thompson and Eglinton, 1978). In addition, the C/N ratios of some sites in northern Gyaring Lake where a small river flows into the lake and in southwestern Ngoring Lake where the Yellow River flows into the lake were greater than 10 (Figures 1 and 3). The grain size analysis results were consistent with the C/N ratio results; that is, the sand content was the greatest at points 12 and 15 near the northern river inlet of Gyaring Lake and at points 22 and 25 near the Yellow River inlet of Ngoring Lake, which was related to the amount of coarse-grained material brought by the river inflow (Figures 3 and 4). The carbon and nitrogen analysis of the soil surface samples in the lacustrine basin also demonstrated that the rivers around the two lakes had an impact on the deposition of organic matter in the lake sediments (Figure 6). In particular, the TOC, TN, and C/N ratio values of surface soil in the basin at points S6 (along the southern lakeside of Gyaring Lake), S7 (near the Duoqu River tributary of the Yellow River around Gyaring Lake), and S11–S17 (near the tributaries around Ngoring Lake) were higher, and their corresponding δ¹³C_org values were negative, which was basically consistent with the analysis of surface sediments at adjacent points in the lake area (Figures 1 and 6).

The δ¹⁵N of organic matter in lake sediments can be used to identify dominant nitrogen sources (human-derived nitrogen typically with a high δ¹⁵N composition (between +10‰ and +20‰) and atmospheric N with a lower δ¹⁵N (−15‰ to 15‰)), internal microbial cycling or changes in lake productivity (Botrel et al., 2014; Kendall et al., 2007). The values of δ¹⁵N in Gyaring Lake and Ngoring Lake varied in the range of 3‰–22‰, which indicated that the nitrogen sources of the two lakes are diverse. The relatively higher δ¹⁵N values in northern, southeastern, and northwestern Gyaring Lake where rivers flow into or out of the lake may indicate that in addition to the nitrogen sources generated by atmospheric deposition and aquatic plants in the lake, the Yellow River and small tributaries carry additional exogenous nitrogen into the lake. It can be inferred that the rivers in the SAYR have a significant impact on the distribution of nitrogen isotopes in the surface sediments of the lakes. Previous studies have shown that water residence time and lake shape also affect internal nitrogen processing by microbes on sediment δ¹⁵N (Botrel et al., 2014; Finlay et al., 2013; Harrison et al., 2009). In the northern, central and southern parts of Ngoring Lake, where the water depth is deeper, the δ¹⁵N values were significantly positive (Figure 2). The dissolved inorganic nitrogen and total nitrogen loading in deep water may be higher, and the retention time of lake nitrogen may be prolonged due to the larger water volume in the deep-water area, resulting in the weakening of nitrogen isotope fractionation and the positive value of nitrogen isotopes in sediments (Woodward et al., 2012).

The average δ¹³C_org values in Gyaring Lake were more positive than those in Ngoring Lake from the perspective of spatial distribution (Figures 2 and 9). It is possible that more organic matter contributed by terrestrial C₃ plants (δ¹³C, −33‰ to −21‰) enters Gyaring Lake relative to Ngoring Lake (Cheng and Ye, 2019; Farquhar et al., 1982; Meyers, 1994). Since the Yellow River flows into Gyaring Lake first and then flows into Ngoring Lake, the terrestrial carbon and nitrogen concentrations carried by the Yellow River are likely to be greatly reduced and detained in Gyaring Lake, as the lake is upstream of Ngoring Lake (Zhang et al., 2010). In recent years, the section of the Yellow River between Gyaring Lake and Ngoring Lake and the water outlet of the Yellow River in Ngoring Lake have been obstructed several times, and the closure of Ngoring Lake has been further exacerbated by the obstruction at the outlet of Ngoring Lake, which weakens the input of carbon and nitrogen from Gyaring Lake into Ngoring Lake to some extent (Dai et al., 2006; Zhang et al., 2010). The highest contents of sand at points 22 and 25 at the Yellow River inlet in the southwestern area of Ngoring Lake correspond to the lowest contents of TOC, TN, δ¹⁵N, clay and silt in the adjacent positions of the lake (Figure 4). This result also indicated that the Yellow River, after flowing through Gyaring Lake, carried a number of exogenous materials into the downstream Ngoring Lake, but there was not much organic matter deposited in this lake. Moreover, the tributaries to the Gyaring Lake basin are much more numerous than those entering Ngoring Lake, which corresponds to the disproportionately larger allochthonous origins of some bulk organic matter entering Gyaring Lake relative to Ngoring Lake (Shang et al., 2006). The correlation analysis also showed a negative correlation between the carbon and nitrogen isotopes in Ngoring Lake (Figure 7), which indicates that there are fewer tributaries of Ngoring Lake with relatively weak interference from the basin in comparison to Gyaring Lake (Shang et al., 2006; Shen et al., 2010). In addition, the water level of Ngoring Lake is much deeper than that of Gyaring Lake, and the deepest position is more than 20 m (Supplemental Table S1, available online). Previous studies have shown that changes in lake water levels affect the carbon and nitrogen isotope values of aquatic plants (Burns and Walker, 2000; Liu et al., 2013, 2019; Robinson et al., 1997). According to our research results, the spatial distribution of aquatic plants is related to the change in water depth (Supplemental Table S1, available online). For example, in the deep lake area, the main plant type is floating plants, while submerged plants and floating-leaved plants are mostly distributed in the lake areas with a water level less than 10 m, and the carbon isotope values in the surface sediments at the same location are relatively heavy (−21.03‰ to −23.25‰) (Supplemental Table S1, available online). The substantially negative δ¹³C_org values and more positive δ¹⁵N in the middle of Ngoring Lake are also closely related to the maximum water level in this area, which further demonstrates that water depth has a great influence on the δ¹³C_org and δ¹⁵N in sediments.

Figure 9.

Scatter chart showing the distribution characteristics of δ¹⁵N and δ¹³C_org in lake surface sediments and soil surface samples of Ngoring Lake and Gyaring Lake.

The organic carbon and nitrogen isotopes of Gyaring Lake and Ngoring Lake were generally lower than those of other lakes on the Qinghai–Tibet Plateau (Aichner et al., 2012; Ji et al., 2005; Pu et al., 2013), which is closely related to the higher elevation and harsher environment in the SAYR on the Qinghai–Tibet Plateau, coupled with its lower vegetation coverage, more fragile ecosystem, low precipitation, and weak surface runoff (Pu et al., 2020). Thus, the transportation of land-derived nutrients and organic matter from the watershed to the lakes is limited. The δ¹⁵N distribution of the surface soil in the basin was relatively uniform and was generally lower than the value of sediments in the two lakes (Figure 6). Only points S5–S6 (southern Gyaring Lake), S8 (southwestern Gyaring Lake), S13–S14 (southeastern basin of Ngoring Lake and the northeastern outlet of the Yellow River, respectively), and S17 (northwestern Ngoring Lake) were lower than the average value (4.61‰), and these collection points are far from the lake shore and thus weakly affected by the river. The δ¹⁵N value of the surface soil in the scatter map was even lower, indicating that the nitrogen isotope value in the terrestrial grassland environment is lower than that of the lake environment in the Qinghai–Tibet Plateau (Botrel et al., 2014; Pu et al., 2020; Woodward et al., 2012). The values of carbon and nitrogen isotopes in the surface soil of the basin were more negative than those of the two lakes (Figure 9), which also indicates that the basin interferes with the lakes to some extent. This is precisely why this region is a relatively ideal research area for reconstructing the paleoclimate and environment.

Conclusion

This study of the present δ¹³C_org and δ¹⁵N analyses of Gyaring Lake and Ngoring Lake indicated the consistency and endogenous characteristics of the organic matter sources in the two lakes. However, there were obvious spatial differences in the distribution of δ¹³C_org and δ¹⁵N in the surface sediments of Gyaring Lake and Ngoring Lake. The δ¹³C_org values in the surface sediments of the two lakes were significantly positive in the lake areas near the outflow and inflow of the Yellow River. The distribution of δ¹⁵N in the two lakes was significantly different. Among them, in the outflow and inflow areas of Gyaring Lake, the exogenous organic matter carried by the rivers had a greater impact on the δ¹⁵N value, which was significantly positive, while δ¹⁵N was more positive in the deep-water area of Ngoring Lake. The study of the δ¹³C_org and δ¹⁵N in the surface sediments of Gyaring Lake and Ngoring Lake provides a favorable empirical basis for the reconstruction of paleoclimate and paleoecology on a long-term timescale in the Qinghai–Tibet Plateau in future research.

Supplemental Material

sj-xlsx-1-hol-10.1177_09596836221126125 – Supplemental material for Spatial distribution and environmental significance of modern organic carbon and nitrogen stable isotopes in the source area of the Yellow River: A case study of Gyaring Lake and Ngoring Lake

Supplemental material, sj-xlsx-1-hol-10.1177_09596836221126125 for Spatial distribution and environmental significance of modern organic carbon and nitrogen stable isotopes in the source area of the Yellow River: A case study of Gyaring Lake and Ngoring Lake by Longjuan Cheng, Qingfeng Jiang, Ming Ji, Dongliang Ning, Ying Ge, Xiang Kuai and Wenwei Zhao in The Holocene

Footnotes

Acknowledgements

We would like to thank the Yellow River Source Area Management Committee for field assistance. We’d also acknowledge Jianan Zheng, Yufeng Yang, and Yuchen Xu for their help in lab examinations.

Data availability statement

The data for the work in this paper can be downloaded from Mendeley website (available from ).

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by the National Natural Science Foundation of China (Grant numbers 41672349, 42002204), the Second Tibetan Plateau Scientific Expedition and Research Program (STEP) (Grant Number 2019QZKK0202), the Jiangsu Provincial Department of Education (Grant Number 20KJB170021), and the Talent Introduction Project of Nantong University (135421621016).

ORCID iDs

Longjuan Cheng

Ming Ji

Supplemental material

Supplemental material for this article is available online.

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