Lake sediment is an important carbon reservoir. However, little is known on
the dynamics and sources of sediment organic carbon in Bosten Lake. We
collected 13 surface (0–2 cm) sediment samples in Bosten Lake and
analyzed total organic carbon (TOC), total nitrogen (TN), stable carbon
isotopic composition in TOC (
Inland water bodies such as rivers and lakes are unique components on the Earth. In spite of their relatively small coverage (Downing et al., 2006), lakes often receive a large amount of terrestrial materials from the watersheds (Battin et al., 2009; Anderson et al., 2013) and store a significant amount of carbon in the sediments (Ferland et al., 2012; Tranvik et al., 2009). Thus, inland lakes may play an important role in the terrestrial carbon cycle. Compared to the oceans, lakes have active biogeochemical processes with stronger “biological pump”, which often leads to higher sedimentation rates and a large amount of organic carbon (OC) burial at the bottom of lakes (Dean and Gorham, 1998).
There have been a number of studies from North America (Dean and Gorham, 1998), western Europe (Bechtel and Schubert, 2009; Woszczyk et al., 2011), eastern Asia (Khim et al., 2005; Wang et al., 2012), and other regions (Dunn et al., 2008), showing large spatial variability in total organic carbon (TOC) of lake sediment. The magnitude of TOC in surface sediment may depend on many factors, including column water productivity, terrestrial inputs of organic materials, properties of sediment, and rate of microbial activity (Burone et al., 2003; Gireeshkumar et al., 2013). Among them, contributions of autochthonous and allochthonous sources have direct impacts on the spatial distribution, which varies largely across regions (Bechtel and Schubert, 2009; Anderson et al., 2009), partly due to differences in lake productivity and morphology (Barnes and Barnes, 1978). In general, lakes with high productivity have more autochthonous TOC, but lakes with low productivity mainly allochthonous TOC (Dean and Gorham, 1998). There is evidence of littoral sources of TOC in small and shallow lakes but autochthonous sources, derived from planktonic organisms, in larger and deeper lakes, especially fjord lakes (Shanahan et al., 2013; Sifeddine et al., 2011; Barnes and Barnes, 1978).
A number of techniques have been applied to quantify different sources of
sediment TOC (Fang et al., 2014; Hanson et al., 2014; Meyers and Ishiwatari,
1993; Bechtel and Schubert, 2009). One of the common approaches is to use
two- or three-end-member mixing models with combined use of TOC to total nitrogen
(TN) ratio (C : N) and stable carbon isotope in organic material (
Bosten Lake, as the largest lake in Xinjiang of China, is a typical place for studying lake carbon cycle. Previous studies have provided evaluations on water quality (Wu et al., 2013), changes in lake level (Guo et al., 2014), and the controlling factors of carbon and oxygen isotopic composition of surface sediment carbonate (Zhang et al., 2009). A recent study indicated that particulate organic carbon (POC) variability in the water column was affected by allochthonous sources in Bosten Lake (Wang et al., 2014). However, little has been done to assess the dynamics and sources of sediment TOC in Bosten Lake. Therefore, this study was designed to evaluate the spatial distributions of major physical and biogeochemical parameters in the surface sediment, and to quantify the contributions of various sources to the sediment TOC in Bosten Lake.
Bosten Lake (41
Map of Bosten Lake with the water depth and the 13 sampling stations (red dotes). Bathymetry was measured in 2008 by Wu et al. (2013) and bathymetric contours were plotted by using software ArcGIS 9.3 and CorelDraw X3.
For the present study, a Kajak gravity corer was used to collect surface sediments from 13 sites in the main section of Bosten Lake in August 2012 (Fig. 1). The sampling sites covered most parts of the lake, with water depths ranging from 3 to 14 m. The sediment cores were carefully extruded, and the top 2 cm sections were sliced into 1 cm and placed in polyethylene bags that were kept on ice in a cooler during transport and before analyses.
Following Liu et al. (2014), each sediment sample (
Sediment C and N contents were measured using an Elemental Analyzer 3000
(Euro Vector, Italy) at the SKLLSE, Nanjing Institute of Geography and
Limnology, CAS. All samples were freeze-dried and ground into a fine powder,
then placed in tin capsules, weighed, and packed carefully, according to
Eksperiandova et al. (2011). For the analysis of TOC, each sample
(
For the analyses of
We applied a three-end-member mixing model (Liu and Kao, 2007) to quantify
the contributions (
Given that there were limited crops around the lake and most crops' growing
season was less than 5 months each year, we assumed that native plants,
mainly reed (
Distributions of
We measured POC, particulate organic nitrogen (PON), and
Our approach may have uncertainties in determining TOC sources. However, the
uncertainties would be small given that the standard errors in
Correlation analyses were performed using the SPSS Statistics 19 for Windows. Spatial distribution maps were produced using Surfer 9.0 (Golden Software Inc.), and the kriging method of gridding was used for data interpolation.
Figure 2 showed the spatial distributions of the main granulometric variables
of the surface sediment. In general, clay content was low (6–17 %),
showing relatively higher values in the southern part than in the northern
part. The highest clay content was found in the southwest and the lowest in
the northwest section. On the other hand, silt content was much higher (greater
than 80 %) with clearly higher values near the mouths of the Kaidu River
(southwest) and Huangshui River (northwest). The lowest content of silt was
found in the mid-west, between the rivers' mouths, where sand content was
highest (Fig. 2c). As expected, the spatial distribution of
Spatial distributions of
Concentration of TOC was highly variable, with higher values (4.3–4.4 %) found in the northern and eastern sections of the lake (Fig. 3a). There was also high concentration of TOC (4.1–4.2 %) near the mouth of the Kaidu River (southwest). On the other hand, lower TOC concentration (1.8–2.4 %) was observed in the mid-west section. Similarly, TN concentration (ranging from 0.28 to 0.68 %) was lowest in the mid-west and highest in the northwest and east sections (Fig. 3b). Overall, the spatial distribution of TN was similar to that of TOC. The exception was in the northwest area that had a high TN value but low TOC concentration.
Spatial distribution of
Figure 4a showed a large spatial variability in the C : N ratio with a
range from 4.6 to 8.6. In general, C : N ratio was higher in the central
part relative to other parts. The highest C : N ratio was found in the
mid-west and the lowest found in the northwest area. The
Using the three-end-member mixing model, we calculated the contributions of autochthonous and allochthonous sources to the surface sediment TOC. As shown in Fig. 5a, the contribution of lake plankton ranged from 54 to 90 %, with the highest in the western shallow lake area, and the lowest in the southern and eastern deep lake area. The contribution of soils varied between 10 and 40 %, with the highest in the southeast and central south area (Fig. 5b). Apparently, the contribution from native plants was extremely low (< 4 %), with only a few sites showing values of 10–12 % (Fig. 5c). On average, the contributions from lake plankton, soils, and native plants were 66, 30, and 4 %, respectively.
Spatial patterns of the relative contributions for TOC in the 0–1 cm
(color map) and 1–2 cm (dashed lines) sediments.
There were large differences in the spatial distributions of TOC between the
autochthonous and allochthonous sources. Autochthonous TOC revealed highest
value (
Spatial distributions of
The concentration of TOC in the surface sediment of Bosten Lake ranged from 1.8 to 4.4 %, which was relatively higher than those (0.2–2 %) in the Tibetan Plateau (Lami et al., 2010; Wang et al., 2012) and Yangtze floodplain (Wu et al., 2007; Dong et al., 2012), but much lower than those (5–13 %) in the lakes of the Yunnan–Guizhou Plateau (Zhu et al., 2013; Wu et al., 2012). Low TOC contents in the Tibetan Plateau lakes were a consequence of low biological productivity owing to the high altitude and low temperature (Lami et al., 2010). Although lakes in the Yangtze floodplain had higher productivity in the water column due to eutrophication (Qin and Zhu, 2006), most of them were shallow lakes that were subject to frequent turbulence and resuspension of sediments (Qin et al., 2006). In addition, warm–humid climate in the Yangtze floodplain could promote decomposition of POC in the water column and TOC in the sediments (Gudasz et al., 2010), which led to less TOC storage in the surface sediments. On the other hand, lakes in the Yunnan–Guizhou Plateau were deep with higher lake productivity, which had favorable TOC burial conditions (Jiang and Huang, 2004).
Sediment organic compounds are either of terrestrial origins or derived from phytoplankton and zooplankton remains and feces (Meyers, 2003; Meyers and Ishiwatari, 1993; Barnes and Barnes, 1978). A number of studies have demonstrated that TOC in small and shallow lakes is attributable to allochthonous sources but TOC in larger and deeper lakes to autochthonous sources that are derived from planktonic organisms (Shanahan et al., 2013; Sifeddine et al., 2011; Barnes and Barnes, 1978). Our analyses showed that the majority of TOC was autochthonous in the surface sediment of Bosten Lake. We also found a significant negative relationship between TOC and dry bulk density (Table 1), confirming that higher TOC (with lighter weight) would be a result of sedimentation of non-terrestrial organic materials.
Correlation coefficient (
WD: water depth, DBD: dry bulk density,
Our study demonstrated large spatial variability in the TOC of the surface sediment in Bosten Lake, with higher values in the central north and east sections and near the mouth of the Kaidu River, but lower values in the west section and mid-south section (Fig. 3a). Further analyses showed that the highest autochthonous TOC was found near the mouth of the Kaidu River and the highest allochthonous TOC in the east section (Fig. 6). There is evidence of high productivity near the sources of nutrients, such as estuaries owing to extra nutrient input from riverine (Deng et al., 2006; Lin et al., 2002). Nutrient conditions in Bosten Lake may be largely affected by the transportation of the Kaidu River, which has a significant decline from the mouth to the east section. A similar finding was also observed in the Nam Co Lake (Wang et al., 2012).
TOC burial in sediments is a result of sedimentation of POC. Here, we
compared the spatial pattern of autochthonous TOC in the 0–1 cm sediment
with the summer POC reported by Wang et al. (2014), which showed the highest
values of both variables near the mouth of the Kaidu River (Fig. 7).
Statistical analysis indicated that the correlation was not significant
(
Spatial distributions of POC concentrations in summer (color map)
and autochthonous TOC in the 0–1 cm sediment (TOC
The magnitudes and spatial distribution of TOC in lake sediment may reflect
multiple, complex processes (Sifeddine et al., 2011; Woszczyk et al., 2011;
Dunn et al., 2008; Wang et al., 2012). Our analyses showed a significant
negative relationship between the
This study is financially supported by the National Key Basic Research Program (2013CB956602), the Special Environmental Research Funds for Public Welfare of the State Environmental Protection Administration (201309041), the Sino-German Project (GZ867), and National Pioneer Project (XDA05020202). Edited by: B. A. Pellerin