Preliminary study on the effect of cascade dams on ...

Preliminary study on the effect of cascade dams on organic matter sources of sediments in the middle Lancang–Mekong River Chen Zhao, Shikui Dong, Shiliang Liu, Nannan An, Isange Sylvie, Haidi Zhao, Qi Liu & Xiaoyu Wu Journal of Soils and Sediments ISSN 1439-0108 Volume 18 Number 1 J Soils Sediments (2018) 18:297-308 DOI 10.1007/s11368-017-1790-5

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Author's personal copy J Soils Sediments (2018) 18:297–308 DOI 10.1007/s11368-017-1790-5

SEDIMENTS, SEC 2 • PHYSICAL AND BIOGEOCHEMICAL PROCESSES • RESEARCH ARTICLE

Preliminary study on the effect of cascade dams on organic matter sources of sediments in the middle Lancang–Mekong River Chen Zhao 1,2 & Shikui Dong 1 & Shiliang Liu 1 & Nannan An 1 & Isange Sylvie 1 & Haidi Zhao 1 & Qi Liu 1 & Xiaoyu Wu 1

Received: 18 December 2016 / Accepted: 16 July 2017 / Published online: 17 August 2017 # Springer-Verlag GmbH Germany 2017

Abstract Purpose Carbon and nitrogen isotopes in sediments collected from the Manwan and Dachaoshan Reservoirs, which were created by cascade dams along the Lancang–Mekong River, were measured to preliminarily analyse their organic matter sources. Materials and methods Sediment samples were collected from the Manwan and Dachaoshan Reservoirs twice during both the rainy and dry seasons by using a gravitational bottom sampler. One set of samples was collected from the river centres of each reservoir, and the other set was collected from the river edges, 5 m from each bank. Sediments were divided by wet sieving into three classes of water-stable aggregates based on sizes, including macroaggregates, microaggregates, and silt and clay. At the same time, terrestrial plants (Phragmites) and surface soil samples (0–5 cm) were collected from the river bank. The total organic carbon (TOC), total nitrogen (TN), stable carbon isotope (δ13C), and stable nitrogen isotope (δ15N) were measured for sediments, soils, and plants to preliminarily analyse the sources of organic matter in the sediments. Results and discussion Because of the presence of dams, a greater amount of the fine fraction was deposited in the reservoirs, which adsorbed and preserved more organic matter. Although terrestrial sources of organic matter dominate in

riverine systems, aquatic sources have been shown to act as new organic matter sources in the Manwan Reservoir. Input from C3 plants was the major new organic matter source in the Dachaoshan Reservoir, which also had strong inputs from aquatic sources. In the dry season, C3 plant input was significantly reduced and less fine fraction was found in the sediments. The high nitrogen isotope values of the sediments collected during the dry season indicated that strong organic matter decomposition decreased the storage of organic matter. Conclusions The cascade dams permanently changed the aquatic environment and modified regional carbon flows resulting in the diversification of organic matter sources. The major source of sedimentary organic matter was terrestrial, including from plants, soils, and particle organic matter, with the new organic matter absorbed by macroaggregates primarily stemming from aquatic organisms and C3 plants. Samples collected during the dry season exhibited strong decomposition, which further weakened the preservation of organic carbon in sediments.

Responsible editor: Nikolaus Kuhn

The preservation of organic matter in riverine sediments is the end effect of the transportation of organic matter from terrestrial to aquatic environments (Johannessen et al. 2003). At the global scale, more than 90% of carbon transportation is driven by river flow, with the carbon ultimately being buried in marine sediments (Emerson and Hedges 1988). The Manwan and Dachaoshan Dams, the first and second cascade hydropower dams along the middle Lancang–Mekong River completed in 1995 and 2000, respectively, have increased the fragmentation

* Shikui Dong [email protected]

1

School of Environment, Beijing Normal University, Beijing 100875, People’s Republic of China

2

China Unicom System Integration Limited Liability Company, Beijing 100032, People’s Republic of China

Keywords Carbon isotope . Carbon source . Cascade dams . Nitrogen isotope . Organic matter

1 Introduction

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of riparian habitats, trapped sediments in reservoirs, and enhanced carbon sinks (Kummu and Varis 2007; Zhao et al. 2015). In river basins, organic matter originates from aquatic organisms and allochthonous sources, including terrestrial plants and soils transported by surface runoff and soil erosion (Yu et al. 2010). A better knowledge of sedimentary organic matter sources is useful for understanding the process of organic carbon preservation (Bianchi et al. 2002). Stable carbon isotopes (δ13C), stable nitrogen isotopes 15 (δ N), and the elemental ratio of total organic carbon (TOC) to total nitrogen (TN), expressed as C/N, have been widely used as indicators to elucidate the process and source of organic matter sedimentation in aquatic environments (Peterson and Fry 1987; Hu et al. 2006; Chabbi et al. 2007; Yu et al. 2010). The C/N ratio is one of the primary indicators of organic matter sources from aquatic and terrestrial ecosystems (Yu et al. 2010). These ratios for freshwater algae range from 5 to 8, but the values for vascular land plants are greater than 20 (Emerson and Hedges 1988; Meyers 1994). Stable carbon isotopes have been widely used to identify organic matter sources because freshwater organic matter depletes δ 13 C values compared to terrestrial organic matter (Middelburg and Nieuwenhuize 1998; Yu et al. 2010; Gontharet et al. 2014). Freshwater plankton isotopic carbon values reported in a previous study were shown to range from −42 to −24‰ (Harding and Hart 2013). The C3 and C4 plants have different δ13C values: −32 to −21‰ and −17 to −9‰, respectively (Smith and Epstein 1971; Ogrinc et al. 2005). Because non-N-fixing plants assimilate ammonium and nitrate containing 14N, nitrification and denitrification can increase δ15N concentration in soils and sediments (Handley et al. 1999; Kendall et al. 2001). Further, sewage contains nutrients with the δ15N values of −3 to 2.3‰ could alter riverine environments as suggested by Wayland and Hobson (2001). The content of organic matter in sediments is positively correlated with the size of particles such as macroaggregates (>250 μm), microaggregates (53–250 μm), and silt and clay ( 0.05) higher in the dry season. With less rainfall in the dry season and forestland on the band, previous studies about runoff in forestland (Smith

Author's personal copy M Manwan Reservoir, D Dachaoshan Reservoir, RA riverine area, TA transitional area, LA lacustrine area, Mz mean particle size, TOC total organic carbon, TN total nitrogen, IN inorganic nitrogen

Values are presented as the mean ± standard deviation, and the same letters in the same column indicate no significant difference at P < 0.05. The sediments of the riverine area in Dachaoshan Reservoir were not collected in the dry season

1.960 ± 0.741ab

– – –

0.919 ± 0.126a 11.350 ± 0.369a 23.925 ± 10.093b

–

0.667 ± 0.192b

0.820 0.815

10.739 ± 0.666a

8.748 Mean value

17 ± 8b 4

4

DLA

208

1.001 ± 0.113a

1.744 – 2.095 ± 1.289ab 0.410 – 0.872 ± 0.303a 4.282 – 10.000 ± 4.028a 246.965 – 76.400 ± 99.27b 0.476 1.148 ± 0.044a 0.680 ± 0.280b 5.567 3.718 ± 1.895b 11.180 ± 3.485a 4 4 5 Mean value DRA DTA

205 566 ± 261a 73 ± 116b

0.489 0.338 ± 0.176b 1.048 ± 0.311a

2.224 ± 0.899a 0.752 ± 0.167b

2.426 ± 1.637a 0.219 ± 0.026b

0.217 ± 0.051b 0.747 ± 0.104a 2.350 ± 0.835b 8.400 ± 0.913a

1.367 ± 0.058b 569.767 ± 236.346a

224.350 ± 73.605b 27.475 ± 18.729b 0.431 ± 0.201b 0.370 ± 0.176b

0.694 ± 0.352b

5.189 ± 2.436bc 9.025 ± 2.312ab

0.439 ± 0.191b 0.755 ± 0.137ab

1.699 ± 0.755c

5 4 MTA MLA

155 ± 37b 24 ± 16b

3 MRA

526 ± 97a

0.219 ± 0.049b

Mz (μm) IN/TN (%) TN (g kg−1) TOC (g kg−1) Mz (μm)

Number

Rainy season

301

Sampling area

Table 2

Compositions of sediments in different fractions of river during rainy and dry seasons

Dry season

TOC (g kg−1)

TN (g kg−1)

IN/TN (%)

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et al. 2013) have suggested that the low flux and surface runoff weaken organic matter input and soil erosion resulting in the low fine fractions observed in the riverine area and the low TOC contents in the sediment. 3.2 TOC, C/N, δ13C, and δ15N ratio contents in different size fractions of sediments 3.2.1 TOC content Variations in TOC in different size fractions are presented in Fig. 2 (1A, rainy season; 1B, dry season). Because sediments in riverine areas rarely contained silt and clay fractions for detecting TOC after the wet sieve process, there were no parallel samples available for calculating the standard deviation. In the Manwan Reservoir, the TOC contents in the macroaggregates and microaggregates increased from the riverine areas to the lacustrine areas (Fig. 2, 1A). As a result, the TOC of the macroaggregates was significantly higher (p < 0.05) than it was for the other fractions in the lacustrine area and lower than for the other fractions in the riverine area. In the Dachaoshan Reservoir, the TOC contents in the macroaggregates and microaggregates of transitional areas were significantly higher (p < 0.05) than they were in the sediments in riverine areas, and they were also higher (p > 0.05) than the sediments in the Manwan Reservoir. The coefficient of TOC variation in silt and clay was small with a value of 0.37 in the Manwan Reservoir and 0.07 in the Dachaoshan Reservoir. There were no obvious differences in TOC content between the samples collected in the dry season and in the rainy season. Under rapid flow conditions in the riverine area, macroaggregates had a poor ability to preserve fine particles, which Mayer (1994) stressed can result in a poor ability to protect organic matter. Consequently, the silt and clay particles consistently contained more TOC and TN in the riverine areas. According to Six et al. (2002), macroaggregates are capable of absorbing new organic matter and converting it into microaggregates after the organic matter decomposes because of the increasing proportion of fine particles in the sediment. Liao et al. (2006) stated that macroaggregates directly expose organic matter to the environment and do not offer long-term physical protection, while the organic matter buried in the interlayer surfaces of clays and the inner spaces of microaggregates could enhance preservation. 3.2.2 δ15N ratios The spatial and seasonal variations of δ15N in different fractions of sediments are presented in Fig. 2 (2A and 2B). In the rainy season, macroaggregates in the Dachaoshan Reservoir were higher (p > 0.05) in δ15N in the lacustrine area, and the samples collected in the riverine area varied significantly (p < 0.05) from each other. The δ 15 N values of the

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Fig. 2 Variation of TOC, C/N, δ15N, δ13C in sediments (macroaggregate (circles); microaggregate (triangles); silt and clay (crosses)). A Rainy season. B Dry season. The same letters in the same column indicate no significant difference among the same fractional sediments at p < 0.05

microaggregates in the Dachaoshan Reservoir increased from the riverine area to the lacustrine area, and the values were similar between macroaggregates and microaggregates in the

Manwan Reservoir. Except for the samples belonging to the lacustrine area of the Manwan Reservoir collected in the dry season, all sediments exhibited higher (p < 0.05) δ15N values

Author's personal copy J Soils Sediments (2018) 18:297–308 Table 3

303

C/N and δ13C of surface soils, terrestrial plants, aquatic organisms, and particle organic matter

End-members

Sample size

C/N

δ13C (‰)

Reference

Surface soils

7 7

C4 terrestrial plants

7

−24.31 (−27.81 to 18.11) −26.08 (−28.73 to −21.04) −12.78 (−9.89 to −15.11)

–

C3 terrestrial plants

9.66 (6.37 to 13.49) 30.00 (10.66 to 36.39) 32.65 (20.12 to 40.83)

Aquatic organisms

– –

−30.00 (−42.00 to −24.00) −19.59 (−21.53 to −14.12)

(Kendall et al. 2001; Goñi et al. 2003)

Particle organic matter

6.10 (5.00 to 8.00) 4.70 (2.30 to 8.60)

in silt and clay than in other sizes of fractions. The difference between the rainy and dry seasons was that δ15N in the macroaggregates and microaggregates in the riverine areas was significantly (p < 0.05) higher in the dry season, which was consistent with the low C/N ratios in the same samples. The decomposition process of nitrogen isotopes involves complex mechanisms, which cause seasonal and spatial fluctuations (Fox and Papanicolaou 2008). δ15N values can be easily modified by biogeochemical processes including ammonification and nitrification. It has been suggested that 14N components are more easily reused by plants and primary consumers, and thus, they enhance 15N preservation in soils and sediments (Zanden and Rasmussen 1999). According to Caraco et al. (1998), the effect of heterotrophs can increase terrestrial particulates from −4 to 9‰, and Hu et al. (2006) found that high δ15N values often occurred with low C/N ratios. As a consequence, the carbon sources were subjected to decomposition. 3.2.3 C/N ratios As shown in Table 3, C/N ratios of riverine phytoplankton determined in previous studies ranged from 5.8 to 7.6 with a mean value of 6.1 (Kendall et al. 2001; Goñi et al. 2003), while for the terrestrial plants measured in this study, it generally exceeded 16 with a mean value of 20, which is consistent with the results of previous studies (Emerson and Hedges 1988; Meyers 1994). Soils eroded by water flows are the primary transportation mechanism of organic matter from land to sediments (Thorp et al. 1998). The C/N ratios of terrestrial soils in this study area ranged from 6 to 16 with a mean value of 9.6. Particulate organic matter (POM) values reported by Fox and Papanicolaou (2008) and Machiwa (2010) ranged from 2.3 to 8.6 with mean value of 4.7, and the spatial distributions of C/N ratios in both the rainy and dry seasons were similar (Fig. 2, 2A and 2B). As shown in Fig. 2 (3A and 3B), in the rainy season, C/N ratios of macroaggregates in riverine areas were significantly

– –

(Kendall et al. 2001; Machiwa 2010)

lower (p > 0.05) than those in other areas of the reservoirs. The highest C/N ratios of the Manwan and Dachaoshan Reservoirs were found in lacustrine and transitional areas, respectively. Macroaggregates in the Dachaoshan Reservoir had higher (p > 0.05) C/N ratios than those in the same area of the Manwan Reservoir. The microaggregates had a similar spatial distribution to the C/N values of the macroaggregates. The C/ N ratios of silt and clay exhibited less variation than other fractionated sediments. Because of chemical and physical protection, silt and clay contained a stable composition of carbon and nitrogen, leading to little spatial variation in C/N ratios. In the dry season, the macroaggregates and microaggregates collected from riverine and transitional areas had lower (p > 0.05) C/N ratios than the samples collected in the rainy season. In Dachaoshan Reservoir, the C/N ratios of macroaggregates and microaggregates collected from the riverine area were significantly lower (p < 0.05) than the samples collected from the transitional area in dry season. During the decomposition of sedimentary organic carbons, Meyers et al. (1996) stressed that the C/N ratios are altered by the decomposition of organic matter. The labile organic matter of organic-rich sediments with high C/N values can be decomposed easily as suggested by Guenet et al. (2010). Furthermore, Carranca et al. (2009) found that agroecological systems may import organic matter with high C/ N ratios into the reservoir and enhance these ratios even in the riverine area of Dachaoshan Reservoir. As a result, C/N ratios altered by organic matter decomposition need to be carefully considered as an indicator when analysing sources of organic matter. 3.2.4 δ13C ratios As shown in Table 3, C3 terrestrial plants had an average δ13C ratio of −26.08‰ and ranged from −21 to −28‰, and C4 plants had a mean value of −12.78‰ and ranged from −12.45 to −13.22‰. The soils exhibited a range of δ13C ratios from −18.11 to −28.71‰ with a mean value of −24.31‰. In a

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Fig. 3 Distribution of C/N ratios and δ13C with end-members (macroaggregates, microaggregates, silt and clay collected in rainy season (filled circles, filled triangles, filled diamonds); macroaggregates, microaggregates, silt and clay collected in dry season (open circles, open triangles, open diamonds))

previous study, riverine plankton generally assimilated carbon with low δ13C values (average −30‰; range −24 to −42‰) leading to a greater depletion of δ13C than in terrestrial plants (Kendall et al. 2001). As an important part of carbon flux in riverine systems, the δ13C values of POM have been shown to range from −14.1 to −21.5‰ with a mean value of −19.6‰ (Machiwa 2010). The seasonal variations in δ13C ratios were similar to those of C/N ratios in this study. As shown in Fig. 2 (4A), in the rainy season, δ13C of macroaggregates collected in riverine areas were higher (p > 0.05) than in other areas belonging to the same reservoir. The lowest δ13C values of the Manwan and Dachaoshan Reservoirs were found in lacustrine and transitional areas, respectively. Compared with macroaggregates collected from the transitional and lacustrine areas of the Dachaoshan Reservoir, the macroaggregates in these areas of the Manwan Reservoir exhibited lower δ13C values (p > 0.05). As shown in Fig. 2 (4A and 4B), the spatial variation of δ13C in the microaggregates was similar to those in the macroaggregates. Compared with macroaggregates, microaggregates had lower (p < 0.05) δ13C values in sediments collected from the Dachaoshan Reservoir. There were no obvious differences in δ13C between the silt and clay fractions collected in the Manwan and Dachaoshan Reservoirs, and the values ranged from −25.57 to −23.52‰. The δ13C values of silt and clay in the transitional and lacustrine areas of the Dachaoshan Reservoir and the lacustrine area of the

Manwan Reservoir were higher (p < 0.05) than those of the microaggregates, and the spatial variations in δ13C values in the dry season were similar to those in the rainy season. δ13C values decrease in the depths of stratified lakes, which are similar to the lacustrine areas in the reservoirs of this study, because of less respired CO2. Rau (1978) stated that benthic algae tend to uptake more 13C than riverine plankton because of a CO2 boundary layer effect and the presence of bicarbonate. In reservoir food webs, the 13C content in aquatic organisms is determined by the contribution of atmospheric deposition and dissolved organic matter in the carbon cycle (Harding and Hart 2013). According to Li et al. (2013a, b), the contribution of aquatic organisms with depleted carbon is enhanced by the lentic environment caused by dams. As a result, anthropogenic disturbances and the cumulative effect of cascade hydropower stations may be responsible for differences between the Manwan and Dachaoshan Reservoirs. Because of poor capacity to conserve soil, terraced fields input more soil and plant material containing more labile organic matter (Smith et al. 2013), thus improving the δ13C values of the sediments. Because of the transportation of sediments between the reservoirs of cascade hydropower dams, Vukovic et al. (2014) stated that refractory organic matter from terrestrial sources can increase deposition in the reservoirs further downstream. In contrast to δ15N, δ13C is an indicator that can distinguish sources of organic matter, which is not markedly influenced by the decomposition process (Chen and Jia 2009).

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305

Fig. 4 Variations of carbon source percentages of sediments in different sampling areas. (POM, soils, C 3 plants, and aquatic life in macroaggregates, , , , , in the left bar; POM, soils, C3 plants,

and aquatic life in microaggregates, , , , , in the central bar; POM, soils, C3 plants, and aquatic life in silt and clay, , , , , in the right bar). a Rainy season. b Dry season

3.3 Source of organic matter in different fractions

components of organic matter in sediments. The rectangles in Fig. 3 represent the end-members, and the boundaries of the rectangles indicate the maximum or minimum values of C/ N or δ15N. Mean values of the end-members are denoted by the triangle peaks. The triangles shown in Fig. 3 suggest that

To distinguish the different organic matter sources, we selected C/N and δ13C ratios to represent the potential terrestrial and aquatic sources, and we selected five end-member

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organic matter of different fraction sizes was composed of C3 land plants (S1), soils (S2), aquatic organisms (S3), and POM (S4). However, some samples collected from the Manwan Reservoir in the dry season were located out of the range because of low C/N ratios. The contribution from the endmembers to the organic matter of different size fractions can be calculated by end-member mixing equations based on mass balance (Eqs. 3–6). There were four end-members, but only three equations can be used. To calculate the values, four scenarios referring to the four triangles were set (one large triangle and three small triangles, Fig. 3), and each scene consisted of three end-members. Each sample contributed results to three end-members, except the site located outside of the range. Mean values of the two scenes’ results represented the contribution percentages in this study. The contributions of different sources are shown in Fig. 4. Terrestrial sources, including soils, POM, and plants, were the dominant inputs of organic matter. In the rainy season, aquatic sources in macroaggregates were enhanced from the riverine areas to the lacustrine areas of the Manwan Reservoir, while POM inputs became weak. The microaggregates showed lower aquatic proportions than the macroaggregates in the abovementioned areas. According to Six et al. (2002), new organic matter incorporated by macroaggregates in the transitional and lacustrine areas in the Manwan Reservoir were of aquatic origin. There were no significant changes in the percent contribution of silt and clay. In the Dachaoshan Reservoir, the macroaggregates contained more C3 plant inputs than did other fractions in the transitional and lacustrine areas. Compared to the aquatic inputs of the Manwan Reservoir, inputs in the Dachaoshan Reservoir decreased while the contribution ratios of silt and clay fractions remained the same. There were more aquatic inputs in the microaggregates of the transitional areas. More organic matter from terrestrial sources (C3 plants and POM) was found in macroaggregates with less in microaggregates indicating that terrestrial organic matter, rather than aquatic organic matter, may be the new organic matter source. For the riverine areas of these two reservoirs, POM was one of the primary components of terrestrial sources instead of C 3 plants and aquatic organisms. In the dry season, because of the low C/N ratios in the size fractions of riverine sediment in the Manwan Reservoir, the sites in Fig. 3 were outside the triangle formed by end-members. The samples with high δ15N values demonstrated the large amount of organic matter decomposition assisted by impoundment, which ensures the safe production of electricity in the dry season. The contributions of aquatic organic matter and C3 plants to the macroaggregates and microaggregates of the Manwan Reservoir were reduced, but the percent contribution of

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aquatic organisms remained higher in macroaggregates than in other fractions. With less terrestrial organic matter inputs due to less rainfall, the preservation of organic carbon declined. As dams are constructed, the biomass of fishes and plankton increases significantly, especially in riverine areas and in the first half of transitional areas (Li et al. 2013a, b). Compared with terrestrial organic matter, aquatic organic matter is often considered highly labile and can be decomposed easily (Keil 2011). The increased population around the Manwan hydroelectric station, which has been operating for 20 years, has resulted in more terraced fields on the hillside of the riverine area in the Dachaoshan Reservoir. The agroecosystem is often considered a strong source of labile organic matter (Carranca et al. 2009) due to the changes in soil from fertilizers and plant materials. Terraced fields do not conserve soil very well and may cause more soil erosion and enhanced terrestrial inputs (Chabbi et al. 2007). In this study, because the end-members have significant difference, the mixing equations based on mass balance could provide a preliminary analysis. The end-member model used in this study just applies to a four end-member situation. If there are more similar sources of organic matter in future research, the Monte Carlo simulation framework (Collins et al. 2013) could be used to determine the source contributions.

4 Conclusions In the present study, cascade dam construction deposited more fine fractions in the reservoirs and created a lacustrine watershed. Because of the accumulation of fine fraction and of the impoundment by cascade dams, organic matter was buried in the sediments. The significant spatial variation of organic carbon and nitrogen in this study implied that dam construction was the key factor altering the accumulation of organic matter in the sediments. Hydropower dams should remove deposited sediment over time and decrease potential environmental risk. The major source of sedimentary organic matter was terrestrial, including from plants, soils, and particulate organic matter. The new organic matter absorbed by macroaggregates primarily originated from aquatic organisms (Manwan Reservoir) and C 3 plants (Dachaoshan Reservoir). More aquatic inputs were found in the transitional and lacustrine areas than in the riverine areas. The contribution of organic matter inputs in stable silt and clay fractions did not significantly change. In the dry season, samples contained less C3 plant inputs than the samples collected in the rainy season, and dry season samples exhibited strong decomposition that further weakened the preservation of organic carbon in sediments. As sedimentary organic matter is an important potential source of

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greenhouse gasses and is a carrier for persistent organic pollutants, variation in the sources contributing to organic matter preservation influenced by cascade dam construction should receive more attention. Acknowledgements We thank Xukun Su, Lin Tang, and Yuanyuan Li for their assistance in the laboratory. We are grateful to our funding support from the National Key Research and Development Project (Grant No. 2016YFC0502103).

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