The Infuence of Cascade Reservoir Construction on Labile Phosphorus in the of Lancang River

Yao Cheng (  [email protected] ) Hebei University of Engineering Yu Li Hebei University of Engineering Jinkun Wu Hebei University of Engineering Mingming Hu State Key Laboratory of Simulation and Regulation of in River Basin Yuchun Wang State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin Zhongjun Wang State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin Xiaoman Jiang Northwest Agriculture and Forestry University

Research Article

Keywords: Phosphorus, Sediment, Release, Cascade reservoirs, Lancang River, Remobilization

Posted Date: July 21st, 2021

DOI: https://doi.org/10.21203/rs.3.rs-614801/v1

License:   This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License

Page 1/19 Abstract

The infuence of cascade reservoirs construction on labile phosphorus (P) is an important scientifc problem in the Lancang River. The concentration of labile P in cascade deep-water reservoirs were determined, and the infuence of cascade reservoirs construction on the DGT-labile P was analyzed. The construction of cascade reservoirs led to signifcant differences in concentrations of DGT-labile P, which in the upstream of Xiaowan (XW) Reservoir were differences from that in the downstream Nuozhadu (NZD) Reservoir. The P diffusion fuxes in XW Reservoir were − 8.59–250.50 ng·cm− 2·d− 1, and that in NZD Reservoir were 3.82–24.80 ng·cm− 2·d− 1. The P pollution of XW Reservoir was higher, highlighting the importance of controlling P pollution of XW Reservoir. The construction of cascade reservoirs had made the release of DGT-labile P more dependent on the reductive dissolution of Mn oxides. The early transformed bio-availability P (BAP) remobilization into DGT-labile P that made the increase of DGT-labile P/BAP with depth. However, the DGT-labile P/BAP of upstream XW Reservoir was 7.8 times larger than that of downstream NZD Reservoir, which indicated that the construction of cascade reservoirs weakened the remobilization of P in sediment.

Introduction

There are more than 3700 dams on world rivers, which are used for hydropower, water supply or transport (Chen et al. 2019, Li et al. 2012). However, the construction of dams would have a far-reaching infuence on the water balance and river ecosystems in the basin (Li et al. 2013). The upstream reservoir interrupted the continuity of sediment fux and nutrient transport by intercepting suspended solids, while the downstream reservoirs reduced the input of nutrients and sediment, thus affecting the water quality and even the riparian vegetation of the river (Chen et al. 2020, Wei et al. 2011). As an important source of nutrients, sediment plays an important role in the of nutrients (Marip et al. 2020).

Among the nutrients elements in sediment, the circulation of phosphorus (P) in the water environment has been a subject of important scientifc signifcance (Młynarczyk et al. 2013). Gao et al. (2019) observed that P would be released in sediment if the physical and chemical properties of sediment changed in the aquatic system, and Anschutz et al. (2007) found that the early diagenesis would also make sediment change from the sink of P to the source. Since the release of P in sediment is complex and dynamic (Yang et al. 2020), it is necessary to study the geochemical cycle of P in sediment-water interface (SWI). Giles et al. (2016) observed that P released from sediment in a shallow bay in Lake Champlain (Missisquoi Bay, USA) depended on the reduction of Fe oxides (FeOOH) and Mn oxides (MnOOH) to Fe2+ and Mn2+, which had been confrmed in the sediment of Lake Taihu, China by Zeng et al. (2018). Although the release mechanism of P in sediment of shallow lakes or reservoirs has been widely studied, the migration and transformation of P in sediment of deep-water reservoirs and the infuences of cascade reservoirs on P circulation in sediment have been not clear enough.

The Lancang River (LCR) is located on the Lancang-Mekong River upstream. At present, there are eight deep-water cascade hydropower stations in the middle and lower reaches of LCR (Grumbine and Xu

Page 2/19 2011). Due to construction of large cascade dams, the interception of sediment and nutrients has been paid more and more attention. In recent years, scholars have made detailed studies on the effect of cascade reservoirs on the composition and distribution of P in LCR (Liu et al. 2015, Mu et al. 2020). However, these studies had ignored the infuence of cascade reservoirs on DGT-labile P. Therefore, this study took the cascade deep-water reservoirs in the LCR as the object to explore the impact of cascade reservoir construction on DGT-labile P. The purposes of this study are: (1) to study the distribution of DGT- labile P in cascade reservoirs sediment; (2) to analyze the infuencing factors of DGT-labile P release in cascade reservoirs; and (3) to discuss the effect of cascade reservoirs on remobilization of P in sediment.

Materials And Methods Sampling location

The Lancang-Mekong River is the most important transnational river in Asia. It originates from T'ang-ku- la Mountains in Qinghai Province of China, fows through the three provinces of Qinghai, Tibet and Yunnan Province in China, and is called the LCR (Zhang et al. 2020). There is about 2,160 km in length and 5,000 m above sea level, of which 91% of the drop is concentrated in the LCR (Yi et al. 2014). Because of its large altitude span, the river has huge hydropower resources, making hydropower development extremely feasible. The Xiaowan (XW) Reservoir and Nuozhadu (NZD) Reservoir in the study area are multi-functional projects focusing on power generation. Other functions include food control and navigation. In this study, three sampling points were set up at each reservoir to collect sediment samples for analysis and research (Fig. 1).

Water Quality Analysis And Sampling Collection

In December 2019, the temperature, pH, dissolved oxygen (DO), and conductivity (Cond) were measured by Multi-parameter controller (Yellow Springs Instruments Inc., Yellow Springs, OH, USA). The sediment cores were collected from each sampling point using a gravity sampler (6 cm in diameter and 100 cm in length). The DGT device was prepared and arranged in the sediment core by the method of Wang et al. (2016). The sediment core was sliced and stored in plastic ziplock bags. The sediment samples were freeze-dried. After fully grinding and sifting with a standard 100 mesh sieve, it could be used for extraction and analysis.

Chemical Analysis And Data Analysis

The sediment was digested with HNO3-HF-HClO4. ICP-OES was used to measure the content of metal elements—manganese (Mn), calcium (Ca), iron (Fe), aluminum (Al). The grain size of sediment was analyzed by Mastersizer 2000 (Malvern Instruments Ltd, Malvern, UK). It was classifed into three speciations: <4µm (clay), 4–63µm (silt), > 63µm (sand). The extraction and measurement of DGT-labile P referred to the method of Zeng et al. (2018). The P was extracted with 1.0 mol NaOH, which was Page 3/19 measured by molybdenum blue spectrophotometry. The DGT-labile P concentrations were calculated using the equation by Xu et al. (2018). The diffusion fux of DGT-labile P can be calculated using equation by Sun et al. (2019).

Results Physical and chemical properties of the overlying water

Table 1 The physical and chemical properties distribution of the overlying water in the two reservoirs XW NZD

Temperature (℃) 14.4 ± 1.2 17.7 ± 0.2

pH 8.0 ± 0.2 7.7 ± 0.3

DO (mg·L− 1) 4.2 ± 0.9 1.9 ± 0.9 437.6 ± 4.3 391.2 ± 9.9 Cond (µS·cm− 1) The physical and chemical properties distribution of the overlying water in XW and NZD reservoirs are shown in Table 1. The water temperature in NZD Reservoir was higher than that in XW Reservoir, which was 17.7 ± 0.2°C and 14.4 ± 1.2°C, respectively. Compared with the NZD Reservoir, the pH values in XW Reservoir were higher, and the overlying water was alkaline. The DO in NZD Reservoir was lower than that in XW Reservoir, and the values were 1.9 ± 0.9 and 4.2 ± 0.9 mg·L− 1, respectively. The concentrations of dissolved oxygen less than 2 mg·L− 1 were defned as hypoxic (Alvisi and Cozzi 2016), so the DO of the overlying water at NZD Reservoir was hypoxic. The Cond in XW Reservoir was higher than that in the NZD Reservoir, and the Cond values were 437.6 ± 4.3 and 391.2 ± 9.9 µS·cm− 1, respectively. The physical and chemical properties of sediment

According to the texture triangle, the grain size from XW Reservoir covered the felds of silt loam, silty clay loam, and silty clay, and the grain size from NZD Reservoir was generally loamy sand or sandy loam (Fig. 2). Due to the regional sediment supply, the sediment grain size of downstream NZD Reservoir was larger than that of upstream XW Reservoir. Previous research results showed that the sediment in the river usually showed a trend of downstream fning trend (Morris and Williams 1999). However, this fning trend might be changed by the replenishment of regional sediment and dam construction (Catalunya et al. 2006).

The distribution of metal content at each sampling point is shown in Fig. 3. The Mn content in upstream XW Reservoir was lower than that in downstream NZD Reservoir, while the Ca content in downstream XW Reservoir was much higher. In addition, there were little differences in the content of the other two metal elements. In XW Reservoir, the content of four metal elements gradually decreased from the main stream

Page 4/19 to the dam front and then increased to the tributary, while in NZD Reservoir, the content of metal elements in the transition region were higher than that of the dam front and the main stream. The change of metal elements in sediment of the two reservoirs might be related to different sedimentary environments and land use patterns (Wan et al. 2020).

The Variation Of Dgt-labile P Concentrations

The DGT-labile P at XW and NZD reservoirs were 0.12 ± 0.21–0.79 ± 0.21 mg·L− 1 and 0.03 ± 0.01–0.24 ± 0.09 mg·L− 1, respectively (Fig. 4). In this study, statistical analysis was used to describe whether there was a signifcant difference in DGT-labile P of cascade reservoirs. The distribution of DGT-labile P in XW and NZD reservoirs was signifcantly different (α = 0.05). The result indicated that the construction of cascade reservoirs had an obvious infuence on DGT-labile P in sediment.

The DGT-labile P were observed increased with the depth at all the sampling points. It is worth noting that the DGT-labile P at XW-C varied widely, while NZD-C had the lowest concentrations and increased lightly with the depth. The DGT-labile P concentrations increased with the depth might be affected by the early diagenesis. During the early diagenesis, organic matter decomposed and mineralized under redox conditions to form soluble P, and fnally P was released into the pore water of , which led to the increase of P concentrations with the depth (Anschutz et al. 2006).

Discussion The diffusion fuxes of P in SWI

The diffusion fuxes of P in SWI are shown in Fig. 5. The values of the XW Reservoir and NZD Reservoir were signifcantly different. The only negative value appeared in XW-A, and the value of XW-C was much higher than others. XW-C was polluted by cage fsh culture, which had an important impact on the eutrophication of XW Reservoir. Compared with the NZD Reservoir, the diffusion fuxes of P in the XW Reservoir were higher, and the value near the dam head was lower than that in the river region and transition region. The value of diffusion fux can describe whether the sediment is a sink or source of P (Sun et al. 2019). Only XW-A was negative, and all other sampling points were positive, indicating that the DGT-labile P in sediment in other areas was transformed from sediment to overlying water excluding XW- A.

As shown in Table 2, the diffusion fux values of NZD Reservoir were similar to the values of Dongting Lake in Hunan Province, China, but lower than that of other lakes and reservoirs in China and other countries. However, the values of XW Reservoir were larger than that of other lakes and reservoirs in China, which were similar to that of other countries, indicating that although many shallow lakes and reservoirs in China were seriously eutrophic, their internal pollution levels were relatively lower than that of XW Reservoir. In other words, attention should be paid to the control of P pollution in XW Reservoir.

Page 5/19 Table 2 Diffusion fuxes of P in reservoirs and lakes Location P fux Method Reference

(ng·cm− 2·d− 1)

NZD Reservoir, LCR, China 3.82–24.80 DGT This study

XW Reservoir, LCR, China -8.59- DGT This study 250.50

Dongting Lake, Hunan, China -2.70-19.70 DGT (Gao et al. 2016)

Taihu Lake, Jiangsu, China -21-65 DGT (Ding et al. 2015)

Hongze Lake, Jiangsu, China 17.20– DGT (Yao et al. 2016) 79.30

Hongfeng Reservoir, Guizhou, 1–83 DGT (Wang et al. 2016) China

Lake Erie, Ohio, USA 95–179 DET (Matisoff et al. 2016)

Lake Peipsi, Estonia and Russia 2.30- discrete sediment pore (Tammeorg et al. 103.80 water 2015)

Lake Okeechobee, Florida, USA 83–240 Aerobic and anaerobic (Das et al. 2012) condition

Lake Pontchartrain, Los 30–106 Aerobic and anaerobic (Roy et al. 2012) Angeles, USA condition

Effect Of Cascade Reservoirs On Release Of P

Due to the effect of cascade reservoir construction, most of the sediment was deposited in the upstream XW Reservoir, which led to regional sediment replenishment as the main sediment source of the downstream reservoir (Mu et al. 2020). The grain size from NZD Reservoir was coarser and contained high content of Al and Mn, and the grain size from XW Reservoir was mainly fne particles, and the content of Fe and Ca were relatively higher than that in NZD Reservoir. This indicated that the differences in physical and chemical properties in the sediment between XW and NZD reservoirs were related to the source of reservoir sediment. According to the above research, there was an obvious difference in the DGT-labile P between upstream and downstream reservoirs. The construction of cascade reservoirs not only led to different physical and chemical properties of sediment between the two reservoirs, but also led to signifcant differences in DGT-labile P between the two reservoirs.

The release of P in sediment involves the joint action of various geochemical processes, among which is closely related to geochemical cycles of Fe and Mn (Defforey and Paytan 2018, Smith et al. 2011). In this

Page 6/19 study, it could be observed that the DGT-labile Fe and Mn in XW and NZD reservoirs had the same variation trend with DGT-labile P excluding XW-C (Fig. 6). Similarly, DGT-labile Fe and Mn at other sampling points also had signifcant correlation with P (p < 0.05). The results indicated that FeOOH and MnOOH were reduced to Fe2+ and Mn2+, and led to the release of P (Chen et al. 2019). XW-C is located in the fsh culture area, only a small part of the excessive bait was fed by fsh, and most of the bait containing P was deposited at the bottom of the reservoir, which led to different vertical distribution and low correlation between DGT-labile P with Fe and Mn in XW-C.

The slopes of linear regression equation can refect the degree of infuence of DGT-labile Fe and Mn on P (Ding et al. 2016, Zeng et al. 2018). We could observe that the slopes between P with Fe in XW Reservoir were larger than that in NZD Reservoir, and the slopes between P with Mn in XW Reservoir were also larger than that in NZD Reservoir, which might be related to different sediment sources of the two reservoirs (Fig. 7). The sediment of NZD reservoir was supplemented by regional sediment, the release of DGT-labile P was more depended on the infuence of sediment input, which led to different control factors of P. It is worth noting that in XW and NZD reservoirs, the slopes of P and Fe were larger than that of P and Mn. The results indicated that Fe oxides had stronger adsorption capacity for P in the sediment. In other words, the reductive dissolution of MnOOH dominated the release of P, which would take precedence over FeOOH (Postma and Appelo 2000). This is consistent with study by Li et al. (2021) on the coupling of Fe and Mn with P in the sediment.

Effect Of Cascade Reservoirs On Remobilization Of P

Bio-availability P (BAP) is the sum of active P forms in sediment, including P that can be directly utilized by and P that can be converted into potential bioavailable P (Dan et al. 2020). DGT-labile P is an easily released or movable P component in sediment (Tao and Yali 2017). The BAP can be considered as the main source of the DGT-labile P. Correlation analysis also showed that there was a high correlation between DGT-labile P and BAP in each reservoir (p < 0.05). The DGT-labile P/BAP could be seen as the release of DGT-labile P by the unit BAP in the sediment, and it was not related to the absolute content of BAP in the sediment. The vertical variation of BAP and DGT-labile P/BAP in the two reservoirs is shown in Fig. 8. Two important conclusions can be drawn from the picture.

Firstly, with the increase in depth, BAP content was stable, indicating that the BAP input from outside was consistent and stable in the historical process of reservoir sediment deposition. However, the DGT-labile P/BAP in the two reservoirs had a clear tendency to increase with depth. The results show that with the increase of deposition time, more BAP was transformed into DGT-labile P, which was released into pore water. In other words, early diagenesis changed BAP remobilization into DGT-labile P. Secondly, the BAP in downstream NZD Reservoir was higher than that in upstream XW Reservoir, whereas the DGT-labile P concentrations in downstream NZD Reservoir were higher than that in upstream XW Reservoir. The DGT- labile P/BAP in upstream XW Reservoir was 7.8 times larger than that in downstream NZD Reservoir. This showed that, on the one hand, the cascade reservoir highlighted the importance of regional sediment.

Page 7/19 This could also be supported by previous studies. On the other hand, the cascade reservoirs weakened the remobilization of P in the sediment.

Conclusion

The effect of cascade reservoir construction on P in sediment was studied. The results show that the concentrations of DGT-labile P in the upstream of XW Reservoir had an obvious infuence than that in the downstream NZD Reservoir. The P diffusion fux values of XW Reservoir were − 8.59–250.50 ng·cm− 2·d− 1, which were higher than that of NZD Reservoir by 3.82–24.80 ng·cm− 2·d− 1. The P diffusion fuxes of XW Reservoir were higher than that of other lakes and reservoirs in China, and were similar to that of foreign lakes, which indicated that the P pollution of XW Reservoir was higher, highlighting the importance of controlling P pollution in XW Reservoir. Because of the construction of cascade reservoirs, the control factors of P release were different. The construction of cascade reservoirs had made the release of DGT-labile P more dependent on the reductive dissolution of MnOOH. Early diagenesis activated BAP into DGT-labile P, which led to an obvious trend of increasing DGT-labile P/BAP with depth. The DGT-labile P/BAP of upstream XW Reservoir was 7.8 times larger than that of downstream NZD Reservoir, which related that the construction of cascade reservoirs weakened the remobilization of P in the sediment.

Declarations

Author ContributionYao Cheng: Writing - Original Draft, Conceptualization. Yu Li: Writing - Original Draft, Formal analysis. Yuchun Wang: Conceptualization, Methodology. Jinkun Wu,

Mingming Hu, Zhongjun Wang, Xiaoman Jiang: Investigation and Formal analysis.

Funding This research was fnancially supported by the National Natural Science Foundation of China (U1802241, U204021, 92047204, and 11371117), Innovative Research Group of Hebei Natural Science Foundation (E2020402074), University Science and Technology Research Project of Hebei, China (ZD2019005) and Graduate Innovation Foundation of Hebei Province (CXZZBS2020152 and CXZZSS2021090).

Data availability statements The datasets involved in this study are available from the corresponding author on reasonable request.

Ethics approval and consent to participate Not applicable

Consent for publication All authors agree with the content to submit the paper.

Competing interests The authors declare that they have no known competing fnancial interests or personal relationships that could have appeared to infuence the work reported in this paper.

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Figures

Figure 1

Page 12/19 Research area and sampling points

Figure 2

Distribution of grain size at each sampling point

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Distribution of metal content at each sampling point

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The vertical variation of DGT-labile P in the XW and NZD reservoirs

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The diffusion fuxes of P in SWI

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The vertical variation of DGT-labile P, Fe and Mn at the XW and NZD reservoirs

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Linear regression analysis of DGT-labile P with Fe and Mn in the two reservoirs

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The vertical variation of BAP and DGT-labile P/BAP at the XW and NZD reservoirs

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