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Keywords:

  • characteristics;
  • nitrogen;
  • sediments;
  • Xiangxi Bay

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussions
  7. Conclusions
  8. Acknowledgements
  9. References

Spatial and temporal distribution characteristics of total nitrogen and ammonium nitrogen in the sediments, pore water and overlying water of Xiangxi Bay in China Three-Gorge Reservoir were investigated. In surveys, the sampling was undertaken from six sites of Xiangxi Bay on three dates: 29 March 2009, 28 March 2010 and 17 August 2010. Mean values of total nitrogen and ammonium nitrogen contents in the sediments and pore water of Xiangxi Bay all increased with time, especially in summer. There were intimate relationships between internal nitrogen loading and water ecosystem. Correlation coefficients R were 0.7408 (between total nitrogen in pore water and total nitrogen in sediments) and 0.7483 (between total nitrogen in sediments and chlorophyll-a in surface water), respectively. Total nitrogen concentration differences between pore water and overlying water were all positive and had good correlation with chlorophyll-a concentration of Xiangxi Bay. The release of nitrogen in sediments had an important impact on phytoplankton growth in Xiangxi Bay.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussions
  7. Conclusions
  8. Acknowledgements
  9. References

The sediment plays an important role in water ecosystem as it is both an important source of various dissolved substances and a sink for particulate materials (Masuda & Boyd 1994). Microbial degradation of organic detritus within the sediment and on suspended particles regenerates nutrients and may release pollutants (Mortimer et al. 1998). So, the sediment is a ‘potential source’ of nutrients such as nitrogen and phosphorus in water (Stimson & Larned 2000; Lu et al. 2005; Steinman et al. 2009).

Three-Gorge Dam (TGD) in China is the world's largest dam, measuring 2335 m long and 185 m high, and the reservoir created by it has an area of 1080 km2 in 2009 (Wu et al. 2003). It has installed a total generating capacity of 22 400 MW (100 billion kWh), dramatically increasing Chinese energy supply. Meanwhile, the dam dramatically increases the flood control ability of Yangtze River from the present 10-year to a 100-year frequency flood (Zhang & Lou 2011). However, since the reservoir was filled to an altitude of 135 m above sea level in June 2003, the reservoir bays of Three-Gorge Reservoir (TGR) was showing symptoms of eutrophication, and algal blooms often occur in these bays (Xu et al. 2011). The Xiangxi River, which lies 38 km upstream from the Dam, is the largest tributary in the Hubei portion of TGR. This river is 94 km long, with a watershed of 3099 km2 (between 110°25′ and 111°06′E long., 30°57′ and 31°34′N lat.) (Ye et al. 2003). With impoundment of TGR, the downriver stretch of Xiangxi River was inundated, and Xiangxi Bay was formed. Then, the water flow velocity has dropped from the original 0.43–0.92 m/s (Tang et al. 2004) to 0.0020–0.0041 m/s (Wang 2005). So when water temperature increased in spring, there were algal blooms with prolonged retention time and rich nutrients in Xiangxi Bay.

Based on Reynolds' (1984) research, the growth of phytoplankton is governed by phosphorus when nitrogen phosphorus ratio (N/P) in freshwater is higher than 30 while governed by nitrogen when N/P < 8. Because Xiangxi basin is in high phosphorus background region, the drainages of phosphorus diggings and factory discharges play the most important role in the point source pollutants of phosphorus. So, some scholars (Ye et al. 2006; Li et al. 2008) found that the range of N/P in Xiangxi Bay was low or middle level (0.06–24.20), and there would be nitrogen-governed type. In recent years, the distributions and influences of nitrogen in water body of Xiangxi Bay have been studied (Luo et al. 2009; Dai et al. 2010; Yang et al. 2010; Wang et al. 2011). However, the influence of nitrogen in sediments was not considered in earlier-mentioned work. The spatial and temporal variability of nitrogen in sediments of Xiangxi Bay and its roles in biogeochemical cycling processes still need to be fully studied. Does the internal nitrogen loading influence the water body of Xiangxi Bay and cause phytoplankton growth? Whether nitrogen concentration differences between pore water and overlying water are positive and nitrogen in sediments could be released to the water body? So, the main objectives of this paper were: (1) to investigate total nitrogen (TN) and ammonium nitrogen (NH4-N) characteristics in the sediments, pore water and overlying water of Xiangxi Bay; (2) to discuss their relationships and the release possibility of nitrogen in sediments; and (3) to study the influences of internal source nitrogen to chlorophyll-a (Chl-a) concentration in Xiangxi Bay. The results would be helpful in developing effective management for sustainable ecological, economic and social development in Xiangxi Bay.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussions
  7. Conclusions
  8. Acknowledgements
  9. References

Sampling and sample preparation

The sampling was undertaken from six sites of Xiangxi Bay (Fig. 1) on three dates: 29 March 2009, 28 March 2010 and 17 August 2010. Sites XD1–XD5 are on the Xiangxi River. Site GL is located at the downstream of Gaolan River, which is the largest tributary of the Xiangxi River. Water samples were collected at 0.5 m depth from surface in the middle of the river using a 5-L Niskin sampler (Hydrobios-Kiel, Altenholz, Germany). Sediments with a 15-cm overlying water column were collected using acid-washed PVC core tubes (diameter 65 mm). The overlying water was siphoned off, filtered and stored at 4°C for analysis. The top 5 cm of sediment cores were segmented and stored in air-sealed plastic bags at 4°C. Pore water was separated from the sediments by centrifugation (3000 rpm, 10 min) followed by filtration of the supernatant (passing a Whatman 0.45 μm pore-size filter). Prior to analysis the sediment samples were freeze-dried and ground to pass through a 100-mesh sieve.

figure

Figure 1. Sediment sampling sites in Xiangxi Bay.

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Analysis

TN of sediments

Sediment (0.1500 g) was put into a 50-mL glass tube, and 20 mL oxidative agent (0.1875 M NaOH, 0.0741 M K2S2O8) was added. The mixture was shaken and then digested for 30 min at 135°C in high-pressure sterilizing kettle. After digestion, the solution was cooled, centrifuged and filtered by Whatman 0.45 μm pore-size filter. The content of NO3- in filtrate was determined as the TN content by colorimetry. A blank was processed simultaneously.

NH4-N of sediments

Sediment (1.2500 g) was put into a 50-mL glass tube, and 25-mL NaCl solution (2 M) was added. The solution was shaken for 4 h, centrifuged (3000 rpm, 15 min) and filtered by Whatman 0.45 μm pore-size filter. The NH4+ in filtrate was determined using spectrophotometry. A blank was processed simultaneously.

Organic matter of sediments

Organic matter contents of sediments were determined by potassium dichromate oxidation titration method.

Water samples

TN and NH4-N of pore water and overlying water were determined in the laboratory using State Environmental Protection Administration (SEPA) standard methods (Jin & Tu 1990). For Chl-a analysis, water samples filtered through Whatman filters were extracted with cold 90% acetone and estimated by spectrophotometer (Lewitus et al. 1998).

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussions
  7. Conclusions
  8. Acknowledgements
  9. References

Nitrogen characteristics of the sediments

Mean values of TN contents in sediments were 244.13 (March 2009), 348.49 (March 2010) and 770.61 mg/kg (August 2010), respectively. Mean values of NH4-N in sediments were 19.60 (March 2009), 20.59 (March 2010) and 77.01 mg/kg (August 2010). TN and NH4-N contents in sediments increased with time, especially in summer. TN contents of site XD3 in 2010 summer increased 162.63% than that in 2010 spring and reached to the maximum (1202.65 mg/kg). TN contents in sediments increased from down to upper reaches of Xiangxi Bay in 2010 spring but had the distribution characteristic of ‘middle high, both ends low’ in 2010 summer (Fig. 2). NH4-N contents in sediments increased from down to upper reaches of Xiangxi Bay both in 2010 spring and summer (Fig. 3). TN and NH4-N contents of tributary site GL in sediments are all near the mean values of different seasons in Xiangxi Bay.

figure

Figure 2. Total nitrogen (TN) contents in sediments of Xiangxi Bay.

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Figure 3. Ammonium nitrogen (NH4-N) contents in sediments of Xiangxi Bay.

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Nitrogen characteristics of pore water and overlying water

Mean values of TN concentrations in pore water were 7.8667 (March 2009), 12.0839 (March 2010) and 16.7683 mg/L (August 2010), respectively. Those in overlying water were 1.5274 (March 2009), 1.7160 (March 2010) and 0.7967 mg/L (August 2010). So, TN concentration of pore water in summer was higher than that in spring, while TN concentration of overlying water in summer was lower than that in spring (Fig. 4). In 2010 spring, TN concentrations of pore water and overlying water all had the distribution characteristic of ‘middle high, both ends low’ and reached to the maximum 18.4682 and 1.9080 mg/L in site XD3, respectively. In 2010 summer, TN concentrations of pore water and overlying water in down reaches of Xiangxi Bay were higher than upper reaches, and the maximum values were 19.3760 (pore water) and 1.3350 mg/L (overlying water), all in site XD1. TN concentrations of pore water in tributary site GL are all near the mean values of different seasons in Xiangxi Bay, while TN concentrations of overlying water in site GL in 2010 spring (1.8700 mg/L) and 2010 summer (1.0262 mg/L) were higher than the mean values in Xiangxi Bay.

figure

Figure 4. Total nitrogen (TN) concentrations of pore water (a) and overlying water (b) in Xiangxi Bay sediments.

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Mean values of NH4-N concentrations in pore water were 4.2220 (March 2009), 4.4423 (March 2010) and 5.1730 mg/L (August 2010). The changes of NH4-N concentration in pore water among different seasons were not significant, but NH4-N concentration of overlying water in 2010 summer had dramatically increased (Fig. 5). Mean values of NH4-N concentrations in overlying water were 0.1997 (March 2009), 0.1850 (March 2010) and 0.4108 mg/L (August 2010), respectively. In particular, NH4-N concentrations of overlying water in 2010 summer had increased 322.77 (site XD2), 198.03 (site XD3) and 138.85% (site GL) compared with that in 2010 spring.

figure

Figure 5. Ammonium nitrogen (NH4-N) concentrations of pore water (a) and overlying water (b) in Xiangxi Bay sediments.

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Organic matter characteristics of the sediments

Mean values of organic matter contents in the sediments of Xiangxi Bay were 1.61 (March 2009) and 1.87% (March 2010), and increased significantly to 2.94% in 2010 summer (August 2010). In site XD3, organic matter content in 2010 summer had increased 172.88% than that in 2010 spring (Fig. 6). Organic matter contents in sediments of Gaolan River (site GL) were 2.67 (March 2010) and 3.25% (August 2010), which were the maximum in the corresponding time period.

figure

Figure 6. Organic matter contents of the sediments in Xiangxi Bay.

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Discussions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussions
  7. Conclusions
  8. Acknowledgements
  9. References

The relationships of nitrogen contents among sediments, pore water and overlying water were investigated. TN concentrations in pore water had positive correlation with TN contents in the sediments (correlation coefficient R = 0.7408, Fig. 7). There was a linear relationship between NH4-N concentrations in pore water and that in overlying water (R = 0.7567, Fig. 8). The response relationship between TN contents in sediments and Chl-a concentrations in surface water was obvious (R = 0.7483, Fig. 9). This indicated that there were intimate relationships between internal nitrogen loading and water ecosystem.

figure

Figure 7. Relationship between total nitrogen (TN) in pore water and TN in sediments of Xiangxi Bay.

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Figure 8. Relationship between ammonium nitrogen (NH4-N) in pore water and NH4-N in overlying water of Xiangxi Bay.

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figure

Figure 9. Relationship between total nitrogen (TN) in sediments and chlorophyll-a (Chl-a) in surface water of Xiangxi Bay.

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Internal nitrogen loading of Xiangxi Bay were in close relations with organic matter contents in the sediments. The linear correlation coefficient between organic matter and TN (NH4-N) contents in the sediments was 0.8072 (0.8160) (Figs 10 and 11).

figure

Figure 10. Relationship between organic matter and total nitrogen (TN) contents in sediments of Xiangxi Bay.

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figure

Figure 11. Relationship between organic matter and ammonium nitrogen (NH4-N) contents in sediments of Xiangxi Bay.

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TN concentration differences between pore water and overlying water (△TNPO) were all positive (Fig. 12). Mean values of △TNPO were 6.3392 (March 2009), 10.2847 (March 2010) and 15.9542 mg/L (August 2010), respectively. The release probability of nitrogen from sediment to water was comparatively high and increased with time. Lorentz model between △TNPO and Chl-a concentration in surface water was established (coefficient of determination R2 = 0.6151, Fig. 13). There was good correlation between △TNPO and Chl-a concentration. This means that the release of nitrogen in sediments has an important impact on phytoplankton growth in Xiangxi Bay.

figure

Figure 12. Spatial and temporal distribution of total nitrogen (TN) concentration differences between pore water and overlying water in Xiangxi Bay. △TNPO, TN concentration differences between pore water and overlying water.

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figure

Figure 13. Relationship between total nitrogen (TN) concentration differences between pore water and overlying water (△TNPO), and chlorophyll-a (Chl-a) in Xiangxi Bay.

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Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussions
  7. Conclusions
  8. Acknowledgements
  9. References
  • (1)
    Spatial and temporal distribution characteristics of TN and NH4-N in the sediments, pore water and overlying water of Xiangxi Bay were investigated. Mean value of TN in the sediments increased from 244.13 (in March 2009) to 770.61 mg/kg (in August 2010), while NH4-N in sediments increased from 19.60 (in March 2009) to 77.01 mg/kg (in August 2010).
  • (2)
    There were intimate nitrogen relationships between sediments and water ecosystem. TN concentrations in pore water had positive correlation with TN contents in the sediments (R = 0.7408). There was a linear relationship between NH4-N concentrations in pore water and that in overlying water (R = 0.7567). The correlation coefficient R between TN in sediments and Chl-a in surface water was also high (0.7483).
  • (3)
    TN concentration differences between pore water and overlying water were all positive (4.3141–18.0409 mg/L) and had good correlation with Chl-a concentration of Xiangxi Bay. The coefficient of determination (R2) of Lorentz model between △TNPO and Chl-a concentration in Xiangxi Bay was 0.6151. So, the phytoplankton growth in Xiangxi Bay should be influenced by the release of nitrogen in sediments.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussions
  7. Conclusions
  8. Acknowledgements
  9. References

This work was funded by National Natural Science Foundation of China (No. 51179095), National Science and Technology Support Program of China (No. 2008BAB29B09), Major Science and Technology Program for Water Pollution Control and Treatment in the National Twelfth Five-Tear Plan of China (2012ZX07104-002-04) and Hubei Province Ministry of Environmental Protection, China (No.2008HB08). We thank Daobin Ji, Zhengjian Yang, Jun Ma, Song Kong, Niansan Hu, Yu Zhang, Jingfeng Xu, Shuyong Hu and Xia Yang for their assistance in the field and lab.

Footnotes
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References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussions
  7. Conclusions
  8. Acknowledgements
  9. References
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