Study on Phosphorus Characteristics in Sediments of Xiangxi Bay, China Three-Gorge Reservoir ()
1. Introduction
Reservoir eutrophication has become a serious environmental problem in China. Biological productivity in reservoir is strongly related to the concentration of phosphorus (P). It has been shown that the sediment can act as an internal source of phosphorus for the overlying water [1] -[3] . P can be transferred from water to sediment through biochemical and physical reactions such as ion exchange, adsorption, and precipitation [4] , and also could be released from sediments as the overlying water quality changing [5] -[8] .
China Three-Gorge Dam (
2335 m
long and
185 m
high) is the world’s largest dam, and the reservoir created by it has an area of
1080 km
2 in
2009 [9] . The Xiangxi River, which lies
38 km
upstream from the Dam, is
94 km
long with a watershed of
3099 km
2 (between 110˚25'E and 111˚06'E long., 30˚57'N and 31˚34'N lat.) [10] . With impoundment of Three-Gorge Reservoir (TGR), the water flow velocity in Xiangxi Bay dropped from the original 0.43 - 0.92 m/s [11] to 0.0020 - 0.0041 m/s [12] . As water temperature increased in spring, there were algal blooms with prolonged retention time and rich nutrients in
Xiangxi
Bay
.
In Xiangxi Bay, the distributions and influences of phosphorus in water body have been studied [13] -[16] . However, the spatial and temporal variability of phosphorus and phosphorus bioavailability in sediments of
Xiangxi
Bay
still need to be fully studied. So the objective of this study was to investigate total phosphorus and phosphorus fractions characteristics in the sediments of
Xiangxi
Bay
. The phosphorus relationships among sediment, pore water and overlying water were also studied.
2. Materials and Methods
2.1. Sampling and Sample Preparation
The sampling was undertaken from six sites of Xiangxi Bay (Figure 1) on 29 March 2009 and 28 March 2010.
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Figure 1. Sediment sampling sites in Xiangxi Bay.
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
. 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.
2.2. Sediment Analysis
2.2.1. Total Phosphorus (TP)
Freeze-dried sediment (
0.7000 g
) was put into a 50-ml glass tube, and digested with potassium persulphate and 30% v/v sulphuric acid. After digestion, the solution was cooled, centrifuged and filtered by Whatman 0.45 μm pore-size filter. Then phosphorus was determined using the ammonium molybdate-ascorbic acid (AMAA) method [17] . A blank was processed simultaneously.
2.2.2. Phosphorus Fractions
The contents of different phosphorus fractions were determined using the sequential extraction scheme suggested by Psenner et al. [18] with the modifications of Hupfter et al. [19] (Figure 2), which based on differences in reactivity of solid phases to different extractant solutions. The extraction procedure divided inorganic phosphorus (IP) fractions into loosely sorbed P (NH4Cl-P), redox-sensitive P (BD-P), metal oxide bound P (NaOH-P) and calcium bound P (HCl-P). The difference between TP and IP is the residual P (Res-P) fraction, which contains organic P and refractory P compounds.
2.3. Water Samples
Dissolved total phosphorus (DTP) of pore water and overlying water were determined in the laboratory using AMAA method.
All samples were analyzed in triplicates and the data were expressed as the average.
3. Results and Discussion
3.1. Sediment TP Characteristics
Because Xiangxi river basin is in high phosphorus background region, TP contents in sediments of Xiangxi Bay were high, which maximum value reached 1941.29 mg/kg (XD1, March 2010). Mean values of TP contents in sediments were 1533.09 mg/kg (March 2009) and 1600.48 mg/kg (March 2010), respectively. As the water flow velocity from upper to down reaches of
Xiangxi
Bay
decreased, sedimentation of phosphorus increased as moving to the estuary. TP contents in sediments decreased from down to upper reaches of Xiangxi Bay except TP content of site XD
4 in
2010 (Figure 3). In site XD4 (March 2010), TP contents increased 31.32% than that in 2009 spring and reached to 1886.65 mg/kg. TP contents of tributary site GL in sediments were 1359.65 mg/kg (March 2009) and 1484.88 mg/kg (March 2010), which all near the mean values of TP in
Xiangxi
Bay
.
3.2. Sediment Phosphorus Fractions
For mean values of all the sediment samples phosphorus fractions were in the order: Res-P > HCl-P > NaOH-P > NH4Cl-P > BD-P. But there were different sequences in different sites (Figure 4): XD1 (March 2009) with sequence of Res-P > HCl-P > NaOH-P > BD-P > NH4Cl-P; XD3 (March 2010) with sequence of HCl-P > NaOH-P > Res-P > NH4Cl-P > BD-P; XD5 (March 2009 and March 2010) with sequence of HCl-P > Res-P > NaOH-P > NH4Cl-P > BD-P; GL (March 2009 and March 2010) with sequence of NaOH-P > HCl-P > Res-P > NH4Cl-P > BD-P. It was reported that in heavily polluted lakes, the rank order of NaOH-P > HCl-P was found [1] , while the oppositive order of HCl-P > NaOH-P was in mesotrophic lakes [20] . So the rank order of phosphorus fractions suggests that the sediments of
Xiangxi
Bay
are mostly mesotrophic, except for site GL.
NH4Cl-P represents the loosely sorbed P in the sediments [20] [21] . In
Xiangxi
Bay
, mean values of NH4Cl-P contents in sediments were 95.06 mg/kg (March 2009) and 131.01 mg/kg (March 2010), respectively. NH4Cl-P contents of all sampling sites in 2010 were higher than that in 2009 except XD2 (Figure 4(a)). In 2010, the highest NH4Cl-P amount was observed in XD4 (187.15 mg/kg), which was almost two times higher than that in site XD1 (94.11 mg/kg). The percentage of NH4Cl-P contributing to TP increased from down to upper reaches of Xiangxi Bay in 2010 and was in the range of 4.57% - 8.47% (March 2009) and 4.85% - 13.22% (March 2010) for all sampling sites (Figure 4(b)).
The BD-P, mainly including P bound to Fe-hydroxides and Mn compounds, represents the redox-sensitive P fraction [22] . In dissolved oxygen (DO) depletion environment, the BD-P was released from anaerobic sediments and acted as an internal P source to the overlying water [23] . In Xiangxi Bay, the mean BD-P amounts in sediments were the lowest among the five P fractions and exhibited high variability in various sampling sites.
![]()
Figure 3. TP contents in sediments of Xiangxi Bay.
The BD-P amounts in the sediments ranged from 57.80 mg/kg (XD4) to 96.45 mg/kg (XD3) in 2009, which contributed 3.15% - 7.03% of TP, and from 55.64 mg/kg (XD2) to 163.34 mg/kg (XD4) in 2010, which contributed 3.40% - 11.77% of TP (Figure 4). DO depletion, high pH level or bacterial activity can enhance contribution of the BD-P fraction [20] .
NaOH-P is phosphorus bound to metal oxides and can be released for the growth of phytoplankton when anoxic conditions prevail at the sediment-water interface [24] . NaOH-P amounts in the sediments ranged from 139.49 mg/kg to 477.65 mg/kg, which contributed 7.11% - 32.17% of TP. Mean values of NaOH-P contents in sediments were 95.06 mg/kg (March 2009) and 131.01 mg/kg (March 2010), respectively. The highest amount was detected in site GL (432.59 mg/kg in 2009 and 477.65 mg/kg in 2010). NaOH-P was used for the estimation of available P in the sediments and was indicator of algal available P [25] . So the risk of phosphorus release in GL was high for algal bloom.
HCl-P was assumed to mainly consist of apatite P, including P bound to carbonates and traces of hydrolysable organic P. This phosphorus fraction was deemed as a relatively stable fraction in the sediments [26] . HCl-P amounts in the sediments ranged from 410.56 mg/kg to 748.38 mg/kg, which contributed 22.02% - 39.33% to TP with the average of 32.94% (Figure 4(b)). The high HCl-P content was owing to high portions of calcium mineral P in
Xiangxi
Bay
.
3.3. Phosphorus Characteristics of Pore Water and Overlying Water
Mean values of DTP concentrations in pore water were 0.2236 mg/L (March 2009) and 0.2854 mg/L (March 2010), respectively. Those in overlying water were 0.0811 mg/L (March 2009) and 0.0968 mg/L (March 2010). DTP concentrations of pore water in all sampling sites in 2010 were higher than those in 2009 except XD2 (Figure 5(a)). The maximum DTP concentration of pore water was 0.3648 mg/L (site XD4, March 2010). DTP concentrations of overlying water increased from XD2 to XD5, which reached to the maximum values in site XD5 (0.1130 mg/L in 2009 and 0.1550 mg/L in 2010) (Figure 5(b)).
The relationships between DTP of pore water and different P fractions in the sediments are shown in Figure 6. It can be concluded that the DTP concentrations of pore water were strongly in positive correlations with NH4Cl-P (R = 0.9752, P < 0.01) and BD-P (R = 0.8516, P < 0.01). The DTP concentrations of overlying water were also in positive correlations with NH4Cl-P (R = 0.7306, P < 0.01) and BD-P (R = 0.7150, P < 0.01) (Figure 7). This indicates that NH4Cl-P and BD-P may be easily released from the sediments in
Xiangxi
Bay
, and they were main fractions of the release phosphorus source in the sediments and of the sources for the overlying water.
![]()
(a)![]()
(b)
Figure 5. DTP concentrations of pore water (a) and overlying water (b) in Xiangxi Bay sediments.
![]()
![]()
Figure 7. Relationships between DTP of overlying water and different P fractions in Xiangxi Bay sediments.
4. Conclusion
Spatial and temporal distribution characteristics of TP and phosphorus fractions in the sediments of Xiangxi Bay were investigated. The contents of TP and different phosphorus fractions in the sediments varied greatly in sampling sites. TP contents ranged from 1111.29 mg/kg to 1941.29 mg/kg with the mean value of 1533.09 mg/kg in 2009 and 1600.48 mg/kg in 2010. Phosphorus contents in NH4Cl-P, BD-P, NaOH-P, and HCl-P ranged from 82.33 to 187.15 mg/kg, 55.64 to 163.34 mg/kg, 109.63 to 477.65 mg/kg, and 410.56 to 748.38 mg/kg, respectively. For mean values of all the sediment samples, the rank order of phosphorus fractions was: Res-P > HCl-P > NaOH-P > NH4Cl-P > BD-P, which indicated that the sediments of Xiangxi Bay were mesotrophic. Dissolved total phosphorus of pore water and overlying water all have positive correlations with NH4Cl-P and BD-P. NH4Cl-P and BD-P may be easily released from the sediments and contributed mostly as the release phosphorus source in the sediments of Xiangxi Bay. The results reported herein would be helpful in developing effective management to control internal phosphorus-loading release in the
Xiangxi
Bay
sediments.
Acknowledgements
This work was funded by National Natural Science Foundation of China (No. 50679038, 51009080), National Water Special Project of China (2008ZX07104-004) and Science Foundation of China Three Gorges University (No. 1112046). 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.