To explore the environment influence of the sediments in Xiangxi Bay (China Three-Gorge Reservoir), spatial and temporal distribution characteristics of total phosphorus (TP), phosphorus fractions, dissolved total phosphorus (DTP) of pore water and overlying water in the sediments were investigated. In surveys, the sampling was undertaken from six sites of Xiangxi Bay on 29 March 2009 and 28 March 2010. TP contents ranged from 1111.29 mg/kg to 1941.29 mg/kg with the mean value of 1533.09 mg/kg in 2009 spring and 1600.48 mg/kg in 2010 spring. Five fractions of sedimentary phosphorus, including loosely sorbed phosphorus (NH 4Cl-P), redox-sensitive phosphorus (BD-P), metal oxide bound phosphorus (NaOH-P), calcium bound phosphorus (HCl-P), and residual phosphorus (Res-P), were separately quantified. DTP of pore water and overlying water all have positive correlations with NH 4Cl-P and BD-P, which indicated that NH 4Cl-P and BD-P were the main fractions that can easily release phosphorus in the sediments of Xiangxi Bay.
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 [
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 [
In Xiangxi Bay, the distributions and influences of phosphorus in water body have been studied [
The sampling was undertaken from six sites of 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.
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 [
The contents of different phosphorus fractions were determined using the sequential extraction scheme suggested by Psenner et al. [
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.
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 (
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 (
NH4Cl-P represents the loosely sorbed P in the sediments [
The BD-P, mainly including P bound to Fe-hydroxides and Mn compounds, represents the redox-sensitive P fraction [
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 (
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 [
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 [
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 (
The relationships between DTP of pore water and different P fractions in the sediments are shown in
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.
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.