The sampling was undertaken from six sites of Xiangxi Bay (Figure 1) on 29 March 2009 and 28 March 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 . 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.

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 (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 .

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.

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 .

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.