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Depending on the dynamic equilibrium between the solution and solid phases, phosphorus can either be adsorbed or desorbed from sediments. Determining the sediment EPC0 value can reveal the direction of soluble reactive phosphorus (SRP) flux (House & Denison 2002). In this study, the EPC0 values of the five sediments were calculated with the modified models. The results of the modified Langmuir model better reflect the relationship between EPC0 values in the sediments and SRP in the interstitial water than those of the modified Freundlich model. D1 is located in the outlet of Dongting Lake and the estuary of the Yangtze River. The flow velocity in the lake was high (0.42 m/s), and the disturbance of the sediments was severe. Furthermore, the sediments in D1 were arenaceous and clean, with relatively lower TP contents than the other sediments. Thus, the calculated results of EPC0 for D1 (Table 2) were significantly lower than those of other sediments in 2011 and 2013. The monitoring data (Table 4) indicated that the SRP concentrations of the interstitial water were higher than that of EPC0. Thus, phosphorus sorption occurred from the overlying water, and the sediments of D1 served as a sink of phosphorus. This phenomenon could explain why D1 did not undergo eutrophication in the eastern part of Dongting Lake. On the contrary, the TP contents in sediments in D2 and D3 were high, and the values of EPC0 were substantially higher than the SRP concentration of the interstitial water in the eastern part of Dongting Lake. Thus, the two districts had a high phosphorus concentration in water.

Table 4

EPC0 and SRP concentration in interstitial water of the study area

 EPC0 (mg L−1)
SRP in interstitial water (mg L−1)
Sample2011201320112013
D1 0.032 0.025 0.126 0.101 
D2 0.228 0.406 0.172 0.267 
D3 0.243 0.203 0.138 0.141 
D4 0.137 0.612 0.091 0.096 
D5 0.127 0.127 0.082 0.078 
 EPC0 (mg L−1)
SRP in interstitial water (mg L−1)
Sample2011201320112013
D1 0.032 0.025 0.126 0.101 
D2 0.228 0.406 0.172 0.267 
D3 0.243 0.203 0.138 0.141 
D4 0.137 0.612 0.091 0.096 
D5 0.127 0.127 0.082 0.078 

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