# Characteristics and Release Risk of Phosphorus from Sediments in a Karst Canyon Reservoir, China
## 4 Discussion
### Sediment Bioavailability and Risk of Phosphorus Release  
Sediment TP contents below 600 mg-kg\({}^{-1}\) are considered environmentally safe, while TP content between 600 mg-kg\({}^{-1}\) and 2000 mg-kg\({}^{-1}\) reflects moderate pollution, and TP content higher than 2000 mg-kg\({}^{-1}\) indicates severe pollution [49]. In this study, the maximum sediment TP content observed was 438.04 mg-kg\({}^{-1}\), and the mean values of WF1-3 were lower than 600 mg-kg\({}^{-1}\), indicating that the overall sediment TP content in the Wanfeng Reservoir is at a safe level. However, solely measuring phosphorus contamination by comparing sediment TP levels and standard values may not accurately assess the risk of sediment phosphorus release. To better evaluate the status of sediment phosphorus release risk in the Wanfeng Reservoir, this study used the ratio of biologically active phosphorus (BAP) to TP as an indicator [11].  
BAP represents the amount of bioavailable phosphorus released as dissolved phosphate that can be utilized by aquatic organisms, thus serving as an indicator to evaluate the potential for sediment phosphorus release [50]. The H2O-Pi and NaHCO3-Pi forms of phosphorus are easily released into the water, while Fe/Al-Pi is susceptible to migration and transformation under changes in pH and redox potential. Therefore, this study considers the sum of H2O-P, Fe/Al-P and NaHCO3-P as the potential BAP. The BAP content and the proportion of BAP in TP were calculated from the measured results as the phosphorus contamination index of the sediment.  
The BAP content in the sediments of the Wanfeng Reservoir ranged from 66.97 to 201.46 mg\(\cdot\)kg\({}^{-1}\), with an average content of 130.59 mg\(\cdot\)kg\({}^{-1}\). The phosphorus pollution index ranged from 55.6% to 59.6%. The spatial distribution of sediment BAP content in the reservoir area exhibits significant heterogeneity, with the highest pollution index observed at WF3 (Figure 7). The phosphorus contamination index of surface sediments ranged from 57.5% to 57.9%, indicating relatively similar contamination levels among the sites. The spatial distribution pattern is characterized as WF3 \(>\) WF1 \(>\) WF2, consistent with the spatial differentiation of TP. The pollution index of WF3 is higher than that of WF1 and WF2 due to its proximity to areas of human activities, which has a more pronounced impact on the sedimentary environment. Additionally, WF3 is located at the confluence of Mengli River and Hongran River, where the water level is relatively low and the water environment is unstable. These conditions can lead to alternating redox reactions in the sediments, causing the diffusion of inorganic phosphorus and resulting in significantly higher levels of surface active phosphorus compared to WF1 and WF2.  
The research results show that the risk of phosphorus release from sediment in the Wanfeng Reservoir is high. Sediment phosphorus has a significant impact on the trophic state of the water, and the release of various phosphorus forms may become a potential risk for water quality deterioration in the reservoir in the future. On one hand, the areas surrounding the reservoir are heavily influenced by human activities, resulting in a higher exogenous contamination load, with phosphorus in the sediment primarily present in easily degradable forms. Additionally, Wanfeng Reservoir is a typical karst canyon reservoir with alkaline water conditions. Under alkaline conditions, the sorption capacity of oxidizable metals such as Fe/Mn for phosphate is reduced, leading to enhanced phosphate release [49]. Therefore, future water ecology management in karst canyon reservoirs should focus on regulating human production activities and reducing the risk of endogenous phosphorus release.  
Figure 7: _Cont._