In a recent study published in Water Research, a team led by Prof. LI Qingman from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences has proposed a multi-level assessment framework for the risk of internal phosphorus (P) release, based on dilute-HCl extracted P (dHCl-P), providing a practical tool for evaluating sediment P risk in freshwater ecosystems.

P is one of the key limiting nutrients that drive eutrophication in aquatic ecosystems. Even after external nutrient inputs are effectively controlled, the continuous release of bioavailable P from sediments can sustain elevated nutrient levels in the water column, thereby hindering the ecological recovery of lakes and reservoirs.

Developing a simple, reliable, and ecologically meaningful method to assess the risk of internal P release from sediments is therefore of great significance for water environment management and aquatic ecosystem restoration.

To address this issue, the researchers investigated sediments collected from four representative freshwater bodies in China, namely Honghu Lake, Xiashan Reservoir, Zhanghe Reservoir, and Yudong Reservoir. They evaluated the relationships between dHCl-P and sediment reductive type, sediment matrix type, total P content, P fraction composition, and P concentrations in the overlying water.

The researchers found that dHCl-P is primarily controlled by sediment reductive type rather than by total P content or sediment matrix type alone. In particular, RedOr-Sⁿ sediments contained substantially higher levels of dHCl-P than RedOr-Fe(II) sediments, indicating a greater potential for internal P release.

Further analysis revealed that dHCl-P mainly consists of labile P fractions, including exchangeable P, Fe(II)-bound P, and Ca-bound P. As sediment reduction intensifies, Fe(III) oxides undergo reductive dissolution, which weakens the P retention capacity of sediments and promotes the accumulation of labile P. These findings indicate that dHCl-P is not only an extraction-based chemical index, but also an effective indicator of P mobilization risk under redox-driven sedimentary processes.

The study further found that dHCl-P is significantly correlated with both total dissolved P and soluble reactive P in the overlying water. Compared with conventional indicators, dHCl-P shows a stronger capacity to reflect the sedimentary P pool that can be potentially released to the water column.

Based on the findings, the research team has established a four-level framework for assessing the risk of internal P release by integrating the sediment reductive type, the proportion of dHCl-P in total P, and the reduction degree of iron oxides.

This framework provides a unified and practical approach to identifying the risk of sediment P release across different freshwater systems and can serve as a valuable reference for managing lakes, reservoirs, and other eutrophic waters.

This study offers a new technical pathway for internal P risk assessment in freshwater environments. By linking a simple dilute-acid extraction index with sediment redox conditions, labile P fractions, and P responses in overlying water, the work provides important support for risk identification, management prioritization, and the development of targeted remediation strategies.

The study was conducted by the IHB in collaboration with the Weifang Xiashan Reservoir Management Service Center and other partners.