Quantifying the impact of damming on phosphorus reallocation: Finer particles offset the reduction in soluble reactive phosphorus (SRP) by decreasing suspended sediment concentration.

In this research, an innovative approach to quantify the impact of damming on phosphorus (P) reallocation between suspended sediments (SS) and water was proposed. P allocation can be described by the surface complexation model, with the impact of damming quantified by four variables: P load, suspended sediment concentration (SSC), particle size, and pH. Iron/aluminium (Fe/Al) oxide-adsorbed P (Fe/Al-P) was identified as the exchangeable P during adsorption/desorption equilibrium with a series of heterogeneous sediment samples from two large Asian rivers, the Mekong River and the Yellow River. In both rivers, the Fe/Al-P concentration increased from the tail towards the dam of the reservoirs, primarily attributed to the decrease in particle size from the tail towards the dam of the reservoirs. The Fe/Al-P concentration in the Lancang River was higher than that in the Yellow River, ranging from 14.5 to 119.9 mg kg and from 14.5 to 22.1 mg kg, respectively. The soluble reactive P (SRP) concentration decreased with decreasing SSC, while finer suspended sediment particles containing more Fe/Al-P greatly offset the reduction in SRP concentration. When the maximum Fe/Al-P concentration in the finest particles of SS was assumed to be 100 mg kg, the P equilibrium concentration (c) decreased from 0.028 mg L to 0.008 mg L when the SSC decreased from 64 g L to 1 g L for SS with a median grain size (D) of 32 μm and an Fe/Al-P concentration of 11 mg kg. However, c increased from 0.008 mg L to 0.021 mg L when the D of SS decreased from 32 μm to 4 μm with an SSC of 1 g L and an Fe/Al-P concentration of 76 mg kg for 4-μm SS. The SRP concentration is sensitive to the Fe/Al-P concentration in SS, and the P allocation ratio between sediments and water is comparable.