Research on partition of phosphorus in the Three Gorges Reservoir on the Yangtze River

Partition of phosphorus (P) plays an essential role in its ecological effect in surface waters. Yet limited river sampling hinders our understanding for it. P partition between suspended sediments (SS) and aqueous phase in the mainstem of the Three Gorges Reservoir (TGR) on the Yangtze River were studied based on data during 2004-2019. The results reveal that the percentage of DP (dissolved phosphorus) in TP (total phosphorus) (i.e, λ (DP/TP)) decreased remarkably with increasing concentrations of SS, and the empirical equation by nonlinear fitting is λ (DP/TP) = (SS/50 + 80)/(SS + 98) (SS: mg/L, Model I). When SS increased from several mg/L to 180 mg/L, λ (DP/TP) decreased sharply from averagely 0.80 to 0.25. In the range of SS﹥ ~ 400 mg/L, λ (DP/TP) tended to be relatively steady remaining between 0.05 and 0.20 with an average of 0.12. The partition coefficient (K_p) of P between SS and aqueous phase was found to decrease with rising SS and C_e (aqueous concentration of P, i.e., DP).The empirical equation based on SS is K_p (L/g) = 1000 × (49 × SS + 900)/(SS² + 4000 × SS) (SS: mg/L, Model II). When SS increased from <3 mg/L to ~50 mg/L, K_p decreased rapidly from averagely 88 to 23 L/g, and when SS exceeded 50 mg/L, the pace of decreasing of K_p slowed down. The equation based on C_e is K_p (L/g) = 45.88-194.44 × C_e (mg/L) (Model III). When C_e increased from 0.025 to 0.25 mg/L, the average K_p decreased from 50 to 7.0 L/g. Compared with the influence of variation in SS and C_e, the influence of temperature change on K_p can be ignored. New models are advantageous over previously reported ones, and they can be used to better predict P partition and determine whether SS is a sink or a source.