Sediment-dependent phosphorus transport in dammed rivers: The critical role of pretreatment method

The long-term ecological impact of total phosphorus (TP) in fluvial systems is well-established, prompting its inclusion in water quality regulations worldwide. Due to the strong affinity between sediment and phosphorus, certain regulatory protocols recommend pretreating water samples using methods such as shaking or clarification. Recent studies have noted discrepancies between these methods, but their comparisons were experimental and did not consider sediment conditions altered by dam construction. This study aims to investigate how the shaking and clarification methods differ under varying sediment conditions associated with inflow-outflow scenarios. Using the globally representative, dam-regulated Yellow and Yangtze rivers as case studies, we conducted comparative analyses of TP determined by both methods, based on consecutive field measurements and long-term hydrological data. The shaking method indicated substantial reductions in TP (up to 63.75%) from inflow to outflow sites. In contrast, the clarification method yielded much smaller reductions, and during flood seasons it even showed an increase (up to 35.89%). These discrepancies in methodology may lead to substantial underestimation of reservoir TP retention from the perspective of reservoir-induced sediment regime shifts. By integrating theoretical analysis with field data, we demonstrate that this discrepancy is inevitable: the clarification method significantly underestimates TP under inflow condition characterized by coarse sediment or high sediment concentration (S), whereas this underestimation becomes negligible under outflow condition featuring fine sediment or low S. In order to keep a consistent measurement in TP under varying sediment conditions and thereby evaluate the TP retention effect of dams more objectively, we recommend adopting the shaking method for complete TP quantification. This work provides fundamental insights into TP measurement that are crucial for managing river ecosystems.