Impact of Simulated Water Level Regulation on Sediment Nutrient Release

Representative sediment samples rich in nitrogen and phosphorus (both continuous and intermittently submerged) were used to conduct dynamic water level (WL) regulation experiments with various WL velocity modes (0, 3, and 6 cm/day). The experiments lasted three WL regulating cycles (6 months), and each single cycle included four WL dynamic phases: decline, stable, ascend, and re-stable. During the experiment, a greater nutrient stock caused higher nutrient release fluxes for continuously submerged sediments when compared to corresponding intermittently submerged sediments regardless of WL regulation. Moreover, continuous submerged sediment nutrient release showed a similar “U” pattern to the intermittently submerged sediment, and nutrient concentrations within the water phase generally increased with rising WL and decreased with dropping WL. Rapid WL regulation such as 6 cm/day promoted nitrogen release, and slow WL regulation at 3 cm/day favored phosphorus leaching. When three WL regulation cycles were finished, WL regulation of 6 cm/day resulted in 18 and 25 % decline of sediment mean organic matter (OM) content for continuous and intermittently submerged sediment, respectively. However, increased WL regulation cycles impacted on sediment nitrogen and phosphorus stock in different manners. For example, a WL regulation of 6 cm/day led to a 582 mg/kg decline and 322 mg/kg increase for intermittently submerged sediment in terms of total nitrogen (TN) and total phosphorus (TP) content, respectively. Results indicated that direct WL regulation insignificantly affected sediment nutrient release, but changed the overlying water conditions such as pH and redox potential (redox), and then indirectly changed the nutrient release dynamics.