Seasonal variation regulate the endogenous phosphorus release in sediments of Shijiuhu Lake via water-level fluctuation.

Freshwater lakes undergo substantial alterations of the phosphorus (P) cycle in the water-sediment ecosystem due to thermal change. The impact process of seasonal fluctuation on P cycling in sediments has been scarcely investigated. P forms in sediments from a freshwater lake in China were analyzed using sequential extraction technique. The vertical distribution of soluble reactive P (SRP), Fe, and S in the interstitial water was measured using diffusion gradient technique (DGT). Fick's Law and DIFS model were used to obtain the diffusion fluxes of SRP and the kinetic parameters in the water-sediment system. The results showed that total P (TP) concentrations in the solid sediments varied from 207.5, 266.6 and 130.3 mg/kg to 614.7, 1053.1, and 687.6 mg/kg in winter, spring, and summer, respectively. The concentrations of individual P forms in spring were higher than those in other seasons, with Fe-bound P (Fe-P) concentration being the highest across all seasons. Notably, significant variations of SRP concentrations were found in the interstitial water between sedimentary depths of approximately 2 cm and 6 cm, particularly in the summer. Furthermore, higher diffusion fluxes of SRP through the interface were found in summer. A stable anaerobic environment failed to develop in spring with high water level, preventing the desorption of solid Fe-P and diffusion of Fe into the water due to the afflux and deposition of P-containing particulate into deeper sediment layers along with organic material. Under extreme high-temperature in summer, decreased rainfall and rising temperatures boosted the activity of aquatic organisms in the water, thereby reducing P fixation by sediments and leading to P release. This process increased the risk of P excess and potential eutrophication in the water. Generally, clarifying the resupplying processes of endogenous P in sediment systems experiencing seasonal variations is critical for eutrophication management of lakes.