Water level variation substantially affects the trophic levels in a lake. The primary objective of this research is to investigate the impact of water level fluctuation on sediment and phosphorous (P) dynamics in Tonle Sap Lake (TSL), Cambodia. Water samples were collected from eight cross sections in a lake at 3-month intervals for 3 years, during the low-water period (March and June) and high-water period (September and December) from December 2016 to June 2019. Water quality parameters—temperature, conductivity, salinity, dissolved oxygen, pH, oxidation reduction potential, chlorophyll-a (Chl-a), and blue green algae—were measured using an EXO(R) sensor. The sedimentation and resuspension rates of the sediment were measured using a sediment trap. Sediments were collected during the low-water period of March 2017. The sediment resuspension condition was simulated via centrifugation (150 rpm, end-to-end mechanical shaker, room temperature 25 °C for 24 h) to assess phosphorous dynamics. The P content in the bed sediment was fractionated to obtain loosely bound, metal oxide bound, apatite bound, and organic bound P. Sediment resuspension was greater during the low-water period (< 1 m) than that during the high-water period (> 4.8 m). The sites exhibited significant variation (p < 0.05) in terms of dissolved oxygen, pH, oxidation–reduction potential, Chl-a, and depth during low- and high-water periods. In general, the Chl-a concentration throughout the sampling campaigns ranged between 1.7 and 9.0 μg/L, suggesting the existence of a mesotrophic state in the TSL. The zero equilibrium P concentration of sediment in TSL was greater when the sediment was under the resuspension condition (18.9 ± 3.0 μg/L) than under the static condition (7.7 ± 1.1 μg/L). In TSL, during resuspension (low-water) conditions, sediments act as a source and release P (apatite bound, loosely bound, metal oxide bound, and soluble reactive P). However, in static (high-water) conditions, sediments act as a sink by adsorbing P from the overlying water.
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