In the context of a reservoir, the anoxia that develops in the bottom sediment induces the release of phosphorus (P) into the overlying water, thus supporting eutrophication. Most studies focusing on P dynamic in an aquatic environment fail to consider the "truly" dissolved and colloidal fractions, hence the colloidal P has gone largely unexplored. The aim of this study was to investigate the release of sedimentary P under oscillating aerobic, anoxic and aerobic conditions, in taking into account the colloidal (10 kDa-1 µm) and truly dissolved (< 10 kDa) fractions. Laboratory incubations of wet sediment originating from a dam reservoir were performed over 63 days, consisting of 25 days of aerobic conditioning (lasting 2 periods) and 38 days of anoxia. Results showed that oxic conditions induced a very limited release of phosphorus, both in truly dissolved and colloidal forms. In turn, the development of anoxic conditions caused a large release of P, mainly in the colloidal fraction, representing about 90 % of the total water-mobilizable P (PWM < 1 µm). The initial release of truly dissolved P during the anoxic stage gradually diminished over time, possibly due to the formation of secondary minerals or re-adsorption processes. Approximately half of the PWM released during anoxia persisted under subsequent oxic conditions and consisted solely of colloidal P. The dynamics of PWM were primarily influenced by two main factors: (i) the reductive dissolution of iron, which released both dissolved and colloidal P, and (ii) the release of indigenous organic matter, which impacted the stability of the released colloids through bridging mechanisms.
Keywords: Anoxia; Colloid; Dam reservoir; Phosphorus; Redox; Sediment.
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