Eutrophication and algal blooms have recently been found in the backwater areas of some tributaries in the Three Gorges Reservoir (TGR), for which phosphorus (P) is an important driving factor. However, P species in the TGR sediments at different water levels were little known. In this study, five P species of the TGR sediments during the high and low water level periods were analyzed with a sequential extraction method. The total P (TP) concentrations were 714.88 ± 37.86 μg/g and 697.57 ± 111.49 μg/g at the low and high water levels, respectively. The concentrations of P species decreased in the orders detrital P > authigenic P > organic P > iron-bound P > exchangeable P at the low water level and detrital P > organic P > authigenic P > iron-bound P > exchangeable P at the high water level. P in the TGR sediment sourced mainly from the upstream input at the low water level but from the inputs of tributaries and hillslope soils at the high water level. The bioavailable P (BAP) possessed > 29.5% of total P in the two periods. The total storages of total P and BAP were estimated to be about 1.34 × 106 t and 1.77 × 105 t, respectively during 2003-2017. It was further found that the BAP concentration significantly increased from periods I (2003-2009), II (2010-2014), to III (2015-2017), while the deposition flux and storage of BAP were the highest in period II. Our findings provide new insight into the P cycle and benefit eutrophication treatment in the TGR.
Keywords: Bottom sediment; Controlling factor; Eutrophication; Phosphorus species; Three Gorges Reservoir; Water level.