Phosphorus (P) fractionations in the surface sediment of Sancha Lake in China's southwestern Sichuan Province were examined to assess the potential P release at the water-sediment interface and to understand its seasonal (2009-2010) and historical dynamics (1989-2010) in the surface water. Elevated P concentrations were detected in the sediment at main reservoir inflow, south canal of the Dujiangyan irrigation network, and intensive cage fish farming area, accounting for 32 and 40% of current total P discharges. The highest total P concentration (11,200 μg P g-1) was observed in the upper sediment below intensive fish farming area with a specific enrichment of HCl-P (51% of total P) mainly from fish feeds and feces. These sediments had larger MgCl2-P pools with higher diffusive P fluxes (0.43-0.47 mg m-2 d-1) from surface sediment than those from other areas (0.25-0.42 mg m-2 d-1). The general small proportion of MgCl2-P (5.7-10%) and low diffusive P fluxes from surface sediment (<0.02% of sediment P storage (0-1 cm)) indicate low mobility and slow release of P from sediments. The sediment as an internal P source led to a 3-4-year lag for P concentration decrease in the surface water after restriction of anthropogenic P discharges since 2005. Thus, the peak P concentration in April and September could be explained as a combined effect of supplementing internal loading via reductive processes in sediments and seasonal water vertical circulation in the early spring and fall. Policy played a crucial role in reducing P inputs to the lake.
Keywords: Aquatic ecosystem; Environmental protection; Eutrophication; Reservoir; Sediment; Socio-economic development.