Historical and seasonal dynamics of phosphorus mobility in Sancha Lake of Southwest China’s Sichuan Province

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.