Phosphorus internal loading and sediment diagenesis in a large eutrophic lake (Lake Chaohu, China)

Sediment phosphorus (P) release and retention are important in controlling whole-system P dynamics and budget in eutrophic lakes. Here we combine short- (seasonal) and long-term (years to decades) studies to quantify the internal P loading and P release potential in the sediments of Lake Chaohu and explore their controlling mechanisms. In the west region of the lake, short-term P diffusive fluxes ranged from 0.2 mg/m²·d^-1 to 6.69 mg/m²·d^-1 (averaged 2.76 mg/m²·d^-1) and long-term net P release ranged from 2.25 mg/m²·d^-1 to 8.94 mg/m²·d^-1 (averaged 5.34 mg/m²·d^-1); in the east region, short-term P diffusive fluxes varied from 0.73 mg/m²·d^-1 to 1.76 mg/m²·d^-1 (averaged 1.05 mg/m²·d^-1) and long-term P release ranged from 0.13 mg/m²·d^-1 to 4.15 mg/m²·d^-1 (averaged 1.3 mg/m²·d^-1). Both short- and long-term P releases were in the same order of magnitudes as the external P inputs (3.56 mg/m²·d^-1). Comparison of the long-term and short-term sediment P release indicates that while the high summer P release in the east might only represent a snapshot value, the sediments in the west contribute to large P release for years or even decades, impeding water quality recovery under lake management. Mobilization of surface sediment legacy P accounted for 81% of short-term P release. The long-term release was dominated by remobilization of iron bond P (BD-P) (average 52.1%) at all sites, while Aluminium-bound P (NaOH-rP) exhibited partly reactive and potentially mobile, releasing P to the water column in most sites in the west. Our study demonstrates the importance of sediments as P sources in lake Chaohu. The combination of short- and long-term P release studies can help understand the roles of sediments in regulating the water quality and eutrophication.