This study aims to establish the relative importance of sediment organic phosphorus (Po) to the total P and the major classes of organic molecules that contribute to sediment Po, determined by measuring their susceptibility to enzymatic hydrolysis, across a suite of lakes ranging from oligotrophic to eutrophic status. The results showed that Po accounted for 21-60% of total P, and bioavailable Po accounted for 9-34% of Po in the sediments. The bioavailable Po includes mainly labile (H2O-Po) and moderately labile (NaOH-Po) P forms. For H2O-Po (accounting for only1.4% of Po), 53% (average) was labile monoester P, 28% was diester P and 17% was phytate-like P. For NaOH-Po (accounting for 9-33% of Po), 32% was labile monoester P, 33% was phytate-like P and 18% was diester P. The composition of bioavailable Po, determined by enzyme assays, was related to the lake nutrient levels, which implies that sediment bioavailable Po could act as an effective indicator for lake eutrophic status. With the increase of lake nutrient levels, bioavailable Po content and alkaline phosphatase activity in the sediment all increased, indicating that Po represents an important and bioavailable source of P that increases with eutrophication, and could contribute to internal loading and resistance of eutrophic lakes to remediation. This implies that eutrophic lakes would maintain long-term eutrophic status and algal bloom phenomena even after the external input of P was controlled and the total P concentration of water has declined. Thus, in order to reduce the release risk of sediment P more efficiently and effectively, sediment P control technique should focus not only on reducing the total P and inorganic P, but should also pay close attention to the removal of bioavailable Po.
Keywords: Bioavailability; Contribution; Organic phosphorus; Sediment.
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