Bioavailability of organic phosphorus in an eutrophic lake: Insights from an in-situ experiment

The bioavailability of organic phosphorus (Porg) in surface sediments of an eutrophic lake was studied in an in-situ experiment. On the temporal scale, soluble reactive phosphorus (SRP) in the surface water showed an initial decrease and then increased in western and middle parts of the lake. In contrast, the SRP remained about 0.22 mg L-1 throughout the year in the eastern area of the lake. Four Porg compounds, phosphonate, orthophosphate monoesters, phospholipids and deoxyribonucleic acid phosphorus, were detected using solution P-31 nuclear magnetic resonance spectroscopy. Orthophosphate monoesters were the dominant Porg in the sediment extracts, higher than the orthophosphate diesters deoxyribonucleic acid phosphorus and phospholipids. The SRP and Porg showed obvious seasonal variations in different lake regions. The surface water SRP had a negative relationship with sedimentary Porg in the western and middle lake regions. From spring to autumn, SRP in the water column decreased while Porg in the surface sediment increased; from autumn to winter, SRP in the water column increased and Porg in the surface sediment decreased. According to aquatic metabolism, we inferred that excessive external phosphorus input was the "trigger" and the aquatic plants were the "pump" for Porg biogeochemical cycling in the shallow lake. Therefore, appropriate human intervention, such as the harvest of aquatic plants, is vital for eutrophication control in freshwater ecosystems.