As the running time of reservoirs is increasing, a large number of reservoirs are becoming eutrophicated. Organic phosphorus (OP) is a key factor in eutrophication. However, the mechanism and extent to which organic matter degradation affects P recycling in water column of large deep reservoirs are unclear, especially for the newly-built ones. In this study, different forms of carbon (C) and P in the water column of Hongjiadu Reservoir were investigated. The contents of particulate organic carbon (POC) and particulate organic phosphorus (POP) both decreased with depth in summer, indicating that organic matter was degraded during the deposition of particulates. In contrast, the contents of POC and POP varied slightly with depth in winter. This difference may result from the double thermal stratification and the corresponding double oxygen stratification in summer. The POC/POP ratios were lower in the epilimnion and increased with depth, suggesting that P was preferentially regenerated relative to C during organic matter degradation. The contents of particulate inorganic phosphorus (PIP) and POP were significantly negatively correlated, indicating that POP transformed into PIP in deeper water. The double thermoclines and oxyclines in Hongjiadu Reservoir lead to very low dissolved oxygen (DO) concentrations in the hypolimnion, which should receive sufficient attention. If water becomes hypoxic, enhanced P release during organic matter degradation will promote phytoplankton growth, leading to higher phytoplankton biomass and more severe DO depletion. Thus, a positive feedback loop may form among hypoxia, enhanced P release, higher primary productivity, and more severe hypoxia, accelerating P recycling in large deep reservoirs. Once if eutrophication occurs in these reservoirs, it will be very difficult to restore the water ecosystem. Thus, it is particularly important to prevent the occurrence of eutrophication and the formation of positive feedback loop as early as possible. This highlights the importance of both reducing external loading and improving DO level in large deep reservoirs.

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