Sediment sink-source shifts drive intensified internal phosphorus (P) release during the rainy season in a eutrophic...

Soluble reactive phosphorus (SRP) depletion can be observed during prolonged algal blooms in shallow lakes, yet the role of sediments in this process remains inadequately understood. To elucidate the regulatory role of sediments in SRP cycling during bloom development, we conducted field campaigns throughout the 2024 rainy season in Lake Dianchi-a highly eutrophic plateau lake characterized by persistently low SRP concentrations. Water quality, sediment properties, SRP fluxes, and adsorption characteristics were analyzed at 30 sites to assess spatiotemporal shifts in sediment source-sink behavior. Water column SRP declined significantly from 15 ± 14 μg l^-1 in the early rainy season to 4 ± 1 μg l^-1 in the late rainy season (p < 0.001), coinciding with a pronounced rise in sediment Fe-P content (R² = 0.442, p < 0.001), suggesting adsorption induced by external P inputs. Despite high sediment total P (2142 ± 833 mg kg^-1) and a large reactive P fraction (44.0% ± 16.3%), adsorption capacity remained substantial (Q_max: 1223-3643 mg kg^-1), attributed to calcareous sediments enriched with exogenous Fe-Ti minerals. Incubation experiments captured a dynamic transition in sediment SRP release flux (R_SRP) from sink to source across Lake Dianchi, with mean R_SRP increasing from 0.72 ± 5.02 to 1.74 ± 1.69 mg m^-2d^-1 from the early to late rainy season. R_SRP was governed by water SRP concentration, reactive sediment P pool size, and P-binding strength, and was further amplified by temperature. These findings demonstrate the dual regulatory role of sediments: they buffer external P pulses while storing reactive P that fuels internal loading and sustains biological uptake. Such tightly coupled water-sediment feedback during the rainy season poses a major challenge for the restoration of eutrophic lakes such as Dianchi.