Phosphorus (P) stored in lake sediments as legacy P can serve as a long-term internal source of eutrophication if environmental conditions shift. One key sedimentary P sink is vivianite (Fe₃(PO₄)₂·8H₂O), a ferrous phosphate mineral that forms under anoxic, iron-rich conditions. However, vivianite is unstable in sulfidic environments, where sulfide (S²⁻) preferentially binds to Fe(II), potentially releasing P back into porewater. In this study, we examined the stability of naturally formed vivianite under in situ sulfidic conditions in Lake Arendsee, a eutrophic lake in northern Germany with elevated sulfate concentrations and deep vivianite-rich sediment layers. Using sediment-filled dialysis chambers (peepers), vivianite-bearing sediment was exposed for 2 and 9 months to varying natural porewater conditions across a redox gradient. Our results show that sediment exposed to sulfidic conditions near the sediment–water interface experienced significant solid phase P loss—up to 41 %—accompanied by increased S content and decreased vivianite signal, as confirmed by Mössbauer spectroscopy and X-ray diffraction (XRD). Most P released originated from vivianite, indicating that sulfide-induced vivianite dissolution was the primary P-mobilization pathway. These findings demonstrate that vivianite can act as a dynamic P source when exposed to sulfidic porewater, with implications for long-term P cycling and lake management. As sulfate inputs to freshwater systems continue to rise, processes that destabilize vivianite may compromise the effectiveness of internal P retention. Consequently, lake restoration strategies must account for sulfur-driven P release mechanisms.