Vivianite as a phosphorus source in lake sediments: Importance of increased sulphate reduction on phosphorus mobilisation

Abstract Purpose Eutrophication of freshwater systems is primarily driven by excessive nutrient inputs, particularly phosphorus (P). While external nutrient control has been emphasized, the prediction and management of internal P loading from sedimentary sources remain complex. This study examines the role of vivianite (Fe(II) 3 (PO 4 ) 2 ·8H 2 O), a P-bearing mineral in anoxic sediments, in contributing to internal P release under sulfidic conditions. Materials and methods A mesocosm experiment was conducted using sediment cores from Lake Arendsee, Germany. The cores were exposed to elevated sulfate concentrations to induce sulfate reduction, simulating anoxic and sulfidic conditions. Both water column chemistry and sediment solid-phase analyses were performed. Phosphorus release from vivianite-rich sediments was monitored, along with changes in iron (Fe) mineral phases using sequential extraction and X-ray diffraction. Results and discussion Increased sulfate reduction rates significantly mobilized P from vivianite-rich sediments, leading to elevated soluble reactive P levels in the water column. A marked decrease in vivianite content and an increase in sulfide-bound Fe species were observed in the sediments. These findings demonstrate that vivianite in Fe-rich sediments serves as an important internal P source under sulfidic conditions, exacerbating P release. Conclusions This study highlights the role of sulfur cycling in internal P loading and suggests that increased sulfate inputs may enhance eutrophication by mobilizing P from buried vivianite. Effective management of eutrophication should consider both external inputs and internal P sources like vivianite.