Iron focusing and sulphur-driven mineral formation regulate phosphorus sequestration in Lake Stechlin (NE Germany)

Abstract Iron (Fe) plays a key role in lake ecosystems by regulating the availability of phosphorus (P) as a limiting factor for primary production. This study investigates how microbial sulphate reduction in Lake Stechlin, a dimictic and initially oligotrophic lake in northeast Germany (zₘₐₓ = 69.5 m), has altered the mobility of Fe and P over several decades. Using long-term monitoring data combined with sedimentological investigations, the study examines long-term geochemical focusing, defined as the net accumulation of redox-mobilised Fe in the deepest basin driven by reductive dissolution, lateral transport, and re-precipitation. Under oligotrophic conditions prior to the 1960s, focusing of Fe toward the deepest site likely involved: (1) reductive dissolution of sedimentary Fe oxides in shallow areas, (2) diffusion of dissolved Fe into overlying water, (3) re-oxidation/complexation and stepwise lateral transport within the water column, and (4) final deposition and burial at the deepest zone. Chemical and mineralogical evidence indicates that Fe burial was facilitated by authigenic formation of stable Fe(II) minerals, most likely ankerite (CaFe[CO 3 ] 2 ) and vivianite (Fe 3 [PO 4 ] 2 ·8H 2 O). Fe enrichment at the deepest site has strongly weakened, coinciding with intensified pyrite (FeS 2 ) formation in anoxic littoral and profundal sediments. This shift was likely driven by sufficient supply of organic matter and elevated sulphate inputs, which stimulated microbial sulphate reduction. Elevated S/Fe ratios indicate that sulphide efficiently competed with P for binding to Fe, potentially increasing susceptibility to external and internal P loading.