Sulfur controls the fate of many geochemical elements in lake sediments, including iron, phosphorus and environmentally important trace elements. We measured the speciation of porewater and sediment-bound sulfur (aqueous sulphate and sulfides, elemental sulfur, iron monosulfide, pyrite, organic sulfur) and supporting geochemical variables (carbon, oxygen, iron) in the sediments of a perennially oxygenated and a seasonally anoxic basin of an oligotrophic lake in Québec, using a combination of pore-water analyses, sequential extractions and X-ray absorption near edge structure. A non-steady state early diagenetic model was developed and calibrated against this extensive dataset to help unravel the pathways and quantify the rates of S transformations. Results suggest that the main source of S to the sediments is the settling of organic ester-sulfate (R-O-SO3-H). Hydrolysis of these compounds provides an additional source of sulfate for anaerobic microbial oxidation of sedimentary organic matter, releasing sulfide to the pore-water. Reduced solid-bound S species accumulate as thiols (R-SH) and iron sulfides in the perennially oxygenated and seasonally anoxic basin, respectively. The model-estimated rate constant for R-SH formation is lower than previously estimated for this particular lacustrine site, but similar to that proposed for marine shelf sediments. The solid sediment S profiles, however, carry the imprint of the time-dependent sulfate input to the lake. Iron sulfide enrichments formed during past decades of elevated atmospheric SO4 deposition are presently dissolving. In the sediments of the perennially oxygenated basin this reaction hampers the build-up of Fe(III) (oxy)hydroxide near the sediment-water interface.

We expanded the existing one‐dimensional MyLake model by incorporating a vertically resolved sediment diagenesis module and developing a reaction network that seamlessly couples the water column and sediment biogeochemistry. The application of the MyLake‐Sediment model to boreal Lake Vansjø illustrates the model's ability to reproduce daily water quality variables and predict sediment‐water column exchange fluxes over a long historical period. In prognostic scenarios, we assessed the importance of sediment processes and the effects of various climatic and anthropogenic drivers on the lake's biogeochemistry and phytoplankton dynamics. First, MyLake‐Sediment was used to simulate the potential impacts of increasing air temperature on algal growth and water quality. Second, the key role of ice cover in controlling water column mixing and biogeochemical cycles was analyzed in a series of scenarios that included a fully ice‐free end‐member. Third, in another end‐member scenario P loading from the watershed to the lake was abruptly halted. The model results suggest that remobilization of legacy P stored in the bottom sediments could sustain the lake's primary productivity on a time scale of several centuries. Finally, while the majority of management practices to reduce excessive algal growth in lakes focus on reducing external P loads, other efforts rely on the addition of reactive materials that sequester P in the sediment. Therefore, we investigated the effectiveness of ferric iron additions in decreasing the dissolved phosphate efflux from the sediment and, consequently, limit phytoplankton growth in Lake Vansjø.

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