Sulfate reduction and diffusion in sediments of Little Rock Lake, Wisconsin

Abstract

Rates of sulfate diffusion and reduction were measured in sediments of Little Rock Lake, an oligotrophic, soft‐water lake in northern Wisconsin. Laboratory measurements of kinetics of sulfate reduction found half‐saturation constants (20–30 µmol liter−1) and Q10 values (2.6) similar to values reported in the literature. Sulfate reduction under in situ conditions in sediment cores was limited by sulfate and followed similar uptake kinetics as in laboratory experiments. Some variation in kinetic parameters was evident as a function of location in the lake. No seasonal variation was observed in sulfate reduction rates in the lake sediments, and littoral and pelagic sites exhibited similar rates. Rates of sulfate reduction were much higher than fluxes of sulfate calculated from pore‐water profiles. Pore‐water profiles also indicated little difference in diffusive fluxes among pelagic and littoral sites and among seasons. The discrepancy between diffusive fluxes and sulfate reduction rates is ascribed to high rates of oxidation of reduced sulfur. Nonlinear rates of sulfate reduction and calculated turnover times of sediment sulfide pools support the hypothesis that sulfide oxidation occurs nearly as rapidly as sulfate reduction.