Seasonal dynamics of sulfate and hydrogen sulfide near the sediment-water interface of an oligotrophic acid lake

Summary

The seasonal evolution of porewater SO^2- ₄ and ∑EH₂S was studied in the shallow (z = 4.5 m) and hypolimnetic (z = 15.5 m) sediments of an oligotrophic acid lake. At both sites the porewater profiles are characterized by a pronounced seasonal and horizontal variability. The SO^2-₄ profiles of the shallow station indicate SO^2-₄ flux to the sediment during summer and winter, and SO²₄ flux from the sediment during spring and fall when sediment temperature is low and the overlying waters are fully oxygenated. At the deeper station, the profiles indicate SO^2-₄ flux to the sediment during summer, fall and winter, and SO^2-₄ release from the sediment, shortly after spring turnover. Hydrogen sulfide usually show a maximum concentration of 0.2 – 8 μmol · 1⁻¹ located at the level of SO^2-₄reduction in the sediment column. The SO^2-₄ maxima present near the interface during periods of SO^2-₄ release from the sediment appear to be supported by the oxidation of reduced inorganic sulfur. The estimated minimum annual diffusive sulfur flux to the sediment, based on the seasonal SO^2-₄ and H₂S profiles and on Fick's first law is 3–4 times higher than the ²¹⁰Pb-derived present rate of total S accumulation. The pronounced seasonal and horizontal variability in SO^2-₄ profiles, and the discrepancy between calculated and measured S flux to the sediment suggest that porewater studies may not be appropriate to quantitative estimations of SO^2-₄ reduction in this type of lake.