Sulfur cycling in the water column of Little Rock Lake, Wisconsin

The S cycle in the water column of a small, soft-water lake was studied for 9 years as part of an experimental study of the effects of acid rain on lakes. The two basins of the lake were artificially separated, and one basin was experimentally acidified with sulfuric acid while the other served as a reference or control. Spatial and seasonal patterns of sulfate uptake by plankton (53-70 mmol m-2 yr-1), deposition of sulfur to sediments in settling seston (53 mmol m-2 yr-1), and sulfate diffusion (0-39 mmol m-2yr-1) into sediments were examined. Measurements of inputs (12-108 mmol m-2 yr-1) and outputs (5.5-25 mmol m-2 yr-1) allowed construction of a mass balance that was then compared with rates of S accumulation in sediments cores (10-28 mmol m-2 yr-1) and measured fluxes of S into the sediments. Because of the low SO2-4 concentrations (μmoleL-1) in the lake, annual uptake by plankton (53-70 mmol m-2 yr-1) represented a large fraction (\u3e 50%) of the SO2-4 inventory in the lake. Despite this large flux through the plankton, only small seasonal fluctuations in SO2-4 concentrations (μmole L-1) were observed; rapid mineralization of organic matter (half-life \u3c 3 months) prevented sulfate depletion in the water column. The turnover time for sulfate in the water column is only 1.4 yr; much less than the 11-yr turnover time of a conservative ion in this seepage lake. Sulfate diffusion into and reduction in the sediments (0-160 μmole m-2d-1) caused SO2-4 depletion in the hypolimnion. Modeling of seasonal changes in lake-water SO2-4 concentrations indicated that only 30-50% of the diffusive flux of sulfate to the sediments was permanently incorporated in solid phases, and about 15% of sulfur in Settling seston was buried in the sediments. The utility of sulfur mass balances for seepage lakes would be enhanced if uncertainty about the deposition velocity for both sulfate aerosols and SO2, uncertainty in calculation of a lake-wide rate of S accumulation in sediments, and uncertainty in the measured diffusive fluxes could be further constrained