Spatial Variation of Sediment Sulfate Reduction Rates in a Saline Lake

Devils Lake in North Dakota is a terminal, multibasin, saline lake with an overall surface area that is currently approximately 44,520?ha (110,000?acres). Lake elevation has increased by more than 7?m within 10?years, and vast areas of prairie and cropland have been flooded. The lake is rich in sulfate, and water column sulfate concentrations are relatively uniform within each of the five major basins, but increase from 3.1?mM (300?mg/L) in West Bay to 31?mM (3,000?mg/L) in East Devils Lake. Sediment cores were collected from three of the basins at different water depths, and used in laboratory studies to evaluate the spatial distribution of sulfate-reducing bacteria (SRB) activity in the lake sediments. The high sulfate concentrations within the experimental sediment cores suggest that the activity of SRB is limited by the availability of suitable electron donors rather than by the availability of sulfate and that SRB activity can be defined by a zero-order volumetric rate constant (K0). Experimentally determined K0 values ranged from 11?to?88?mmol SO42? m?3?day?1. The water depths from which sediment cores were collected in Devils Lake are related to the elapsed time since inundation by the rising lake level. It was found that time since inundation influences the observed K0 value. Mean K0 values for cores from an average depth of 4.8?m (submergence time of about 5?years), and 9.4?m (submergence time of about 28?years) were 62 and 17?mmol SO42? m?3?day?1, respectively. The significant difference (two-tailed t-test, p<0.05) suggests that SRB activities in the Devils Lake sediments change with submergence times. A uniform sulfate reduction rate applied to all Devils Lake sediments is therefore only a crude approximation of reality.