Determination of sediment oxygen demand by direct measurement and by inference from reduced species accumulation

Rates of sediment oxygen demand (SOD) were determined for hypereutrophic Onondaga Lake, New York (USA), by direct and indirect means. The direct approach, laboratory measurements made on intact sediment cores, yielded an average SOD rate of 1.68�0.56 g O2 m-2 day-1. This was essentially equivalent to that determined indirectly by estimating the sediment flux of reduced chemical species (1 .64�0.14 g O2 m-2 day-1). Sulfide, methane, ammonia, and ferrous iron contributed 50, 42, 7, and <1% of the total sediment flux of reduced species, respectively. The sulfide percentage is unusually high for freshwater systems, reflecting the sulfate-rich nature of the lake. Variability in the sediment flux of reduced species over the 3-year study was modest (12%), and rates were consistent with those measured in other systems of similar trophic state. Rates of SOD closely approximated the areal hypolimnetic oxygen deficit of the lake. More than 70% of the SOD was explained by the oxidation of reduced chemical species. These findings suggest that recovery of the oxygen resources of the lake will be mediated primarily through reductions in the delivery of particulate organic matter to the sediments and the rate at which in-place reserves of particulate organic matter are stabilized. The processes of algal respiration and aerobic mineralization of organic matter in the water column, which exhibit a more rapid response to remediation measures, will play a lesser role.