Extensive contamination of aquatic ecosystems with mercury (Hg) has led to a growing interest in developing in situ management strategies to repress Hg bioaccumulation in aquatic biota in reservoirs. This study used experimental chamber incubations to assess the impact of three potential treatments, oxygen addition, nitrate addition and aluminum addition, to reduce the flux of toxic methylmercury (MeHg) from profundal reservoir sediment. The study sites, Almaden Lake and Guadalupe Reservoir, are located downstream of the historic New Almaden mining district in Santa Clara Valley, California, USA. In the first experiment (experiment 1), replicate chambers from both sites were incubated sequentially under aerobic and anaerobic conditions. At both sites, mean anaerobic fluxes of MeHg were higher than aerobic fluxes (Almaden: 11.0 vs. 2.3 ng/m2-d; Guadalupe: 22.3 vs 5.5 ng/m2-d), and anaerobic MeHg fluxes correlated with rates of sediment sulfate uptake, highlighting the linkage between MeHg production and microbial sulfate reduction. Under aerobic conditions, sediment from Guadalupe Reservoir released Hg(II), iron and sulfate, suggesting the oxidative dissolution of Hg-bearing sulfide minerals. A follow-up study at Almaden Lake (experiment 2) found that mean MeHg fluxes under aerobic conditions (5 ng/m2-d) and anoxic (nitrate-rich) conditions (1.7 ng/m2 $d) were lower than anaerobic conditions (174 ng/m2-d), but aluminum addition had little effect (105 ng/m2-d) on MeHg flux. In both anaerobic and aluminum treated chambers, MeHg flux turned negative during the second half of the incubation, suggesting that highly reduced, sulfidic conditions lowered net methylation, possibly by enhancing demethylation or repressing Hg(II) bioavailability for methylation.