Reservoirs are vital components of California’s water infrastructure that allow a population of 40 million to thrive in its drought-prone climate. These engineered impoundments provide additional benefits beyond storing water for irrigation and potable supply. By containing runoff during the rainy season, reservoirs provide incidental flood protection downstream. Reservoirs can benefit the environment by storing water that maintains consistent outflow to creeks that would otherwise go dry. Reservoirs also serve recreational needs of communities by providing local opportunities for fishing, boating, picnicking, and hiking. Many of those who fish in reservoirs rely on their catch to provide themselves and their families with an inexpensive, nutritious meal. However, California’s legacy of mining has contaminated fish throughout the state, putting anglers and their families at risk for mercury poisoning.About half of reservoirs in California exceed regulatory standards for safe mercury levels in fish set by the State Water Resources Control Board. Some coastal California reservoirs are contaminated by waste material from legacy mercury mining operations. Others have received quicksilver lost to the environment during the California Gold Rush. Even seemingly pristine lakes can host fish with high mercury concentrations that result from deposition of mercury released into the atmosphere from fossil fuel combustion around the world.Though mercury is typically present in trace concentrations in reservoir sediments, reservoirs create conditions that are conducive to the production and bioaccumulation of neurotoxic methylmercury. When inflow decreases and air temperatures rise in the springtime, reservoirs undergo thermal stratification. The warm, buoyant epilimnion floats atop the cold, dense hypolimnion, blocking off bottom waters from oxygen input from the atmosphere and photosynthesis. Aerobic bacteria consume the remaining dissolved oxygen in the hypolimnion, creating anoxic conditions in bottom waters and at the sediment-water interface. Under anoxic conditions, some anaerobic bacteria can convert inorganic mercury present in the water and sediments into methylmercury. Methylmercury is a potent neurotoxin that can cause neurological damage, cardiovascular disease, and reproductive impairment in humans and wildlife. Methylmercury is released into the water column where it concentrates in phytoplankton by millions of times its concentration in water. Methylmercury is concentrated further up the food chain in zooplankton, planktivorous fish, and predatory fish, reaching dangerously high (>0.5 ppm) concentrations in fish that are consumed by humans and wildlife.The complexity of mercury cycling and bioaccumulation in reservoir systems presents a wide array of potential management strategies. Reservoir managers could decrease methylmercury levels in fish by limiting the microbial conversion of inorganic mercury to methylmercury, by lowering its introduction into the food web, or by controlling trophic transfer between organisms. This dissertation investigated these three different aspects of reservoir mercury management in a series of field and laboratory studies. The overall objective of this dissertation was to investigate methods for controlling methylmercury production and bioaccumulation in reservoirs contaminated by the former New Almaden Mining District, North America’s largest historical mercury mine.Chapter 1 is a manipulated field experiment to evaluate the effects of hypolimnetic oxygenation on mercury cycling and fish tissue mercury in contaminated reservoirs. Using a 15-year dataset consisting of water quality and fish tissue measurements collected prior to and during reservoir oxygenation, I conducted statistical analyses to identify key changes in water quality and fish tissue data. Results indicated declining trends in fish mercury in two of four reservoirs, likely the result of increased primary productivity and not due to the inhibition of methylmercury production. Chapter two consists of a set of sediment slurry incubations aimed at evaluating the use of manganese oxide and activated carbon sediment amendments for mercury control through redox buffering and sorption. Manganese oxide and activated carbon amendments were equally effective in decreasing methylmercury concentrations in sediment porewater. Manganese oxide amendments were rapidly reduced to Mn2+, warranting further optimization to increase longevity. Chapter three is an observational study aimed at characterizing plankton dynamics and methylmercury bioaccumulation in the pelagic food web. An increased understanding of bioaccumulation in lower-trophic level organisms is necessary when considering potential ecological mercury management options (e.g., fish stocking). Overall, results demonstrated the seasonal patterns of MeHg introduction into the pelagic food web and how complex and counteracting factors may control the magnitude of MeHg bioaccumulation. Collectively, the three chapters of this dissertation provide key information that will inform future mercury management efforts in the study reservoirs.