Hydrologic, Climatological, and Biogeochemical Controls on Thermal Structure and Anoxia in Four Hypereutrophic Drinking Water Reservoirs

Includes bibliographical references (pages 27-37).Decadal (1990-2011) trends in reservoir thermal stability, anoxia, and Secchi disk depth were documented for four urban water supply reservoirs with differing morphology and land use in their watersheds. The initial hypothesis was that changes in anoxia, after correcting for interannual variability in stability, would be greatest for reservoirs with large increases in urban and residential land uses in their watersheds. Anoxia increased significantly at two reservoirs, after controlling for stability, suggesting that allochthonous and autochthonous nutrient loading were key controls on development and extent of anoxia. Reservoir depth, not air temperature, was the dominant control on stability. Contrary to expectation, water quality deteriorated at the reservoir with the most pristine watershed. The onset of deterioration coincided with the introduction of a forage fish (threadfin shad), which may suppress density of zooplankton and enhance translocation of phosphorus from benthic\ud sediment to the photic zone. Water quality also decreased at the reservoir with the most highly urban and agricultural watershed. Reservoir water quality is sensitive to nutrient addition and can deteriorate over time, even in pristine catchments