| The physical processes underlying successful aeration to prevent winterkill are not well understood. Several descriptive and quantitative measurements were performed in aerated lakes concerning lake stratification, water movement, energy use, polynya size, aeration efficiency, methane oxidation and oxygen depletion in aerated and unaerated lakes. The following hypotheses were tested: (1) Successful aeration induces large-scale (whole-lake) mixing as part of the aeration process; and (2) Surface aerators induce greater oxygen transfer than air diffusers due to creation of a greater surface area for air-water contact. During the ice covered period vertical and horizontal gradients of oxygen and temperature were compared between three lakes treated with point-release air injection, three with mechanical surface aerators and three lakes which served as unaerated controls.; All three unaerated lakes assumed typical inverse thermal stratification and dissolved oxygen (DO) fell to near zero mg L-1 throughout the entire water column by early January. Both types of aeration caused thermal destratification with the zone of influence of either technique reaching the most distant shorelines (up to 1100 m). Discrete near-field oxygenated cells were not identified using either technique. However, some decrease in oxygen was observed at distances greater than 350 m in surface aerated lakes and greater than 800 m in air injected lakes. Velocity measurements in the polynya and dye studies confirmed that air injection induced greater velocity and more rapid lake circulation than surface aeration. In addition, these measurements revealed a new conceptual model of near-field and whole-lake circulation patterns. The depth of oxygen and temperature uniformity was clearly set by the depth of the diffuser or by water depth below the surface aerator. Morphometric features such as small depressions or distinct basins separated from the aerated basin by a shallow sill stratified and tended toward anoxia as winter progressed.; Oxygen depletion rates varied substantially in relation to freeze up timing and periods of snow accumulation. Aeration startup enhanced oxygen depletion rates by up to 4-fold. Although methane accumulation varied among lakes it may profoundly affect the oxygen budget and aerator performance. Measurement of oxygen transfer and depletion rates allowed the calculation of empirical mass balance equations to describe the aeration process, the dynamic nature of oxygen depletion and the impediment that methane imposes on aeration success. (Abstract shortened by UMI.)... |
