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4241LNOT12_S11_PP - Environmental Geochemistry GLY...

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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2011 1 LECTURE 12 - HYPOXIA Note: Slide numbers refer to the PowerPoint presentation which accompanies the lecture. Hypoxia, slide 1 here One of the more important chemicals parameters in natural waters is the amount of dissolved molecular oxygen, often abbreviated as DO. Oxygen strongly affects the chemical redox potential, and is vital to many forms of life. Hypoxia, slide 2 here Physical factors affecting the amount of oxygen dissolved in water are temperature, pressure, and, for marine waters, salinity. Dissolved oxygen is often expressed as the amount of oxygen dissolved in water relative to the maximum amount of oxygen that can dissolve in water at the existing temperature, pressure and salinity. This level is the saturation value, so such levels are referred to as percentages of saturation value. Healthy waters generally have a minimum of 75-80% of the saturation limit. Biological factors can also influence the level of dissolved oxygen. Bacteria and other organisms in water use up dissolved oxygen, contributing to Biological Oxygen Demand, or BOD. Also contributing to BOD is sewage or organic discharges into bodies of water, which are then oxidized, consuming oxygen. Nutrient runoff from terrestrial sources contributes to increased number and size of aquatic organisms, which use oxygen. Weather can affect oxygen levels. During bright, sunny weather, photosynthesis will occur at high rates, and water will be well oxygenated. If it is cloudy for prolonged periods, photosynthesis will diminish, but respiration will continue, using up oxygen, and causing low DO levels. Static bodies of water, such as lakes or rivers with low current flow rates, usually have far higher densities of rooted plants than rivers with swift currents. High plant densities result in large DO swings. During daylight, large amounts
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Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2011 2 of oxygen are produced. At night, photosynthesis shuts down, but respiration continues, and DO levels may plummet. Hypoxia, slide 3 here Hypoxia occurs when dissolved oxygen in aquatic systems is reduced to the point where it becomes detrimental to aquatic organisms living in the water. This level will vary with species. Fish generally require a minimum of 30% DO in the water. For other species, hypoxia is often defined as between 1 and 30% DO. The term dysoxic is sometimes used to refer to waters which are between hypoxic and anoxic. Hypoxia, slide 4 here Oxygen levels can also be measured in ppm DO. The graphic shows levels that are necessary for fish. The temperature is critical for aquatic organisms. As temperature goes up, metabolic activity increases, and more oxygen is needed. For example, trout need 5-6 times more oxygen at 24° than at 4°C. (Oram, 2010).
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4241LNOT12_S11_PP - Environmental Geochemistry GLY...

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