Environmental Geochemistry, GLY 4241/5243, © David Warburton, 2011
Note: Slide numbers refer to the PowerPoint presentation which accompanies
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