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What factors affect the amount of dissolved oxygen?

Temperature.  The lower the temperature, the higher the amount of dissolved oxygen in a body of water;  the opposite is also true, in other words, the higher the water temperature the lower the amount of dissolved oxygen.  The amount of DO is highly dependent on the temperature.

Altitude.  As altitude increases, the atmospheric (barometric) pressure decreases.  Thus, the amount of oxygen diffused into the water decreases. Look at the graph below. It charts the dissolved oxygen capacity based on altitude and temperature.  Altitude is represented in meters; the scale is from -500 meters (i.e. below sea level) to 2000 meters.  Temperature is represented in degrees Celsius ; the scale is from 1 to 30 degrees Celsius.
Please take a moment to review the chart below. Do you appreciate that as altitude increase the dissolved oxygen capacity decreases and as temperature increases the dissolved oxygen capacity decreases.

3-D Graph of the relationship between dissolved oxygen, temperature and altitude

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