Dissolved Oxygen Collection Protocols
Background
Oxygen is essential for aquatic life as well as terrestrial animals. The
amount of oxygen dissolved in water affects the number and kind of animals
found there. Healthy streams are saturated with oxygen (90 to 110%
saturation) during most of the year.
There are several causes of reduced dissolved oxygen levels in streams. Problems most often occur during the summer low flow period. Warm water holds less oxygen than cold water and shallow water heats up more quickly than deep water. Slow flowing water has little surface turbulence, so little oxygen mixes into the water. Organic wastes such as sewage and agricultural runoff consume oxygen as they decompose. Fertilizers from gardens and farmlands provide nutrients for algae and initially, the algae grow quickly and add oxygen to the water. However, later they decompose and consume oxygen.
During the summer low flow period, dissolved oxygen concentrations often fluctuate during the day. When water temperature increases during the day, the oxygen level decreases by late afternoon. Algal photosynthesis during blooms can increase the oxygen level during the day. Animal and plant respiration can depress the oxygen level during the night.
Planting stream bank vegetation helps to increase oxygen levels in streams. Foliage provides shade and roots absorb nutrients. Controlling sources of sewage, manure, and fertilizers also helps. Fencing keeps livestock away from streams. Building structures in streams helps to aerate the water.
Procedure:
This test is performed using a Hach dissolved oxygen kit. If possible take
samples with the Hach kit when you take the late afternoon temperature
reading. Otherwise, perform the dissolved oxygen test at the same time, and
in the same location as the temperature measurement, and note the time when
the measurement is made.
Follow the directions supplied with the Hach kit. It is important to
perform the test carefully or the results will not be valid. The results
from the Hach test will give you the dissolved oxygen content in
Milligrams/litre (which is the same as parts per million) Temperature has
an effect on the amount of dissolved oxygen water can hold . Cold water
can hold more oxygen than warm water, so it is important to convert Mg/l to
% saturation by using the chart below.
| Temperature °C | __________ |
| Dissolved Oxygen in Mg/litre | __________ |
| % Saturation of Oxygen | __________ |
Instructions:
Use a ruler to connect the oxygen and temperature readings. Read the percent
saturation value where the ruler crosses the middle line.
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