LaMotte Dissolved Oxygen Water Quality - Manual (Page 12)

DISSOLVED OXYGEN FACT SHEET
Oxygen is critical to the survival of aquatic plants and animals, and a shortage
of dissolved oxygen is not only a sign of pollution, it is harmful to fish. Some
aquatic species are more sensitive to oxygen depletion than others, but some
general guidelines to consider when analyzing test results are:
5–6 ppm Sufficient for most species
<3 ppm Stressful to most aquatic species
<2 ppm Fatal to most species
Because of its importance to the fish’s survival, aquaculturists, or “fish farmers,”
and aquarists use the dissolved oxygen test as a primary indicator of their
system’s ability to support healthy fish.
WHERE DOES THE OXYGEN COME FROM?
The oxygen found in water comes from many sources, but the largest source is
oxygen absorbed from the atmosphere. Wave action and splashing allows more
oxygen to be absorbed into the water. A second major source of oxygen is
aquatic plants, including algae; during photosynthesis plants remove carbon
dioxide from the water and replace it with oxygen.
Absorption
Oxygen is continuously moving between the water and surrounding air. The
direction and speed of this movement is dependent upon the amount of
contact between the air and water. A tumbling mountain stream or windswept,
wave-covered lake, where more of the water’s surface is exposed to the air, will
absorb more oxygen from the atmosphere than a calm, smooth body of water.
This is the idea behind aerators: by creating bubbles and waves the surface area
is increased and more oxygen can enter the water.
Photosynthesis
In the leaves of plants, one of the most important chemical processes on Earth
is constantly occurring: photosynthesis. During daylight, plants constantly take
carbon dioxide from the air, and in the presence of water convert it to oxygen
and carbohydrates, which are used to produce additional plant material. Since
photosynthesis requires light, plants do not photosynthesize at night, so no
oxygen is produced. Chemically, the photosynthesis reaction can be written as:
Light + nCO
2
+ nH
2
O
⎯ → ⎯
(C
2
HO)n + nO
2
Light + Carbon
Dioxide
+ Water
⎯ → ⎯
Carbohydrate + Oxygen
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