LaMotte Dissolved Oxygen Water Quality Test Kit - Manual (Page 4)

4
DISSOLVED OXYGEN,
PERCENT SATURATION & BOD
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 fi sh. 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 Suffi cient for most species
<3 ppm Stressful to most aquatic species
<2 ppm Fatal to most species
Because of its importance to the fi sh’s survival, aquaculturists, or “fi sh farmers,” and
aquarists use the dissolved oxygen test as a primary indicator of their system’s ability
to support healthy fi sh.
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 + Water Carbohydrate + Oxygen
Dioxide