Dissolved Oxygen in Streams
Why Is It Important?
Like terrestrial animals, fish
and other aquatic organisms
need oxygen to live. As water
moves past their gills (or other
breathing apparatus),
microscopic bubbles of oxygen
gas in the water, called
dissolved oxygen (DO), are
transferred from the water to
their blood. Like any other gas
diffusion process, the transfer
is efficient only above certain
concentrations. So, a certain
minimum amount of oxygen
must be present in water for
aquatic life to survive. In other
words, oxygen can be present
in the water, but at too low a
concentration to sustain
aquatic life. In addition to
being required by aquatic
organisms for respiration, oxygen also is used for decomposition of organic matter
and other biological and chemical processes.
Reasons for Natural Variation
Oxygen is produced during photosynthesis and consumed during respiration and
decomposition. Because it requires light, photosynthesis occurs only during daylight
hours. Respiration and decomposition, on the other hand, occur 24 hours a day. This
difference alone can account for large daily variations in DO concentrations. During
the night, when photosynthesis cannot counterbalance the loss of oxygen through
respiration and decomposition, DO concentrations steadily decline. They are lowest
just before dawn, when photosynthesis resumes.
Dissolved oxygen concentrations increase wherever the water flow becomes
turbulent, such as in a riffle area, waterfall, or a dam. Oxygen concentrations are
much higher in air, which is about 21 percent oxygen, than in water, which is a tiny
fraction of 1 percent oxygen. Where the air and water meet, this tremendous
difference in concentration causes oxygen molecules in the air to dissolve into the
water until saturation is reached. More oxygen dissolves into water when turbulence
caused by rocky bottoms or steep gradients brings more water into contact with the
surface. A similar process happens when you add sugar to a cup of coffee. The sugar
dissolves, but it will dissolve more quickly if you stir the coffee.
Another physical process that impacts DO concentrations has to do with the
temperature of the water and gas saturation. Cold water can hold more gas – that is
DO – than warm water. So, during the summer months when stream water is
warmer, oxygen can be limited by the ability of the water to "soak up" more oxygen
gas. A table comparing oxygen saturation at different water temperatures can be
found in the lake temperature section.
There are other reasons for seasonal variation. During late summer, streamflows can
get very low in the Puget Sound area. Many of the tributaries that provide
oxygenated water to the main stream dry up, and as water moves slowly over what
may previously have been riffles or rapids, there is less opportunity for aeration and
oxygenation. Warmer summer temperatures also cause increased biological activity
(growth, productivity, respiration, and decomposition), and therefore greater daily
variability in DO.
Expected Impact of Pollution
Pollution tends to cause a decrease in stream oxygen concentrations. This change
can be caused by addition of effluent or runoff water with a low concentration of DO
or chemical or biological constituents that have a high oxygen demand – that is they
require large amounts of oxygen before they can be thoroughly decomposed. The
latter is often the more typical and more serious case.
The demand for oxygen doesn’t occur directly where the effluent or runoff water is
discharged but instead somewhere downstream where decomposition finally occurs.
This can make it difficult to show a direct relationship between addition of an oxygen
demanding pollution source and decrease in oxygen concentrations. There is a way
to determine the amount of oxygen required for decomposition of a pollutant source
by measuring whet is called the biochemical oxygen demand (BOD). Since this
measurement is not typically included in citizen monitoring efforts it is not covered in
this text. Standard Methods for the Examination of Water and Wastewater is a
commonly used reference manual for water quality studies and provides information
on BOD analysis. The full reference is included in the reference section.
Stormwater runoff also delivers oxygen-demanding substances to streams. When a
watershed becomes developed, greater quantities of pollutants are released and the
total volume of runoff increases. Most conventional pollutants (sediments, nutrients,
organic matter) require oxygen for decomposition or for chemical reactions.
Consequently, DO concentrations often decrease in streams located in a developed
or developing watershed.