Diurnal Variation of
Dissolved Oxygen in Natural Streams
RICHARD D. HOAK, Senior Fellow
and
HENRY C. BRAMER, Fellow
Mellon Institute
Pittsburgh, Pennsylvania
Among the many things that affect the health of natural streams, none is
more important than dissolved oxygen. An ample supply of this dissolved gas
provides a suitable environment tor the aerobic microorganisms that consume
dissolved and suspended organic matter inoffensively. In addition, the steady
growth of aquatic recreation is demanding that in most areas, oxygen levels
be maintained high enough to satisfy the needs of a plentiful fish population.
Streams become anaerobic and unsightly where the demand upon their oxygen
resources is consistently greater than the supply. Fish cannot survive under
such conditions, and the recreational value of streams is impaired.
Bacteria are the primary agents in the dissimilation of organic matter,
and they are highly responsive to changes in the supply of available food.
These organisms begin to multiply immediately when there is a sudden increase of organic material in a stream. Bacterial growth rates are auto-
catalytic, i.e., growth starts slowly, increases rapidly to a maximum rate,
then decreases slowly as a population ceiling is approached. Very little food
is consumed when a population ceiling has been reached, even though there
may be a plentiful supply. There is evidence that this results from actual
overcrowding, and it is known that bacteria tend to die off slowly when their
population reaches a saturation level. Food is consumed most rapidly, and
oxygen demand is highest, when the rate of bacterial multiplication is
greatest.
There is a mechanism in nature which effectively prevents bacteria from
reaching a population ceiling and consequently keeps food consumption near
its maximum rate. There are many organisms in streams, notably the protozoa, for which bacteria are a natural food. Thus, just as bacteria multiply
rapidly in response to an increase in raw organic food, so the organisms that
feed on bacteria quickly increase in number as their special food becomes
more plentiful.
Oxygen in streams is absorbed from the atmosphere and from the photo-
synthetic activity of green plants. The equilibrium solubility of oxygen is
only 9.2 ppm at 20 c. In addition, oxygen diffuses into water very slowly,
and solution of oxygen tends to cease when a surface film becomes saturated.
Oxygen dissolved from the atmosphere is normally dispersed in stream water
as a result of turbulence, whereby successively saturated films become mixed
with the water body. Deep, sluggish rivers are thus unable to benefit as much
from atmospheric oxygen as shallow, fast streams.
Photosynthesis, on the other hand, generates oxygen without respect to
stream turbulence. This activity requires only a suitable abundance of
chlorophyll-containing organisms and water of sufficient clarity to permit
penetration of sunlight. It is not uncommon for waters containing prolific
growths of algae to contain concentrations of oxygen far above the saturation
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