78
Multi-Parameter
Water Quality
TROLL
®
0095110 rev. 007 01/09
TROLL
9500
Operator’s Manual
13 DISSOLVED OXYGEN
$/½#ATMPPM#HLORINE
14.6 mg/L
$/½#ATMPPM#HLORINE
9.09 mg/L
$/½#ATMPPM#HLORINE
6.77 mg/L
$/½#ATMPPM#HLORINE
7.35 mg/L
Safe level for most aquatic life
MG,
* representative of solute concentration
Typical D.O. values
WHAT IS DISSOLVED OXYGEN?
The amount of dissolved oxygen (D.O.) in both natural water and
wastewater is a function of several parameters. D.O. is highly
dependent on temperature and atmospheric pressure. An increase
in temperature causes a decrease in the amount of oxygen that can
dissolve in water. On the other hand, higher atmospheric pressures
result in higher D.O. values. Salinity is also a factor. Oxygen solubility
is greater in freshwater than in saltwater. There are also chemical and
biochemical processes that affect D.O.
Most of the dissolved oxygen in water comes from the atmosphere,
but oxygen from the photosynthesis of aquatic plants is also a key
source. D.O. levels in lakes and other surface water will actually follow
a cyclic or diurnal pattern over the course of a day, rising and falling
as light intensity changes from dawn to dusk.
WHY MEASURE DISSOLVED OXYGEN?
Most aquatic life requires an average D.O. value greater than 5.0 mil-
ligrams dissolved oxygen per liter of water (mg/L) in order to survive.
!LTHOUGHTHEAMOUNTOFDISSOLVEDOXYGENINABODYOFWATERmUCTUATES
due to natural processes, large deviations from the norm are usually
a result of human activity. Changes in D.O. levels are usually the
result of a buildup in organic waste. Organic waste can enter surface
water from sewage treatment facilities, runoff from agricultural feed
lots or domestic areas and from industrial discharge. Organic wastes
often contain nitrates and phosphates. Nitrates and phosphates are
nutrients for aquatic plants and algae, stimulating overproduction
when present in excessive levels. Accelerated growth of blooms
increase the number of photosynthesizing plants, which temporar-
ily increases the amount of dissolved oxygen. However submerged
aquatic vegetation eventually experience a reduction in sunlight
due to increased coverage on the surface. This decrease in sunlight
leads to a reduction in photosynthesis and eventual death. Bacterial
processes take over and consume even more dissolved oxygen. Fish
and other aquatic species die due to lack of dissolved oxygen. This
tragic process is known as eutrophication.