Calculating Oxygen Percent Saturation
and Comments on Controls of Oxygen Saturation
ZO 419/519 Laboratory Exercises
What is Oxygen Percent Saturation?
Oxygen percent saturation compares an observed oxygen concentration to
the absolute solubility of oxygen at a particular water temperature.
This index often takes into account barometric pressure and salinity effects
at the measurement site, but usually ignores effects of water pressure
at depths below a lake or stream surface. Solubility of gases increases
by an amount equal to the surface saturation concentration for about every
10 m increase in depth.
% = ([O2]/[Osat]) x 100
% = percent saturation
[O2] = observed oxygen concentration, and
[Osat] = saturated concentration of oxygen at the local
temperature (and possibly altitude, barometric preessure, and salinity
On-Line Table of Oxygen Saturation Values
The U. S. Geological Survay has privided an adjustable
"DO" table on the World Wide Web with which you can look up the
saturated concentration of oxygen at various water temperatures, barometric
pressures, and conductivities.
Calculating Oxygen Saturation Values
Alternatively, I have fitted a polynomial to oxygen saturation data for
standard pressure and 0 salinity that enables calculation of the 100% saturation
values in a spreadsheet. NOTE: This procedure assumes 1 atm pressure, and
air saturated with water vapor.
The Formula is
100% Sat. O2 Conc. = 14.59 - 0.3955 T + 0.0072 T2
- 0.0000619 T3
T = water temperature in C.
Correction for Altitude
An increase in altitude decreases the 100% oxygen saturation concentration
by about the following percentages:
This is actually an air pressure effect, so the most precise values are
based on measurement of local barometric pressure when measurements were
taken. For Yates Millpond's altitude of about 75 m, the correction
is about - 1% of the 100% saturation concentration.
Altitude (m) Reduction
0 - 600
1.3% per 100m
600 - 1500
1.0% per 100m
1500 - 3050
0.8% per 100m
Deriving the Formula
The formula was obtained by fitting a third order polynomial to data for
100% saturation concentrations (in mg L-1) over a temperature
range from 0 to 30 C. The improvement in fit was substantial between
second and third order polynomials. Errors of estimate were less
than 0.5%, even at temperatures above 30 C. The R2
value for this fit was 0.99998.
Interpreting Oxygen Saturation Data
In most cases, oxygen saturation levels indicate how much biological processes
have affected the water recently. Community respiration, mainly by
bacteria, reduces oxygen concentrations. The warmer the water and
the greater the supply of decomposable organic matter and other bacterial
substrates in the water, the faster oxygen concentrations are reduced.
Phytoplankton photosynthesis, plus some contribution from photosynthesis
of submersed plants and benthic algae around the shallow edges of a lake
or pond, can increase oxygen concentrations above saturated levels, but
only during the day when photosynthesis is occurring.
Small amounts of ground water that are low in oxygen due to decay processes
in the soil seep into Yates Millpond from underwater springs, reducing
the average oxygen concentrations in pond layers that have the same density
as the spring water. Spring water loacally tends to be about 11 oe
12 C, so under most stratified conditions in this pond, the spring water
should sink to the deepest parts of the pond. In addition, conductivities
are higher in spring water than in surface runoff, and this factor also
causes the spring water to sink to the deepest areas. While the seepage
contributes insignificantly to pond flow, it will accumulate in the pond
while warmer, less salty surface runoff flows right through, over the dam.
Under loner-lasting, stratified conditions, anaerobic, nutrient-rich, low-oxygen
water may fill the hypolimnion from the bottom upwards.
Water just at the surface of lakes should always be at or close to 100%
saturation because exchange of oxygen with the air by diffusion and wave
turbulence usually occurs at a faster rate than photosynthesis or respiration.
The last time water at any given depth was part of the mixed layer, oxygen
concentrations were saturated, that is, brought into equilibrium with the
air. Once a layer of water is isolated by thrmal stratification,
it's oxygen concentration begins to change as a net result of biological
In enriched or contaminated lakes, changes due to biological activity
may exceed physical exchange rates even at the surface, and saturation
levels may be much higher or lower than 100% at any depth, even near the
surface. Supersaturation is frequently observed near the surface
of Yates Millpond on hot, still days.
Maintained by Sam Mozley, email@example.com.
Last modified on Auguist 7, 2002