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
     where,

%    = percent saturation
[O2] = observed oxygen concentration, and
[Osat] = saturated concentration of oxygen at the local temperature (and possibly altitude, barometric preessure, and salinity or conductivity).

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
where,
 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.

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 processes.

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.

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Maintained by Sam Mozley, s_mozley@ncsu.edu.

Last modified on Auguist 7, 2002