Dissolved Oxygen (DO) is a measure
of the amount of elemental oxygen which is the form required by animals
and aerobic bacteria. The meter
uses an oxidation-reduction reaction to detect the presence and quantity
of oxygen. Units are parts per million (ppm) of O2 or
milligrams of O2 per liter of sample. Dissolved oxygen
values are extremely temperature dependent as solubility increases
with decreasing temperature.
Since the atmosphere is about 21% O2,
atmospheric O2 is available to dissolve into surface waters
to a level dictated
by the temperature controlled solubility. O2 concentrations
less than this are observed when animals and aerobic bacteria are
utilizing
O2 in large amounts during aerobic respiration of organic
matter (and ammonium for nitrifying bacteria). O2 concentrations
greater
than the solubility values are observed when plants are producing
O2 via photosynthesis. The concentration of O2 relative
to its solubility value is expressed as a percent saturation (O2 concentration
observed/O2 solubility concentration x 100). River water values vary from 40%
(4 ppm) in the summer to 80% (8 ppm) in the winter, reflecting the
high concentration of dissolved organic matter which supports a lot
of bacterial respiration year round. Waters with heavy algal growth
(usually related to nutrient pollution) can have percent saturations
as high as 200%! The state’s DO limit is 5 ppm for the Waccamaw
River. Values less than this are observed for most of the summer.
Fish are said to experience stress at DO levels less than 6 ppm.
|