Some aquatic plants and animals have special ways of getting oxygen directly from the air, but most remove oxygen dissolved in the water. Because oxygen dissolved in water is far less abundant than oxygen mixed in the air, the actual amount of oxygen present is an important water quality parameter. As a general rule, the more oxygen dissolved in water the better the water quality. A consistently high oxygen content allows a body of water to support more numbers and variety of aquatic organisms. Survival under low oxygen conditions is a specialization of a limited number of species.
There is a limit to how much oxygen water can hold before it is saturated. This amount, called the oxygen solubility or saturation value, is not fixed, but depends upon oxygen pressure in the air, water temperature, and dissolved salts present. Solubility is greater for fresh water than salt water, and greater for cold water than warm water. Under some circumstances, oxygen concentration can exceed the saturation value, and the water is then said to be supersaturated. How oxygen solubility relates to field measurements is discussed below.
Water Quality Standard
Hawaii (US) Fresh water stream -- DO shall not decrease below 80% of saturation. Estuary -- DO shall not decrease below 75% of saturation. Embayment -- DO shall not decrease below 75% of saturation. Open coastal water -- DO shall not decrease below 75% of saturation. Republic of Palau Class 1 & 2 -- Dissolved oxygen concentrations shall not vary by more than 25% from natural conditions. Class 1 -- Except for concentrations attributable to natural causes, DO concentration shall not be less than 6.0 mg/l or 75% of saturation. Class 2 -- Except for concentrations attributable to natural causes, DO concentration shall not be less than 5.0 mg/l.
A number of methods are available for measuring dissolved oxygen in water (APHA, 1992) and these are discussed in WATERSHEDSS. Convenient for most purposes is the Membrane Electrode Method that uses an electronic device to quantify DO present in water in which it is immersed. A meter is connected to a membrane electrode probe, and the probe inserted into the water. Readings may be given in either mg/l, ml/l, or percent saturation. Some instruments give the oxygen tension (mm Hg). A milligram per liter (mg/l) reading is the weight of oxygen in a liter of water. A milliliter per liter (ml/l) reading is the volume of oxygen (at STP) in a liter of water. Percent saturation is the amount of oxygen present divided by the amount representing saturation, multiplied by 100. Thus, when the amount of DO measured is equal to the saturation value, saturation is 100%. Readings should not be made in percent saturation because this is a relative measure. Instead, temperature and salinity should be recorded and percent saturation determined later by calculation.
Membrane Electrode Method: This method is ideal for field testing.
Measuring DO by membrane electrode probe (oxygen meter) is a field method requiring instrument calibration by the field technician. Some meters automatically adjust for temperature, salinity, and air pressure, but many do not and these values must be entered into the instrument, or readings later corrected by reference to tables of oxygen solubility. Calibration is accomplished by setting the meter reading to 100% saturation with the wetted probe held in air, and any appropriate settings or corrections made. Calibration procedures must be followed each time the meter is turned on to insure accurate measurements. Calibration procedures should be repeated each time the meter is moved any considerable distance (such as between distant stations).
Although we can correct for local differences in air pressure, this is not generally a concern when making field measurements, except where elevation becomes a consideration. The drop in pressure from sea level to 600 meters (about 2000 ft) is 4% for each 300 meters (1000 ft) and this will reduce the solubility by about 5% at typical Hawai'i temperatures for each 300 m rise. Above 600 m to about 1,500 m (up to 5000 ft) the change in air pressure is roughly 3% per 300 m rise; then 2.5% per 300 m betwen 1,500 and 3,000 m or 10,000 ft (Boyd, 1979).
Correction for temperature is very important and accurate water temperature readings are necessary when making DO measurements.
Correction for salt content is necessary in marine and brackish waters encountered along and off the coast. Solubility decreases about 5% for each 9 g/l of salt (= 9 ppt)(Boyd, 1979).
Record all of the following information when making a DO measurement in the field:
Date / Time / Your Initials STATION No. or LOCATION DATA METER TYPE / PROBE TYPE or NUMBER CALIBRATION: TIME/Your Initials MEASUREMENT: O2 READING in mg/l. TEMPERATURE: in degrees Celsius SALINITY: (ignore in fresh waters)