Oxygen Saturation and Dissolved Oxygen

Dissolved oxygen (DO) is the amount of oxygen (O₂ ) dissolved in the water. Dissolved oxygen is one of the best indicators of water quality. People need oxygen in the atmosphere to survive, and aquatic animals such as fish need dissolved oxygen in the water to survive.

The amount of dissolved oxygen that the water can hold depends on the temperature and salinity of the water. Cold water can hold more dissolved oxygen than warm water and fresh water can hold more dissolved oxygen than salt water. So the warmer and saltier the water, the less dissolved oxygen there can be. The maximum amount of dissolved oxygen that the water can hold is called the saturation value. Dissolved oxygen measurements are given as a percent of saturation (%) or in units of milligrams per liter (mg/l).

Oxygen enters the water at the surface of the water where exchange between the atmosphere and the water can take place. Waves and wind help put oxygen into the water. Dissolved oxygen is also put into the water as a byproduct of phytoplankton photosynthesis. Animals such as fish, breathing in the water, consume dissolved oxygen. Dissolved oxygen is also used in the break down of organic matter. As organic matter sinks to the sea floor it begins to decompose. Bacteria in the water use oxygen to break down this organic material. When there is a lot of organic debris, the dissolved oxygen in the deeper water can be used up.

Above 5 mg/l dissolved O₂, most plants and animals have plenty of oxygen. When the dissolved oxygen is low, below 3 mg/l, the water is called hypoxic. If all the dissolved oxygen is used up, below 0.5 mg/l, the water is called anoxic. Under hypoxic conditions, many plants and animals may not survive. No plants and animals that require oxygen can survive in anoxic conditions.

Table 18 below identifies the available DO at various temperatures, Celsius. As stated earlier, colder waters have more DO than warmer waters e.g., at 5 °C the water has 10.5 mg/l of DO, whereas at 30 °C, there is only 6.4 mg/l.

Table 18: Approximate Dissolved Oxygen Saturation Values (At a salinity of 30ppt) (USEPA)

Dissolved oxygen levels in the main channel of the Illinois River in 1966 are below saturation throughout the entire length of the river and have been since 1911. Levels are below 1.0 mg/l at the Peoria and La Grange pools. As stated earlier, fish and other aquatic life require oxygen levels of 5.0 mg/l to survive. The current conditions are not suitable for wildlife.

It is the low dissolved oxygen concentrations, in conjunction with other stresses such as silt and toxic materials that has contributed to the die-off of fingernail claims and snails in the middle section of the river in the mid-1950s. The loss of this important food source that has resulted in a reduction in the number of diving ducks migrating along the Illinois River and the decline in the condition of the commercially valuable carp (Sparks & Starrett 1975).

The low oxygen levels have a direct impact on fish species as well (Figure 32). Studies show that oxygen levels at 35% saturation reduce the survival of larval largemouth bass by 13.7%, and at 70% saturation and below retard growth of larval bass. At the Lower Bath Chute in the La Grange Pool, a good location for largemouth bass, oxygen levels are at 35% saturation in 1970 and have been 4 or 5 five years since 1963.

Figure 32: Relationship Between Oxygen Saturation, Temperature, and Conditions for Fish (Meck 1996)

Another measure of oxygen is parts per million. Minimum requirements vary by fish species, with trout requiring more dissolved oxygen than goldfish or carp. Studies show that 5 parts per million of dissolved oxygen is the lower limit for maintaining a desirable fishery. Other studies demonstrate that oxygen levels cannot drop below 8 parts per million for more than 8 hours a day, or at no time can they be below 3 parts per million (Mills 1966). Table 19 illustrates the problem that exists throughout the Illinois River from 1911 through 1965. Oxygen levels are reported as low as 0.01 parts per million, with numerous recordings being in the range of 2.0-3.0.

Table 19: Summary of Minimum Dissolved Oxygen Determinations Near Surface (Mills 1966)

Dissolved Oxygen in Parts per Million

Conditions upstream from Peoria are significantly better in 1965 as compared to 1922, despite the increased population in the Chicago area. Mills contributes these improvements to the construction of sewage treatment plants that begins in 1922. It is estimated that in 1922, the combined domestic and industrial pollution emptied into the Illinois River is the equivalent to that of a population of 6,211,471 people. By 1960, the amount of pollution is reduced that of a population of 2,417,000. Other factors that lead to improved oxygen levels are the lock and dam system built in the 1930s which slowed the movement of water downstream, and the adoption of water pollution laws beginning in the 1950s (Mills 1966).

The area downstream of Peoria shows some of the lowest oxygen levels in 1964 and 1965. This is attributed to the increase in population of Peoria-Pekin, and the increase in industrial wastes. Despite the improvements in oxygen levels shown in Table 19, most of the river has less than 5 parts per million of dissolved oxygen, the minimum for a healthy fishery.