Dissolved Oxygen Sensors Flashcards
What are the approx. levels of DO for healthy water?
Healthy water should generally have dissolved oxygen concentrations above 6.5-8 mg/L and between about 80-120 %.
Why is the DO important to know?
Dissolved oxygen (DO) is one of the most important indicators of water quality. It is essential for the survival of fish and other aquatic organisms.
A dissolved oxygen level that is too high or too low can harm aquatic life and affect water quality.
What can a high DO level indicate?
Supersaturated water can cause gas bubble disease in fish and invertebrates. Significant death rates occur when dissolved oxygen remains above 115%-120% air saturation for a period of time.
What can a low Dissolved Oxygen level indicate?
Oxygen levels also may be reduced when there are too many bacteria or algae in water (see Biochemical Oxygen Demand). After the algae complete their life cycle and die, they are consumed by bacteria. During this decay process the bacteria also consume the oxygen dissolved in the water. This can lead to decreased levels of biologically available oxygen, in some cases leading to fish kills and death to other aquatic organisms.
How does O2 get into the water?
Oxygen dissolves in surface water due to the aerating action of winds, and by precipitation events. Oxygen is also introduced into the water as a byproduct of aquatic plant photosynthesis.
What does temperature tell you about the potential DO levels?
The colder water is, the more oxygen it can hold. As the water becomes warmer, less oxygen can be dissolved in the water.
What does the salinity (determined by the conductivity) tell you about the potential DO levels?
Fresh water can absorb more oxygen than salt water.
What measurement values are used to express DO?
Dissolved oxygen concentration levels may be expressed as milligrams per liter (mg/L) or parts per million (ppm). Dissolved oxygen saturation is expressed as a percentage.
Approximately how much oxygen do bottom feeders, oysters, crabs, and worms require?
Crabs, bottom feeders, oysters and worms require around 1-6mg/L.
Approximately how much oxygen do shallow water fish require?
Shallow water fish require 4-15mg/L.
What is Gas Bubble Disease?
Gas bubble disease (GBD) is the fish equivalent of the “bends” in human scuba divers. It is the result of the water being supersaturated with oxygen, causing gas to come out of solution in the bloodstream, thereby leading to the formation of emboli (obstruction of an artery). Thus, lesions are common in gills, and the choroid gland of the eye.
What does Henry’s Law indicated about DO levels in a body of water and the air above it?
According to Henry’s Law, the dissolved oxygen content of water is proportional to the percent of oxygen (partial pressure) in the air above it. For example, as oxygen in the atmosphere is about 20.3%, the partial pressure of oxygen at sea level (1 atm) is 0.203 atm.
What happens to DO levels during the various four seasons?
Dissolved oxygen levels often stratify (measure differently at different depths due to temp.) in the winter and summer, turning over in the spring and fall as lake temperatures align (remaining relatively consistent through out the body of water).
How do you calculate DO from % Air Saturation?
To calculate dissolved oxygen concentrations from air saturation, it is necessary to know the temperature and salinity of the sample. Salinity and temperature can then be used in Henry’s Law to calculate what the DO concentration would be at 100% air saturation.
However, it is easier to use an oxygen solubility chart. These charts show the dissolved oxygen concentration at 100% air saturation at varying temperatures, and salinities. This value can then be multiplied by the measured percent air saturation to calculate the dissolved oxygen concentration.
O2 mg/L = (Measured % DO)*(DO value from chart at temperature and salinity)
