Overview
Dissolved oxygen (DO) is a relative measure of the amount of oxygen that is dissolved or carried in a given medium. Dissolved oxygen levels frequently range anywhere from 2 to 14 milligrams per liter (mg/L) even though circumstances can push levels higher or lower than this. Supersaturation can sometimes be harmful for organisms and cause decompression sickness. Most aquatic organisms need 3-5 mg/L to live. This is a typical target level for aquatic ecosystems.
Influencing Factors
The amount of dissolved oxygen in water depends on water temperature, salinity, air pressure, and also the rates of photosynthesis, respiration, and decomposition. Here are a few examples of how these factors affect dissolved oxygen:
Colder water can hold more dissolved oxygen then warmer water.
Freshwater can hold more dissolved oxygen than salt water.
Water at lower altitudes can hold more dissolved oxygen than water at higher altitudes.
Respiration and decomposition remove oxygen from water.
Photosynthesis by plants adds dissolved oxygen to the water.
Dissolved oxygen values in water are often well over 100% air saturation levels. This occurs because photosynthesis is a significant source of oxygen in aquatic ecosystems. Photosynthesis contributes pure oxygen into the water and may do so at a faster rate than it diffuses out of the water into the air. In water that is supersaturated, dissolved oxygen will diffuse out of the water until it reaches 100% air saturation.
Importance
Just as we need oxygen to live, aquatic organisms need dissolved oxygen to live. Fish, invertebrates, bacteria, and plants all need dissolved oxygen for respiration. Respiration is how plants and animals gain energy. During respiration, oxygen is used to break down food to produce energy. Respiration uses oxygen, but produces carbon dioxide, which lowers pH since carbon dioxide produces a mild acid in water. Dissolved oxygen is also needed for the decomposition of organic matter.
Decomposition is a type of respiration by bacteria. Bacteria uses oxygen to break down organic matter, such as dead trees and leaves, to gain energy. Decomposition is important because it recycles nutrients. Similarly to respiration, decomposition uses oxygen and produces carbon dioxide which in turn lowers the pH of water since carbon dioxide produces a mild acid.
Measurement
DO concentrations can be reported as milligrams per liter (mg/L), parts per million (ppm), or as percent air saturation. 1 mg/L is equal to 1 ppm. 100% air saturation refers to the amount of dissolved oxygen that would be in water if it were completely saturated with air.
There are two DO sensor types that are the most common and they inlcude: membrane-covered and optical.
A membrane-covered dissolved oxygen probe can be used to continuously monitor dissolved oxygen. Dissolved oxygen diffuses across the membrane and reacts with the probe surface. The amount of oxygen that diffuses across the membrane is measured and converted into a dissolved oxygen reading. Two common membrane-covered probe technologies are polarographic and galvanic.
Optical dissolved oxygen sensing technologies have recently evolved as a low-maintenance alternative to membrane-covered DO probes. With this method, an optical sensor emits a light, which causes the sensing element to glow. In the presence of dissolved oxygen, the luminescence changes and the sensor measures that change, which is directly proportional to the DO concentration.