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Dissolved Oxygen

What does this mean?

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. Oxygen dissolves in surface water due to the aerating action of winds. Oxygen is also introduced into the water as a byproduct of aquatic plant photosynthesis. When dissolved oxygen becomes too low, fish and other aquatic organisms cannot survive.

The colder water is, the more oxygen it can hold. As the water becomes warmer, less oxygen can be dissolved in the water. Salinity is also an important factor in determining the amount of oxygen a body of water can hold; fresh water can absorb more oxygen than salt water.

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.

Florida's surface water quality standards include minimum values for dissolved oxygen saturation. The standards for freshwater vary in different bioregions of the state:

bioregions map

Dissolved oxygen standards for marine water bodies are expressed as minimums for daily, weekly, and monthly averages.

Detailed water quality standards for dissolved oxygen can be found in Chapter 62-302.533 of the Florida Administrative Code.


How are the data collected? (Methods)

DO can be expressed as a concentration per unit volume, or as a percentage. In aquatic environments, oxygen saturation is a ratio of the concentration of dissolved oxygen (O2), to the maximum amount of oxygen that will dissolve in that water body, at the temperature and pressure which constitute stable equilibrium conditions. Oxygen enters water through several methods, including diffusion from the atmosphere, rapid movement of water (waves, e.g.), or as a byproduct of photosynthesis (generated by seagrass and green algae).

Dissolved oxygen is routinely recorded as part of basic water quality sampling in most surface waters and near-shore coastal systems. There are three common methods for measuring DO. The most practical and consistently accurate method for field measurements employs the polarographic DO sensor. If calibrated correctly, this method provides accurate measurements that can be performed in-situ (i.e., within the water under natural conditions), easily at any depth, and countless times per day. In the laboratory, the Winkler Method is still preferred by some investigators, and is used by many agencies for sensor calibration verification and quality assurance/quality control testing. However, since this method requires a relatively high degree of titration skill, the handling of hazardous reagents (chemicals), and great care with sample collection technique and preservation, it is used in the laboratory only by skilled analysts.


Calculations

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.


Caveats and Limitations

The accuracy of DO measurement is completely dependent upon proper calibration and maintenance (re. DO sensor method), and strict adherence to analytical methodology.

Dissolved oxygen readings of greater than 100% air saturation can occur when photosynthetically-active organisms (plants) produce pure oxygen and/or when there is non-ideal equilibration of dissolved oxygen between the water and the air above it.


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