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Answer: Dissolved oxygen (DO) is a measure of how much oxygen is dissolved in the water, the amount of oxygen available to living aquatic organisms. A small amount of oxygen, up to about ten molecules of oxygen per million of water, is actually dissolved in water.

Where it comes from

Oxygen enters a stream mainly from the atmosphere and, in areas where groundwater discharge into streams is a large portion of streamflow, from groundwater discharge. Water bodies also receive oxygen from aquatic plants. Running water, such as that of a swift moving stream, dissolves more oxygen than the still water of a pond or lake.

What lowers it

• Bacteria in water can consume oxygen as organic matter decays.
• Low oxygen levels (hypoxia) or no oxygen (anoxia) can occur when excess organic materials, such as large algal blooms, are decomposed by microorganisms.
• The concentration of dissolved oxygen is inversely related to water temperature, so levels are at a seasonal low in summer.
• Low oxygen levels often occur in the bottom of the water column and affect organisms that live in the sediments.

Thresholds for fish

While each organism has its own DO tolerance range, generally, DO levels less than 5 mg/L are considered stressful for fish and levels less than 3 mg/L are too low to support fish. DO levels below 1 mg/L are considered hypoxic and usually devoid of life.

How it is measured

For EPA's National Aquatic Resource Surveys, DO is measured with a calibrated water quality probe meter, usually in conjunction with measurements for temperature and pH. DO is a direct indicator of an aquatic resource's ability to support aquatic life.

Claims

• id: C1 claim: DO measures how much oxygen is dissolved in the water. source: S1 evidence: "Dissolved oxygen (DO) is a measure of how much oxygen is dissolved in the water"
• id: C2 claim: Bacteria in water can consume oxygen as organic matter decays. source: S1 evidence: "Bacteria in water can consume oxygen as organic matter decays."
• id: C3 claim: All aquatic animals need DO to breathe. source: S2 evidence: "All aquatic animals need DO to breathe."

Sources

• [S1] Dissolved Oxygen and Water. USGS. https://www.usgs.gov/special-topics/water-science-school/science/dissolved-oxygen-and-water . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Indicators: Dissolved Oxygen. EPA. https://www.epa.gov/national-aquatic-resource-surveys/indicators-dissolved-oxygen . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_a4jdptey,
  author = {{iinventedlight}},
  title = {{Dissolved oxygen: the oxygen aquatic life breathes}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/dissolved-oxygen/},
  note = {Permanent ID iil:note:a4jdptey. License: CC-BY-4.0.},
}

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