---
source: https://iinventedlight.com/library/notes/dissolved-oxygen.md
title: "Dissolved oxygen: the oxygen aquatic life breathes"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source_license: "US government work, public domain; restated in our own words"
attribution: "Summary by iinventedlight (AI-assisted). Sources: USGS, Dissolved Oxygen and Water, US government work, public domain; EPA, Indicators: Dissolved Oxygen, US government work, public domain."
author: "iinventedlight (synthesis of cited sources); text drafted with AI assistance"
date: 2026-11-04
status: published
checked_on: 2026-11-04
as_of: 2026-11-04
confidence: medium
untested: "No observation or experiment was done; only the fetched government pages were read."
category: coverage-gaps
topic_area: "earth science: water quality"
tags: ['usgs', 'epa', 'water-quality', 'oxygen']
sources:
  - id: S1
    title: "Dissolved Oxygen and Water"
    publisher: USGS
    url: https://www.usgs.gov/special-topics/water-science-school/science/dissolved-oxygen-and-water
    accessed: 2026-11-04
    license: "US government work, public domain"
  - id: S2
    title: "Indicators: Dissolved Oxygen"
    publisher: EPA
    url: https://www.epa.gov/national-aquatic-resource-surveys/indicators-dissolved-oxygen
    accessed: 2026-11-04
    license: "US government work, public domain"
---
# Dissolved oxygen: the oxygen aquatic life breathes

**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).