CADDIS Volume 2: Sources, Stressors & Responses
Dissolved Oxygen
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Authors: S.K.M. Marcy, G.W. Suter II, S.M. Cormier
Courtesy of Eric Vance, U.S. EPA.
Dissolved oxygen (DO) refers to the concentration of oxygen gas incorporated in water. Oxygen enters water by direct absorption from the atmosphere, which is enhanced by turbulence (Figure 1). Water also absorbs oxygen released by aquatic plants during photosynthesis. Sufficient DO is essential to growth and reproduction of aerobic aquatic life (e.g., see Murphy 2006, Giller and Malmqvist 1998, Allan 1995).
Advice for deciding whether to include depleted or (less commonly) excessive DO as a candidate cause is provided in this module.
Checklist of sources, site evidence and biological effects
This module addresses low or excessive DO as a proximate stressor, that should be listed as a candidate cause when potential human sources and activities, site observations, or observed effects support portions of the source-to-impairment pathways in the conceptual diagram for DO (Figure 2). A checklist is provided below to help you identify key data and information useful for determining whether to include DO among your candidate causes; for more information on specific sources and activities, site evidence, and biological effects listed in the checklist, click on checklist headings or go to the When to List tab of this module.
Consider listing DO as a candidate cause when the following sources and activities, site evidence, and biological effects are present:
Sources and Activities
- Impoundments
- Municipal waste treatment outfalls
- Industrial point sources
- Agricultural and urban runoff
- Removal of riparian vegetation
- Channel alteration
- Groundwater inflow
Site Evidence
- High plant abundance
- Slow-moving water
- Reduced water volume
- Weather conditions, season, time of day
- High elevation
- Presence of organic waste
- Turbid water
- Foul smelling water
- Yellowish-green, brown, gray, or black water or dark sediments
- Embedded substrate
Biological Effects
- Kills of aquatic life
- Large fish die before small fish
- Species requiring greater concentrations of DO die first
- Characteristic body movements
- Fish gulping air
- High Hilsenhoff Biotic Index (HBI) score
- Replacement of DO sensitive species with fly larvae and worms
Consider contributing, modifying, and related factors as candidate causes when DO is selected as a candidate cause:
- Temperature: High temperatures reduce the solubility of oxygen in water (i.e., warm water holds less DO than cold water).
- Nutrients: High nutrients can lead to excessive plant growth, resulting in DO declines due to respiration and decomposition.
- Sediments: Embedded sediments can prevent DO from permeating interstitial areas.
- Ammonia: Oxygen is consumed as NH3 is oxidized (nitrification), and low oxygen levels increase NH3 levels by inhibiting nitrification.
Consider not listing (eliminating) low DO as a candidate cause when you have evidence from your site about turbulence and DO:
- High turbulence at the site creates consistent aeration (implausible mechanism).
- DO concentrations measured continuously over time at the site confirm that DO concentrations are the same as or greater than DO at sites where biological impairment is not observed (lack of co-occurrence).