WHAT IS DISSOLVED OXYGEN?
The oxygen that makes aquatic life possible does not form bubbles, nor is it the oxygen that is part of the H2O water molecule. It is a separate O2 molecule that is dissolved in the water and invisible to our eyes.
WHY CARE ABOUT DISSOLVED OXYGEN?
All terrestrial and aquatic animals need oxygen to survive. Many aquatic macroinvertebrate species depend on oxygen-rich water. Without sufficient oxygen they may disappear. Even a small change in dissolved oxygen concentration can affect the composition of aquatic communities. Many fish require a certain dissolved oxygen range in order to survive
- Minimum dissolved oxygen concentration for warm-water fish: 5.5 mg/L
- Minimum dissolved oxygen concentration for cold-water fish: 6.5 mg/L
NATURAL FACTORS INFLUENCING DISSOLVED OXYGEN
- Aquatic life- Animals living in water use up dissolved oxygen. Bacteria take up oxygen as they decompose materials. Dissolved oxygen levels drop in a water body that contains a lot of dead, decomposing material.
- Elevation- Since streams get much of their oxygen from the atmosphere, streams at higher elevations will generally have less oxygen.
- Salinity (saltiness)- Salty water holds less oxygen than fresh water.
- Temperature- Cold water holds more dissolved oxygen than warm water.
- Turbulence- More turbulence creates more opportunities for oxygen to enter streams.
- Aquatic Vegetation- Aquatic vegetation and algae directly release oxygen into the water during photosynthesis (during the day). At night, plants actually use oxygen for their metabolism.
- Riparian Vegetation (plants along the stream)- Riparian vegetation shade the stream, decreasing water temperatures, and as temperature decreases dissolved oxygen increases.
HUMAN FACTORS INFLUENCING DISSOLVED OXYGEN
- Clearing Land (e.g., construction, logging) – This may send excess organic matter into streams. Organic matter is decomposed by microorganisms, which use up oxygen in the process.Therefore, if there is a lot of organic waste in the stream, the microorganims use more oxygen than can be replaced in the stream.
- Point source pollution – Sewage effluent and factory discharge into waterways can be decomposed by bacteria as above, leading to areas of low DO downstream.
- Nutrient pollution – Nutrient pollution can cause excess plant and algae growth. As above, the bacteria that eat dead plant material consume oxygen from the water.
- Destruction of riparian areas (e.g., development or overgrazing) decreases the amount of shade and increases the water temperature. Warmer water holds less DO (Dissolved Oxygen) than colder water.
- Learn ways to limit human influences: Protect Your Water
HOW DO WE MEASURE DISSOLVED OXYGEN?
Utah Water Watch- Learn how volunteers across the state measure Dissolved Oxygen. Select the Tier 1 tab for basic/classroom-friendly procedures, or select the Tier 2 tab for more advanced procedures.
Stream Side Science- Explore different lesson plans involving Dissolved Oxygen and see how they apply to the different core curriculums for grades k-12.