The amount of dissolved oxygen in water is influenced by several interacting physical, chemical, and biological factors.
Dissolved oxygen (DO) levels are critical for the health of aquatic ecosystems. They are constantly affected by a balance of processes that add oxygen and those that consume it.
Based on the provided information, dissolved oxygen concentrations are constantly affected by diffusion and aeration, photosynthesis, respiration and decomposition. While water equilibrates toward 100% air saturation, dissolved oxygen levels will also fluctuate with temperature, salinity and pressure changes ³.
Here are the key factors affecting dissolved oxygen in water:
Biological Processes
These processes involve living organisms and significantly impact DO levels.
- Photosynthesis: Aquatic plants and algae release oxygen as a byproduct of photosynthesis, increasing DO levels, especially during daylight hours.
- Respiration: All aerobic organisms in the water (fish, insects, bacteria, plants) consume oxygen through respiration, lowering DO levels.
- Decomposition: When organic matter decomposes, microorganisms like bacteria consume large amounts of oxygen, drastically reducing DO levels, particularly in polluted waters.
Physical Processes
These factors relate to the water's interaction with the atmosphere and its physical properties.
- Diffusion and Aeration: Oxygen from the atmosphere dissolves into the water's surface (diffusion). Processes like wind, waves, and waterfalls (aeration) increase this contact, helping water equilibrate toward 100% air saturation.
- Temperature: Colder water can hold more dissolved oxygen than warmer water. As temperature increases, the solubility of oxygen decreases.
- Salinity: Water with higher salinity (salt content) holds less dissolved oxygen than freshwater.
- Pressure: Atmospheric pressure and hydrostatic pressure (pressure from the water column) can affect the saturation point of dissolved oxygen. Higher pressure generally allows for more dissolved oxygen, although this effect is less pronounced in shallow waters compared to the influence of temperature and salinity.
Summary of Factors Affecting Dissolved Oxygen
Understanding these factors is crucial for managing water quality and supporting aquatic life.
| Factor | Type | Impact on DO Levels |
|---|---|---|
| Diffusion and Aeration | Physical | Increases DO (equilibration with atmosphere) |
| Photosynthesis | Biological | Increases DO (during daylight) |
| Respiration | Biological | Decreases DO |
| Decomposition | Biological | Decreases DO |
| Temperature | Physical | Inverse relationship: Higher temp = Lower DO |
| Salinity | Physical | Inverse relationship: Higher salinity = Lower DO |
| Pressure | Physical | Direct relationship: Higher pressure = Higher DO (saturation point) |
These factors interact continuously, leading to daily and seasonal fluctuations in dissolved oxygen concentrations in natural water bodies. Monitoring these levels is essential for assessing the health of aquatic ecosystems.
