Dissolved oxygen (DO) refers to the amount of oxygen gas incorporated within water. This oxygen is essential for the survival of most aquatic organisms. Fish, invertebrates, and microorganisms all depend on dissolved oxygen for their respiration, much like land animals breathe air. Adequate dissolved oxygen levels indicate healthy water quality and a healthy aquatic ecosystem.
Temperature’s Influence
Water temperature directly impacts the amount of oxygen that can dissolve within it. As temperature rises, water’s capacity to hold dissolved oxygen decreases; colder water retains more. This occurs because increased thermal energy in warmer water causes molecules to move more rapidly, allowing oxygen to escape.
This physical principle has implications for aquatic life. Many aquatic organisms have specific temperature and dissolved oxygen requirements, and a rise in water temperature can lead to oxygen stress or mortality. Dissolved oxygen concentrations are higher in winter and early spring when water temperatures are low, and they decline in summer and fall as water warms.
Biological Processes
Living organisms within a water body influence dissolved oxygen levels through their metabolic activities. During daylight hours, aquatic plants and algae perform photosynthesis, which produces oxygen. This oxygen is released into the water, increasing dissolved oxygen concentrations.
In contrast, all aquatic organisms, including plants and algae during nighttime hours, consume oxygen through respiration. Respiration removes oxygen from the water. Additionally, the decomposition of organic matter by bacteria and other microorganisms consumes oxygen.
Decomposition of dead plants, animals, or other organic materials by decomposers utilizes dissolved oxygen, leading to oxygen depletion, especially in areas with high organic loads. This interplay between oxygen production and consumption creates a natural daily cycle where dissolved oxygen peaks in the afternoon and reaches its lowest point just before dawn.
Water Movement and Aeration
The physical movement of water plays a role in facilitating the exchange of oxygen between the atmosphere and the water body. Currents, waves, and turbulence increase the surface area contact between the water and the air. This contact allows more oxygen from the atmosphere to dissolve into the water.
Natural aeration occurs in various forms, such as the churning of water over rapids and waterfalls, or the agitation of a lake surface by wind. Fast-moving streams and rivers have higher dissolved oxygen levels due to continuous mixing and re-aeration.
In contrast, still or stagnant water bodies, like deep ponds or reservoirs, experience lower dissolved oxygen concentrations. This is due to reduced atmospheric exchange in these calmer environments.