Dissolved oxygen (DO) is the concentration of oxygen gas dissolved in water. This oxygen is not part of the water molecule (H2O) itself, but rather O2 gas dissolved within it. Sufficient dissolved oxygen is fundamental for the survival and reproduction of most aquatic life. Healthy water bodies rely on adequate DO levels to sustain diverse ecosystems and support their biological processes.
Oxygen Transfer from the Atmosphere
Oxygen primarily enters water through direct absorption from the atmosphere. This process, called gas exchange, involves oxygen molecules from the air dissolving into the water at the surface. Driven by differences in partial pressure, oxygen moves from higher concentration in the atmosphere to lower concentration in the water until equilibrium.
The surface area of a water body significantly influences atmospheric transfer. Natural phenomena like wind, waves, and currents enhance this process by continuously mixing the water surface, increasing air-water contact and preventing saturation. Running water, such as in swift-moving streams, tends to dissolve more oxygen than still water due to this turbulence. Artificial aeration, using pumps or air stones, also enhances this process by introducing air bubbles and increasing surface agitation.
Oxygen Production by Aquatic Life
Beyond atmospheric transfer, aquatic plants, algae, and microscopic organisms called phytoplankton contribute to dissolved oxygen levels through photosynthesis. This biological process uses sunlight, carbon dioxide, and water to produce energy, releasing oxygen as a byproduct. Phytoplankton, tiny photosynthetic organisms, are responsible for a substantial portion of oxygen produced in oceans and freshwater ecosystems.
This source of oxygen is particularly notable in well-lit aquatic environments where biological activity is high. During daylight hours, when sunlight is available, aquatic plants and algae actively photosynthesize, releasing oxygen directly into the water. However, the amount of oxygen produced depends on light availability; photosynthesis slows or stops in darkness or during cloudy weather.
Factors Affecting Oxygen Levels
Several environmental factors influence how much oxygen can dissolve and remain in water. Temperature has an inverse relationship with dissolved oxygen; colder water can hold more oxygen than warmer water. As water temperature increases, the movement of gas and water molecules speeds up, allowing oxygen to escape into the atmosphere. Consequently, DO concentrations are often higher in winter and lower in summer.
Salinity, or the salt content of water, also affects oxygen solubility. Freshwater generally holds more dissolved oxygen than saltwater. This occurs because salt ions in the water attract water molecules, leaving fewer free water molecules available for oxygen to bind with, which can drive oxygen out of the solution.
Atmospheric pressure also plays a role; higher atmospheric pressure allows water bodies to retain more dissolved oxygen. Water at lower altitudes, where atmospheric pressure is typically higher, can hold more dissolved oxygen than water at higher altitudes. Conversely, biological consumption processes like respiration by aquatic organisms and decomposition of organic matter by microbes remove oxygen. These processes, particularly decomposition, can significantly reduce net oxygen levels, especially in waters with high organic content.