Dissolved oxygen (DO) refers to the amount of oxygen gas (O2) that is physically dissolved in water. It is the same oxygen molecule found in the air we breathe, but in water, it exists in a dissolved state. The concentration of dissolved oxygen is typically measured in milligrams per liter (mg/L) or parts per million (ppm).
Natural Processes of Oxygen Addition
Oxygen enters water bodies through several natural mechanisms, primarily atmospheric diffusion and photosynthesis. Atmospheric diffusion involves oxygen from the air directly dissolving into the water at the surface. This process is significantly enhanced by turbulence, such as that created by wind, waves, waterfalls, or rapids. Fast-moving water, like in streams or rivers, generally contains more dissolved oxygen than still water.
Aquatic plants, algae, and phytoplankton also play a substantial role in adding oxygen to water through photosynthesis. During this process, these organisms use sunlight, carbon dioxide, and water to produce their own food, releasing dissolved oxygen as a byproduct. Oxygen production from photosynthesis is dependent on sunlight, meaning that dissolved oxygen levels tend to be higher during the day and can decrease at night when photosynthesis ceases and organisms continue to respire.
Engineered Methods for Oxygenation
Human-designed systems are frequently used to intentionally add oxygen to water, especially in controlled environments like aquaculture, wastewater treatment plants, and aquariums. Aeration systems are common engineered methods that physically introduce air into water to promote oxygen transfer. These systems work by increasing the contact area between air and water, allowing more oxygen to dissolve.
Surface aerators agitate the water’s surface, creating splashes and turbulence that facilitate gas exchange. Examples include splash aerators or paddlewheel aerators, which are effective in shallower water bodies. They physically break water apart into small droplets, which then absorb oxygen from the air as they fall back into the water. This method can rapidly add oxygen to the upper layers of water.
Subsurface aerators release air bubbles from below the water surface. These systems typically use a shore-mounted compressor that pumps air through weighted lines to diffusers placed at the bottom of the water body. As the bubbles rise, they transfer oxygen to the water and also create a mixing action that circulates water from the bottom to the surface, where additional gas exchange occurs. Fine bubble diffusers, which produce many small bubbles, are particularly efficient at oxygen transfer due to their increased surface area. Pure oxygen injection systems are another method, dissolving concentrated oxygen gas directly into water, often used when high dissolved oxygen levels are required or space is limited.
Importance of Dissolved Oxygen
Dissolved oxygen is an indicator of water quality and is important for aquatic ecosystems. All aerobic aquatic organisms, including fish, invertebrates, and beneficial microorganisms, require dissolved oxygen for respiration. For instance, fish and zooplankton need oxygen to breathe, similar to how terrestrial animals do.
When dissolved oxygen levels drop too low, a condition known as hypoxia (below 5.0 mg/L), it stresses aquatic life and can lead to impaired growth, altered behavior, and even death, potentially causing fish kills. Low oxygen can also disrupt aquatic food chains and reduce biodiversity. Adequate dissolved oxygen supports the natural breakdown of organic matter by microorganisms, preventing the accumulation of decaying material and generating foul odors. Maintaining sufficient dissolved oxygen levels helps ensure a balanced and thriving aquatic environment.