Dissolved oxygen (DO) refers to the amount of oxygen gas that is physically dissolved in water. Similar to oxygen in the air we breathe, it exists in water, though in much smaller concentrations. Whether high levels of dissolved oxygen are beneficial or detrimental is complex, as the answer depends on the specific environment and the water’s purpose.
What Dissolved Oxygen Is
Dissolved oxygen is a measure of the free, non-compound oxygen molecules available in water. This oxygen is crucial for the survival of most aquatic life, including fish, invertebrates, and microorganisms, as they require it for respiration. Oxygen enters water primarily through diffusion from the surrounding atmosphere and as a byproduct of photosynthesis by aquatic plants and algae. Turbulence, such as waves or flowing rivers, enhances this absorption.
DO concentration is typically measured in milligrams per liter (mg/L) or parts per million (ppm). It is also expressed as a percentage of saturation, which indicates how much oxygen is present compared to the maximum amount the water can hold at a given temperature and pressure. Water’s ability to hold dissolved oxygen decreases as temperature rises, and it can hold more oxygen at lower salinities and higher pressures.
Where High Dissolved Oxygen Is Beneficial
Elevated dissolved oxygen levels are generally favorable in many aquatic environments. In healthy aquatic ecosystems, sufficient DO concentrations support diverse communities of fish and aquatic invertebrates. Many fish species thrive and grow well when DO levels are between 5 and 12 mg/L, with sensitive species like salmonids often needing above 6 mg/L. These levels support robust metabolic processes and successful reproduction.
High dissolved oxygen also plays a significant role in wastewater treatment processes. Microorganisms, particularly aerobic bacteria, rely on oxygen to break down organic waste effectively. Maintaining adequate DO, typically around 2 mg/L in secondary treatment, ensures these bacteria efficiently decompose pollutants, improving water quality before discharge. In aquaculture, proper DO levels (often 5-20 ppm) are important for fish health, growth, and productivity.
When High Dissolved Oxygen Becomes Detrimental
While generally beneficial, excessively high levels of dissolved oxygen, a condition known as supersaturation, can be harmful. When water becomes supersaturated, meaning it holds more oxygen than it normally could at equilibrium, it can lead to health problems for aquatic organisms. For fish, this can cause gas bubble disease (GBD), which is similar to “the bends” in divers. This occurs when dissolved gases, including oxygen, come out of solution within the fish’s bloodstream and tissues, forming bubbles that obstruct blood flow and damage organs.
GBD symptoms in fish include external bubbles on fins, skin, and around the eyes, erratic swimming, and difficulty breathing, potentially leading to death if DO levels remain consistently above 115% to 120% air saturation. Supersaturation can naturally occur during intense photosynthesis by aquatic plants or from significant turbulence, such as below hydroelectric dams. High dissolved oxygen can also accelerate corrosion in industrial systems, particularly metal pipes, leading to infrastructure damage and increased maintenance costs.
The Importance of Context
The determination of whether a high dissolved oxygen level is favorable or not depends entirely on the specific application and environment. There is no single universal level that is considered ideal across all scenarios. For instance, the optimal DO range for a mountain stream supporting trout will differ from that required in a wastewater treatment plant.
The needs of the organisms present are also a primary consideration; some species tolerate lower oxygen levels, while others require near-saturation to thrive. Therefore, understanding the specific context, including the type of aquatic life, the purpose of the water body, and other environmental factors like temperature and salinity, is essential for evaluating dissolved oxygen levels. This nuanced perspective allows for appropriate management strategies to ensure healthy and functional aquatic systems.