Dissolved oxygen (DO) is the amount of oxygen gas present in water. It is dissolved in water, similar to how sugar dissolves, rather than being part of the water molecule (H₂O). Understanding DO levels is important for evaluating water quality.
Understanding Dissolved Oxygen
Oxygen enters water bodies through natural processes. Direct absorption from the atmosphere, enhanced by water movement and turbulence, contributes to DO levels. Aquatic plants also release oxygen into the water as a byproduct of photosynthesis during daylight hours. DO is typically measured in milligrams per liter (mg/L) or parts per million (ppm), indicating oxygen concentration per volume. It can also be expressed as a percentage saturation, showing how much oxygen is present compared to the maximum the water can hold at a given temperature and pressure.
Why Dissolved Oxygen is Crucial for Aquatic Ecosystems
Dissolved oxygen supports the survival of most aquatic organisms. Fish, invertebrates, and many microorganisms rely on DO for respiration, similar to how land animals breathe. Adequate oxygen levels are necessary for their metabolism, growth, and reproduction. DO also aids in the decomposition of organic matter by aerobic microorganisms, recycling nutrients. Therefore, DO levels indicate the health of an aquatic environment.
Ideal Dissolved Oxygen Levels
Ideal dissolved oxygen for aquatic life falls within a specific range. For most healthy aquatic environments, DO concentrations between 6.5 and 8 mg/L are suitable, correlating to about 80-120% saturation. Many fish species thrive in waters with DO levels from 5 to 12 mg/L. Below 5 mg/L, aquatic life, especially fish, experiences stress.
Concentrations below 2 mg/L are hypoxic (insufficient oxygen), and below 1 mg/L are anoxic, often leading to areas devoid of aquatic organisms. Persistent low oxygen can result in impaired growth, reduced reproductive success, and large-scale fish kills. Excessively high DO levels (supersaturation) can also be problematic, potentially causing gas bubble disease.
Factors Influencing Dissolved Oxygen Levels
Several environmental and biological factors influence DO concentration. Water temperature has an inverse relationship with DO solubility; colder water holds more DO than warmer water. Salinity also reduces oxygen solubility, meaning freshwater typically contains more DO than saltwater at the same temperature. Seawater, for instance, holds approximately 20% less oxygen than freshwater under identical temperature and atmospheric pressure. Atmospheric pressure also affects DO levels; higher pressures allow water to retain more oxygen, so waters at higher altitudes generally have lower DO concentrations.
Aquatic plants influence daily DO fluctuations, producing oxygen during photosynthesis in daylight but consuming it through respiration at night. Decomposition of organic matter by microorganisms consumes DO, so excessive organic material can rapidly deplete oxygen levels. Water movement, such as waves, currents, and waterfalls, increases water aeration, allowing more oxygen to dissolve from the atmosphere.