Dissolved oxygen (DO) refers to oxygen gas that has mixed into water. Unlike the oxygen bound within water molecules (H₂O), DO is in a free, molecular form (O₂) that aquatic organisms can use. It is a fundamental requirement for the survival and health of aquatic life, including fish, plants, and microorganisms, as they depend on it for respiration. Maintaining sufficient DO levels is essential for healthy aquatic ecosystems.
Atmospheric Absorption
Oxygen from the atmosphere naturally dissolves into water bodies through a process called gas exchange. This process occurs at the water’s surface, where oxygen molecules from the air transfer into the water. The amount of oxygen that can dissolve is influenced by the partial pressure of oxygen in the air, with higher atmospheric pressure leading to greater solubility.
Water movement enhances the rate at which atmospheric oxygen is absorbed. Waves, currents, rapids, and waterfalls create turbulence and increase the surface area where water comes into contact with the air. This mixing allows oxygen-rich water from the surface to penetrate deeper, replenishing areas with lower oxygen concentrations. Fast-moving water contains more dissolved oxygen than stagnant water due to this increased aeration.
However, if water becomes excessively turbulent, it can lead to supersaturation, where oxygen levels exceed 100% saturation. Extreme supersaturation can stress aquatic organisms by causing gas bubble disease, similar to “the bends” in humans.
Water temperature also plays a role in how much oxygen water can hold. Colder water has a greater capacity to dissolve oxygen than warmer water. As water temperature increases, the solubility of oxygen decreases because water molecules move faster, allowing dissolved gas molecules to escape more easily. For example, pure fresh water at 0°C holds about 14.6 mg/L of oxygen, while at 20°C, it holds approximately 9.1 mg/L. Salinity also reduces oxygen solubility, which is why saltwater holds less dissolved oxygen than freshwater.
Photosynthesis by Aquatic Organisms
Aquatic plants, algae, and microscopic phytoplankton contribute to dissolved oxygen levels in water bodies through photosynthesis. This biological process uses sunlight, carbon dioxide, and water to create energy, releasing oxygen as a byproduct. These organisms contain chlorophyll, which captures light energy.
Submerged plants release oxygen directly into the water. Microscopic algae and phytoplankton are major contributors. Marine phytoplankton alone are estimated to produce over 50% of the Earth’s oxygen.
Oxygen production through photosynthesis depends on light availability. Photosynthesis occurs during daylight hours, releasing oxygen. In the absence of light, at night or in deeper waters, photosynthesis ceases.
During darkness, aquatic plants and other organisms respire, consuming dissolved oxygen, leading to daily fluctuations in oxygen levels. Water depth also affects oxygen production, as light diminishes with depth, limiting activity to shallower zones. Cloudy weather or turbid water can also reduce light, impacting oxygen generation.