Dissolved oxygen (DO) is the concentration of oxygen gas (O₂) dissolved in water, distinct from the oxygen within the water molecule (H₂O). It is a fundamental component of healthy aquatic ecosystems, playing an important role in sustaining the life within them.
Importance for Aquatic Life
Dissolved oxygen is essential for the survival of nearly all aquatic organisms, including fish, invertebrates, and microorganisms. These organisms rely on DO for respiration, a process where oxygen is used to break down food and produce energy. Fish, for instance, extract oxygen from the water using their gills, which are efficient at absorbing oxygen.
Sufficient DO levels are also necessary for the decomposition of organic matter by microorganisms. This decomposition helps recycle nutrients within the ecosystem, contributing to overall water quality. When DO levels drop too low, a condition known as hypoxia, aquatic organisms can become stressed, their growth can be reduced, and their immune systems may weaken, making them more susceptible to disease. Severe oxygen depletion can lead to widespread fish kills and negatively impact the entire aquatic community.
Understanding Optimal and Excessive Levels
While oxygen in water is beneficial, there are optimal ranges for dissolved oxygen levels. For many aquatic species, particularly those supporting coldwater fish, DO concentrations should ideally remain above 6 to 8 milligrams per liter (mg/L). Warm water fish can tolerate slightly lower levels, with objectives not falling below 5 to 6 mg/L. These concentrations ensure aquatic life can perform necessary metabolic processes without stress.
Levels of dissolved oxygen can exceed 100% saturation, a condition known as supersaturation. This can occur when water becomes turbulent, absorbing more oxygen from the atmosphere, or during periods of intense photosynthesis by aquatic plants and algae, which produce oxygen as a byproduct. While high DO is good, excessively high levels can be detrimental. Supersaturation can lead to gas bubble disease in fish, where excess dissolved gases form bubbles in their blood and tissues, similar to “the bends” in divers.
Factors Affecting Dissolved Oxygen
Several factors influence the concentration of dissolved oxygen in water. Water temperature is a primary factor; colder water can hold more dissolved oxygen than warmer water. As water temperature increases, the solubility of oxygen decreases.
Atmospheric pressure also plays a role, with higher pressures allowing water to retain more dissolved oxygen. Salinity affects DO levels, as freshwater generally holds more oxygen than saltwater at the same temperature.
Biological processes impact DO. Photosynthesis by aquatic plants and algae increases oxygen levels during daylight hours, while respiration by all aquatic organisms, including plants at night, consumes oxygen. Water movement and turbulence, such as waves and rapids, enhance the absorption of oxygen from the atmosphere into the water.