Dissolved oxygen (DO) is the amount of oxygen gas present in water. It is important for supporting aquatic life and maintaining healthy environments. Water acquires oxygen from the atmosphere and through the photosynthetic activity of aquatic plants. Adequate dissolved oxygen is necessary for the balance and functioning of aquatic ecosystems.
Dissolved Oxygen as the Breath of Aquatic Life
Dissolved oxygen is necessary for the survival of most aquatic organisms, including fish, invertebrates, and microorganisms. These creatures depend on DO for respiration, converting nutrients into energy for growth, movement, and reproduction. Fish, for example, extract oxygen from the water using their gills, similar to how terrestrial animals breathe air.
Various aquatic species have adapted to absorb oxygen from their surroundings. Bottom-dwelling organisms like worms and certain crustaceans can thrive with lower DO levels (1 to 6 milligrams per liter). In contrast, fish inhabiting shallower waters, such as salmon and trout, require higher concentrations (4 to 15 milligrams per liter) for optimal health. Even aquatic plants and phytoplankton use dissolved oxygen for respiration when light is unavailable for photosynthesis.
Dissolved Oxygen’s Role in Maintaining Healthy Ecosystems
Beyond supporting individual organisms, dissolved oxygen plays an important role in the health and balance of aquatic ecosystems. It is necessary for the aerobic decomposition of organic matter, a process carried out by beneficial bacteria. These bacteria use oxygen to break down dead plants, animals, and waste, preventing pollutant accumulation and recycling vital nutrients back into the ecosystem.
Sufficient DO levels also prevent harmful anaerobic conditions, which occur when oxygen is absent or severely limited. In these oxygen-depleted environments, different bacteria thrive, producing toxic gases like hydrogen sulfide that can alter water chemistry and create unpleasant odors. Dissolved oxygen concentration serves as an important indicator of water quality and an aquatic environment’s vitality, reflecting its capacity to sustain diverse life and process organic material.
Impacts of Depleted Dissolved Oxygen
When dissolved oxygen levels decline significantly (hypoxia) or become entirely absent (anoxia), the consequences for aquatic life are serious. Low DO concentrations stress aquatic organisms, hindering their growth, impairing reproduction, and leading to mortality. This can result in widespread fish kills, where many fish perish due to oxygen deprivation.
Such depletion of dissolved oxygen degrades aquatic habitats, making them unsuitable for many species and reducing biodiversity. The absence of oxygen also allows anaerobic bacteria to proliferate, producing foul-smelling byproducts that compromise water quality and aesthetic appeal. These adverse impacts underscore the importance of maintaining adequate dissolved oxygen levels for the survival and well-being of aquatic ecosystems.