Dissolved oxygen (DO) refers to oxygen gas molecules (O2) mixed within water bodies. This oxygen enters water through direct absorption from the atmosphere, a process enhanced by turbulence. Aquatic plants also contribute oxygen during photosynthesis. DO is fundamental to water quality and supports diverse aquatic life.
How Temperature Influences Dissolved Oxygen
Water temperature inversely affects dissolved oxygen (DO) levels; as temperature rises, DO concentration falls. Higher temperatures cause water and gas molecules to move with greater kinetic energy. This increased motion weakens the bonds holding oxygen in water, allowing more oxygen to escape into the atmosphere.
Like carbonated soda, which loses fizz faster when warm, oxygen is less soluble at higher temperatures. For instance, water at 5°C can contain approximately 55% more dissolved oxygen than water at 25°C. This principle means natural temperature fluctuations, such as seasonal warming, lead to lower oxygen concentrations.
Human activities can also affect water temperature and, consequently, dissolved oxygen levels. The discharge of heated water from industrial facilities and power plants, known as thermal pollution, raises the temperature of natural water bodies. This temperature increase reduces the water’s capacity to hold oxygen, thereby lowering dissolved oxygen levels. Urban runoff can also contribute to this warming effect.
Why Dissolved Oxygen Matters in Water
Adequate dissolved oxygen levels are necessary for aquatic ecosystems. Fish, invertebrates, and microorganisms depend on DO for respiration, similar to how land animals breathe air. When DO levels become too low, aquatic organisms experience stress, leading to reduced growth and reproductive problems. Mobile species may avoid low-oxygen areas, while less mobile organisms can suffer severe consequences.
Dissolved oxygen levels below 5 milligrams per liter (mg/L) stress aquatic life. Below 3 mg/L, oxygen becomes insufficient to support fish, and at levels below 1 mg/L, aquatic environments are devoid of most life. Such severe reductions can cause widespread die-offs of fish and other aquatic animals.
Extreme instances of very low dissolved oxygen are called “dead zones.” These are areas where aquatic life cannot survive due to insufficient oxygen. Dead zones result from nutrient pollution, which triggers algal blooms. When these dense algal blooms die and decompose, bacteria consume large amounts of available dissolved oxygen, depleting the supply for other marine life. Maintaining healthy DO levels is an indicator of overall water quality and ecosystem vitality.