Dissolved oxygen (DO) is the amount of oxygen gas dissolved in water, present in a free, non-compound form. Oxygen enters water through direct absorption from the atmosphere, a process enhanced by turbulence from wind or flowing water. Aquatic plants also contribute oxygen to water bodies during photosynthesis. The presence of dissolved oxygen is important for healthy aquatic environments.
Understanding Dissolved Oxygen and Its Importance
Dissolved oxygen is important for the survival of nearly all aquatic organisms, including fish, invertebrates, and microorganisms. These organisms require oxygen for respiration, a metabolic process that breaks down nutrients to produce energy for growth, reproduction, and overall survival.
The level of dissolved oxygen serves as a primary indicator of water quality and the overall health of an aquatic ecosystem. Water bodies with consistently high levels of dissolved oxygen are considered healthy, supporting diverse aquatic life. Conversely, insufficient dissolved oxygen stresses aquatic organisms, leading to impaired growth, death, disrupted food chains, and reduced biodiversity.
Ideal Dissolved Oxygen Levels for Water Bodies
General ranges for healthy dissolved oxygen levels in water fall between 5 and 8 milligrams per liter (mg/L). Values below 5 mg/L stress aquatic life; below 3 mg/L, most aquatic animals cannot tolerate them, and many fish species show reduced production.
The specific “ideal” dissolved oxygen levels can vary significantly depending on the type of water body and the aquatic species present. For instance, cold-water fish like trout and salmon require higher dissolved oxygen concentrations, often needing levels above 6.5 mg/L, and even higher (e.g., 9.5 mg/L) for early life stages in cold-water ecosystems. In contrast, some warm-water fish, like carp, can tolerate lower oxygen conditions. Organisms like bottom feeders, crabs, oysters, and worms can survive with minimal amounts of oxygen, ranging from 1 to 6 mg/L.
Fast-flowing water bodies, like swift streams, have higher dissolved oxygen levels due to increased aeration from turbulence. Slow-moving or stagnant waters, such as ponds or deep lake bottoms, have lower oxygen concentrations due to limited atmospheric exchange and decomposition processes. For instance, the saturation level of oxygen in pure water at 25°C and 1 atmosphere is around 8.11 mg/L.
Key Factors Affecting Dissolved Oxygen
Several environmental and biological factors influence the concentration of dissolved oxygen in water. Temperature is a key factor; oxygen solubility decreases as water temperature rises, meaning warmer water holds less dissolved oxygen. This inverse relationship can lead to declines during warmer seasons or with heated discharges.
Salinity also affects oxygen solubility, as water with higher salt content holds less dissolved oxygen. Seawater, for example, contains about 20% less oxygen than freshwater at the same temperature and altitude. Atmospheric pressure and altitude similarly impact dissolved oxygen; lower pressure, such as at higher elevations, reduces the amount of oxygen that can dissolve in water.
Aquatic plants and algae influence dissolved oxygen through photosynthesis, producing oxygen during daylight hours. However, they also consume oxygen through respiration at night, leading to daily fluctuations. Respiration by aquatic organisms and bacterial decomposition of organic matter also consume dissolved oxygen, with large amounts of organic material (e.g., dead plants, sewage) significantly depleting levels. Water movement, such as waves, currents, and waterfalls, increases aeration and enhances oxygen dissolution from the atmosphere.
Consequences of Too Little or Too Much Dissolved Oxygen
Water bodies with insufficient dissolved oxygen can experience negative impacts. Low oxygen conditions, known as hypoxia (less than 2-3 mg/L), or the complete absence of oxygen, called anoxia (less than 0.2 mg/L, or defined as less than 1 mg/L), can lead to widespread stress and mortality among aquatic organisms.
Fish and other mobile aquatic animals may try to escape these areas, while less mobile organisms like shellfish cannot, resulting in fish kills and a loss of biodiversity. These low oxygen zones are called “dead zones” as they cannot sustain most aquatic life. Hypoxia can also disrupt aquatic food chains and impair the water’s natural ability to break down pollutants.
While less common, excessively high dissolved oxygen levels, or supersaturation (above 100% saturation or over 110-115% total gas pressure), can also be detrimental. This condition occurs when water contains more dissolved gas than it can normally hold, due to rapid warming of cold, gas-rich water, intense photosynthesis, or high turbulence. In fish, supersaturation can cause gas bubble disease, similar to “the bends” in divers. Gas bubbles form in the fish’s blood vessels and tissues, impeding blood flow and oxygen delivery to organs, leading to tissue damage, blindness, loss of equilibrium, and death.