Dissolved oxygen (DO) refers to the amount of oxygen gas (O2) physically dissolved in water, distinct from the oxygen atoms within water molecules (H2O). It is measured in milligrams per liter (mg/L) or as a percentage of saturation, indicating the proportion of oxygen present relative to the maximum amount the water can hold. Adequate dissolved oxygen is important for the health of aquatic ecosystems. Most aquatic organisms, including fish, invertebrates, and microorganisms, depend on DO for respiration, a process that converts oxygen into energy for survival and reproduction.
Natural Water Conditions
The capacity of water to hold dissolved oxygen is influenced by various physical and chemical properties. Water temperature plays a significant role; colder water can dissolve and hold more oxygen than warmer water. As water temperature increases, oxygen solubility decreases because the increased molecular vibrations reduce the space available for oxygen molecules within the water.
Salinity affects oxygen solubility. Freshwater absorbs more oxygen than saltwater, so as salinity increases, dissolved oxygen capacity decreases. This is partly due to competition from salt ions for intermolecular spaces, which diminishes oxygen’s affinity for water.
Atmospheric pressure also influences dissolved oxygen levels, with lower pressures resulting in less oxygen dissolving into the water. Physical mixing and stratification within a water body also affect DO. Wind, waves, and currents help aerate the water, increasing oxygen absorption from the atmosphere. In contrast, deep or stagnant waters with limited mixing can develop layers where oxygen becomes depleted at lower depths.
Life’s Oxygen Demand
Biological processes consume dissolved oxygen. Respiration by aquatic organisms, including fish, invertebrates, and microorganisms, continuously uses oxygen to metabolize food and release energy. This process occurs day and night, contributing to the demand for oxygen.
Decomposition of organic matter is another consumer of dissolved oxygen. When dead plants, animals, and organic detritus settle, bacteria and fungi break down this material. These decomposers are aerobic, requiring oxygen for their metabolic processes. They consume dissolved oxygen from the surrounding water.
When oxygen consumption by these biological activities exceeds oxygen production, dissolved oxygen levels decline. The combined oxygen consumed by all biological processes is referred to as Biochemical Oxygen Demand (BOD). While aquatic plants produce oxygen through photosynthesis during daylight, they also consume oxygen through respiration at night, contributing to daily fluctuations in DO levels.
Human Activities
Human actions can significantly accelerate the decrease in dissolved oxygen levels. Nutrient pollution, primarily from agricultural runoff and wastewater discharge, introduces excessive nitrogen and phosphorus into water bodies. These nutrients act as fertilizers, leading to rapid growth of algae and aquatic plants, a process known as eutrophication.
When these algal blooms die, their decomposition by bacteria consumes significant amounts of oxygen, quickly depleting available DO. Direct discharge of organic waste, such as untreated sewage or industrial effluent from sources like pulp and paper mills, also contributes to oxygen depletion. These wastes provide a large food source for microorganisms, which multiply and consume oxygen as they break down the organic material.
Thermal pollution, often from power plants or industrial cooling systems that discharge heated water, directly raises the temperature of receiving water bodies. Warmer water inherently holds less dissolved oxygen, exacerbating the problem even without additional organic loading. This temperature increase also accelerates the metabolic rates of aquatic organisms and decomposers, further increasing oxygen consumption.
Consequences for Aquatic Life
Decreased dissolved oxygen levels have serious consequences for aquatic ecosystems. When oxygen concentrations fall below optimal thresholds, below 3 mg/L, the condition is termed hypoxia. A complete absence of oxygen is known as anoxia. Both conditions pose major threats to aquatic life.
Fish and invertebrates become stressed, exhibiting behaviors like moving to surface waters where oxygen levels might be slightly higher. This can lead to reduced growth rates, impaired reproductive success, and increased vulnerability to predators. Prolonged exposure to hypoxic and anoxic conditions can result in mass die-offs of fish, crustaceans, and other aquatic organisms, greatly impacting biodiversity and altering ecosystem structure. The formation of “dead zones” is a direct result of severe and prolonged oxygen depletion, rendering these areas uninhabitable for most aquatic life.