Dissolved oxygen (DO) is the concentration of free oxygen gas molecules mixed into water. This oxygen is necessary for the survival of most aquatic organisms, including fish, invertebrates, and aerobic microorganisms, which depend on DO for respiration. Low DO concentrations, known as hypoxia, stress aquatic life and indicate poor environmental health. Levels below 5 milligrams per liter (mg/L) are considered stressful for fish, and prolonged low DO can lead to mortality events.
The Inverse Relationship with Water Temperature
The solubility of any gas in a liquid is linked to temperature. Oxygen gas molecules are less soluble in warmer water than in colder water, meaning warmer water cannot hold as much dissolved oxygen. For instance, at standard atmospheric pressure, fresh water at 0 degrees Celsius can hold approximately 14.6 mg/L of DO, but this capacity drops significantly to about 7.0 mg/L at 35 degrees Celsius.
This inverse relationship is due to the increased kinetic energy of water molecules at higher temperatures. As water warms, the molecules move faster, making it easier for dissolved oxygen molecules to escape the liquid phase and diffuse back into the atmosphere. This is a direct physical cause of DO depletion, independent of any biological consumption occurring in the water.
This phenomenon is often intensified by thermal pollution, such as the discharge of warm effluent water from industrial processes like power plants. When heated water is released into a river or lake, it immediately reduces the maximum DO concentration the receiving body of water can sustain. The resulting decrease in oxygen-holding capacity puts stress on local aquatic populations, especially species that are more sensitive to temperature changes.
Biological Oxygen Demand from Decomposition
The largest biological mechanism for decreasing dissolved oxygen is the process of decomposition, measured as Biological Oxygen Demand (BOD). BOD quantifies the amount of oxygen consumed by microorganisms, primarily aerobic bacteria, as they break down organic matter. High levels of organic material lead to a rapid increase in their population and a subsequent surge in oxygen consumption.
High BOD is caused by the process of eutrophication, which begins with excessive nutrient loading, typically nitrogen and phosphorus from agricultural and municipal runoff. These nutrients trigger rapid growth of algae and phytoplankton, known as an algal bloom. While living algae produce oxygen during the day, their life cycle is short, and the bloom eventually dies off.
The resulting large volume of dead algal material sinks to the bottom, becoming a source of organic matter for decomposers. Bacteria then work rapidly to digest this carbon-based material, consuming vast amounts of DO from the surrounding water. This microbial respiration can quickly deplete oxygen concentrations, especially in deeper, less mixed waters, creating hypoxic zones often referred to as “dead zones.”
Sources of organic matter that fuel this demand include untreated sewage, animal waste from feedlots, and decaying plant material. When large pulses of this material enter an aquatic system, the resulting microbial activity can lower DO levels below the threshold required for fish to survive. The rate of oxygen consumption is also affected by temperature, as warmer water increases the metabolic rate of the decomposers.
Impact of Salinity and Chemical Oxidation
The chemical makeup of the water itself influences its capacity to hold oxygen. Increased salinity, or the concentration of dissolved salts, reduces the solubility of oxygen gas. This effect occurs because the charged salt ions, such as sodium and chloride, attract the polar water molecules, a process known as the “salting-out effect.”
When water molecules are clustered around ions, fewer are available to hold the oxygen gas molecules in solution. Consequently, saltwater naturally holds about 20% less dissolved oxygen than freshwater. This factor is especially relevant in coastal estuaries where freshwater and saltwater mix, creating a gradient of DO capacity.
Chemical oxidation reactions represent another non-biological mechanism for DO depletion, involving inorganic substances. Certain compounds introduced into the water react directly with dissolved oxygen, consuming it without microbial action. For example, reduced iron and sulfide compounds, often found in industrial effluent or released from deep, anoxic sediments, are strong reducing agents.
These compounds are chemically oxidized by the dissolved oxygen. This form of depletion is measured as Chemical Oxygen Demand (COD) and can be a localized problem near sources of industrial discharge or mining runoff. While this oxygen consumption is generally marginal compared to microbial BOD on a large scale, it can cause severe, immediate DO drops in specific areas.