Dissolved oxygen (DO) is the amount of gaseous oxygen (\(\text{O}_2\)) physically dissolved in water. This dissolved gas is the form of oxygen that aquatic life, such as fish and macroinvertebrates, absorb through their gills to respire. The concentration of DO is a fundamental indicator of water quality and the overall health of an aquatic ecosystem. These levels fluctuate constantly, governed by a complex interplay of physical, chemical, and biological variables.
Temperature and Oxygen Solubility
Water temperature is the most important physical factor influencing the maximum amount of oxygen a body of water can hold. This relationship is inverse: as the water temperature rises, the capacity of the water to retain dissolved oxygen decreases. Increasing the temperature gives the dissolved gas molecules more kinetic energy, allowing them to break bonds with water and escape into the atmosphere.
For example, water at \(0^\circ\text{C}\) can hold approximately 14.6 milligrams of oxygen per liter (\(\text{mg/L}\)), but this capacity drops to about \(7.6\,\text{mg/L}\) at \(30^\circ\text{C}\). This natural reduction in solubility means that aquatic organisms in warmer waters must survive on less available oxygen. The problem is compounded because warmer temperatures simultaneously increase the metabolic rate of aquatic life, increasing their oxygen demand.
In deep lakes, the temperature effect leads to thermal stratification, where water layers form based on density. During summer, a warm, less-dense layer sits atop a colder, denser layer, preventing vertical mixing. The colder, deep water, known as the hypolimnion, is cut off from the surface and atmospheric reoxygenation. If organic material settles there, decomposition consumes oxygen that cannot be replaced, leading to severe DO depletion in the bottom waters.
Biological Activity and Oxygen Consumption
The metabolic processes of organisms represent the second major factor influencing dissolved oxygen levels, driving both production and consumption. Photosynthesis, carried out by algae, phytoplankton, and submerged aquatic plants during daylight, is a primary mechanism for adding oxygen. This process can lead to supersaturation, where oxygen levels temporarily exceed the maximum solubility for that temperature, typically occurring in the late afternoon.
The counteracting force is respiration, performed by every living organism, including fish, insects, bacteria, and plants, which consumes oxygen at all times. The most significant biological consumer of DO is the aerobic decomposition of organic matter (such as dead algae or sewage runoff) by microorganisms. This microbial breakdown creates a strong demand for oxygen known as Biochemical Oxygen Demand (BOD).
When a water body receives excessive nutrients, often from agricultural runoff, it can trigger an algal bloom. While algae produce oxygen during the day, the subsequent consumption of oxygen at night, combined with the massive demand created when the bloom dies and is decomposed, leads to a rapid drop in DO levels. This imbalance causes the lowest oxygen concentrations of the day to occur just before dawn, after a full night of uninterrupted respiration.
Physical Exchange and Atmospheric Influence
The third factor involves the physical mechanisms that facilitate the transfer of oxygen from the surrounding atmosphere into the water. The simplest mechanism is diffusion, where oxygen naturally moves across the air-water surface boundary from the area of higher concentration (the air) to the area of lower concentration (the water). This process is relatively slow in still water, limiting the rate at which oxygen can enter the system.
The rate of oxygen transfer is increased by turbulence and mixing, a process known as reaeration. Forces like wind, waves, currents, or the tumbling action of a waterfall constantly churn the water, maximizing the contact area between the water and the atmosphere. Streams with turbulent flow, such as those with riffles and rapids, are naturally better aerated than slow-moving rivers or stagnant pond waters.
Another external physical influence is atmospheric pressure, which dictates the maximum saturation concentration of oxygen in the water. At higher altitudes, the atmospheric pressure is lower, which reduces the partial pressure of oxygen in the air. Consequently, water bodies at high elevations naturally hold less dissolved oxygen at saturation than those at sea level, even at the same temperature.
The Role of Dissolved Oxygen in Water Health
Understanding the interaction of temperature, biological activity, and physical exchange is important because dissolved oxygen levels are the primary determinant of an aquatic ecosystem’s ability to support life. When DO concentrations fall below the threshold, aquatic organisms experience stress. For most fish species, levels below \(5.0\,\text{mg/L}\) can be stressful.
A condition where DO is severely reduced, typically below \(2\,\text{mg/L}\), is defined as hypoxia. Hypoxic conditions force mobile organisms, like adult fish, to flee the area, or they will suffer physiological damage and potentially die. The complete absence of dissolved oxygen, \(0\,\text{mg/L}\), is termed anoxia.
Anoxic zones are called “dead zones” because they cannot support most complex aquatic life and are instead dominated by anaerobic microbes. Monitoring DO levels is important for environmental agencies to assess water quality, predict the likelihood of fish kills, and manage pollutant inputs that exacerbate oxygen consumption.