Dissolved oxygen (DO) refers to the amount of gaseous oxygen (O2) that is present in water. This oxygen is not part of the water molecule itself (H2O), but rather oxygen gas that has diffused or been introduced into the water from external sources. The presence of dissolved oxygen is fundamental for nearly all aquatic life and plays a significant role in maintaining the health and balance of aquatic ecosystems. Without sufficient dissolved oxygen, many organisms cannot survive.
From the Air Above
One primary way oxygen enters water is through a process called atmospheric diffusion. Oxygen molecules from the air directly transfer into the water at its surface. This exchange occurs as oxygen molecules move from areas of higher concentration (the atmosphere) to lower concentration (the water). The air-water interface acts as the main boundary where this molecular exchange takes place.
This process continues until the concentration of oxygen in the water reaches a state of equilibrium with the oxygen in the overlying air. The amount of oxygen that can dissolve depends on factors such as atmospheric pressure and temperature.
Produced by Water Plants
Another significant source of dissolved oxygen in aquatic environments comes from the biological activity of water plants, algae, and phytoplankton. These organisms perform photosynthesis, a process where they use sunlight to convert carbon dioxide and water into energy-rich sugars. As a byproduct of this energy conversion, oxygen gas is released directly into the surrounding water.
This biological contribution is particularly substantial in well-lit surface waters where photosynthetic organisms thrive. During daylight hours, these aquatic producers can release large quantities of oxygen, significantly increasing the dissolved oxygen levels in their immediate environment.
Stirred In by Nature
Physical forces also help oxygen enter and mix within water bodies. Water movement and turbulence increase the surface area contact between the water and the air, which facilitates more efficient oxygen transfer through diffusion. Natural phenomena such as waves crashing on a shoreline, currents flowing in rivers, and water cascading over waterfalls or through rapids all create this agitation.
Even rainfall can contribute to dissolved oxygen levels by aerating the surface of the water. This constant stirring action helps to distribute the newly dissolved oxygen throughout the water column, rather than just keeping it at the surface. These natural mixing processes are important for maintaining healthy oxygen levels throughout an aquatic habitat.
What Influences How Much Oxygen Stays
Several environmental factors dictate not only how much oxygen can initially dissolve but also how much remains in the water. Temperature is a primary influence, with colder water being able to hold more dissolved oxygen than warmer water. For example, water at 0°C can hold approximately 14.6 milligrams of oxygen per liter, while water at 30°C holds only about 7.6 milligrams per liter. Salinity also plays a role, as fresh water can hold more oxygen than saline or brackish water due to fewer dissolved salts competing for space.
Atmospheric pressure affects solubility, with higher pressures allowing more oxygen to dissolve. Beyond these physical factors, biological processes within the water body can consume oxygen. The decomposition of organic matter by bacteria and the respiration of aquatic organisms, including fish and invertebrates, continuously use dissolved oxygen, thereby reducing its overall concentration.
Why Oxygen in Water Matters
Fish, aquatic insects, and many microorganisms rely on dissolved oxygen for respiration, similar to how land animals breathe air. Without adequate oxygen, these organisms become stressed, making them more susceptible to disease and reducing their ability to reproduce.
Low dissolved oxygen levels, often termed hypoxia or anoxia, can lead to widespread fish kills and significant alterations in aquatic communities. Ecosystems with consistently low oxygen levels struggle to support diverse life. Therefore, monitoring and maintaining healthy dissolved oxygen concentrations are indicators of water quality and aquatic habitat health.