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How Does Dissolved Oxygen Get Into Water?
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Dissolved oxygen (DO) refers to the amount of oxygen dissolved in water. Oxygen enters the water by direct absorption from the atmosphere, a byproduct of aquatic plant photosynthesis, and groundwater discharge.
Dissolved oxygen (DO) is a measure of the concentration of oxygen dissolved in the water, typically measured in parts per million (ppm), milligrams per liter (mg/L), or as a percentage saturation. The amount of dissolved oxygen in the water tells us a lot about water quality and how ecosystems are supported.
How Does Dissolved Oxygen Enter Water?
Dissolved oxygen enters the water in several ways:
- From the atmosphere
- Increased turbulence from streams, rivers, and the ocean
- As a byproduct of photosynthesis from algae and other aquatic plants
- Groundwater discharge
Atmosphere & Turbulence
The Earth’s surrounding air consists of 21% oxygen, which slowly diffuses across the water’s surface or enters via aeration. The amount of oxygen dissolved in water from the atmosphere is in direct proportion to the partial pressure in the gas phase. However, only a small amount of oxygen is dissolved from the atmosphere; it is estimated that only ten molecules of oxygen per million of water, are dissolved in water.
Direct oxygen absorption from the atmosphere is enhanced by turbulence; the more turbulence a river or stream experiences, the more oxygen is absorbed into the water.
Increased turbulence can come from natural processes like waterfalls, rapids, or wind, or by introducing man-made infrastructures such as water wheels and dams.
Oxygen is produced as a waste product when algae and other aquatic plants photosynthesize. In areas that are densely populated with aquatic plants, the DO concentration in the water can become supersaturated (exceeding the 100% saturation level).
Supersaturation occurs when plants produce oxygen more rapidly than it can be used up in the water or released into the atmosphere. Oxygen supersaturation can be an issue, as it displaces other dissolved gases such as nitrogen and phosphorus. Nitrogen and phosphorus are required to support the growth of algae and aquatic plants, which provide both a habitat and a food source for aquatic organisms like fish.
Oxygen production via photosynthesis is dependent on water clarity and sunlight penetration.
Groundwater typically has a low DO concentration, but as it is usually colder than streams or rivers, it can hold oxygen better. When groundwater flows through an aquifer to surface waters, it carries oxygen with it.
Despite the significant contribution of dissolved oxygen from groundwater discharge, it only occurs in areas where groundwater is transported into the stream flow, such as glacial deposits.
How Do Areas Become Depleted Of Oxygen?
Water can quickly become depleted of oxygen, which puts aquatic life under stress. Dissolved oxygen levels can decrease from the following issues:
Discharge Increasing Nutrients
A sudden increase in bio-available nutrients (nitrogen and phosphorus) can stimulate algal blooms which eventually decay and are consumed by bacteria, resulting in a depletion of oxygen in the subsurface water. A prime example of this is the Gulf of Mexico “dead zone”.
Increased Water Temperatures
As temperature levels increase, the amount of dissolved oxygen in water decreases due to the inverse relationship between dissolved oxygen and temperature.
As salinity increases, dissolved oxygen decreases, which is why saltwater holds 20% less dissolved oxygen than freshwater. Ions present in saltwater attract water molecules because the ions want to bond. This leaves oxygen ions available to be captured or released by other elements in the water.
Decreased Water Flow & Deep Water
Dissolved oxygen increases with water movement, therefore, a decrease in flowing water from streams, rivers, and the ocean can influence DO levels in water. When water flow is reduced for a long time, water can become stagnant and oxygen-deprived. This commonly happens in bottom waters in bays and reservoirs, which is an issue for aquatic life that cannot migrate out of the area.
In very deep water, there is not enough sunlight to support photosynthesis, so little oxygen is dissolved into the water and oxygen is continuously consumed via decomposition, depleting DO levels.
To increase water flow in areas that are prone to becoming stagnant, human-induced methods are installed.
Why Do We Measure Dissolved Oxygen In Water?
Aerobic aquatic organisms require oxygen to survive. For example, fish require dissolved oxygen levels between 5-6 ppm. Low DO levels (<3 ppm) can impair or kill aquatic life, and large fluctuations over short periods increase stress.
How Do We Measure Dissolved Oxygen In Water?
The Winkler method directly measures how much oxygen dissolves in water. It involves adding chemicals to the water to react with the oxygen molecules, to form an acidic solution. A neutralizing agent added drop-by-drop neutralizes the solution until the water becomes clear. The amount of neutralizing agent used indicates how much oxygen was in the water.
An electronic meter doesn’t measure oxygen directly. Instead, it uses a dissolved oxygen probe containing electrodes to measure the partial pressure of oxygen in the water, expressed as a DO concentration in mg/L. For continuous measurements, a DO meter is a better choice.
If percentage saturation is required, it can be calculated by dividing the DO concentration measured by the probe by the saturation level and then multiplying by 100.
Dissolved oxygen gets into water from the atmosphere, increased turbulence, as a byproduct of photosynthesis from aquatic plants, and groundwater discharge. Oxygen is then removed when organisms respire and by organic decomposition.
If you have any questions regarding dissolved oxygen in water or which dissolved oxygen kit will suit your testing needs, do not hesitate to contact our world-class team at Atlas Scientific.
Dissolved Oxygen Probes & Sensors
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Parts pеr million (ppm) is a unit typically used to еxprеss thе concеntration of a substancе in a solution or mixturе. Howеvеr, ppm can also bе usеd to dеtеrminе parts and quality in thе automotivе industry. Thе ppm valuе is calculatеd by dividing thе mass or volumе of thе substancе by thе total mass or