What Is The Relationship Between Dissolved Oxygen And Salinity?

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Dissolved oxygen and salinity are key factors when measuring the quality of water. As salinity increases, dissolved oxygen exponentially decreases. This is why saltwater holds 20% less dissolved oxygen than freshwater. A good example is dissolved oxygen levels near the equator; dissolved oxygen levels are lower at the equator because salinity and temperature are higher.

Dissolved oxygen (DO) is not only the most important indicator of water quality, but measuring dissolved oxygen levels is also critical for the survival of fish and other aquatic organisms. 

Dissolved oxygen refers to how much oxygen is absorbed into a water body. Oxygen has a limited solubility range (6-15 mg/L) in water, however, factors such as water temperature, atmospheric pressure, and salinity can cause fluctuations outside oxygen’s solubility range. 

The Earth’s surface is covered in 71% water, 96.5% of that is in the ocean. Salinity is an important factor when determining how much oxygen a body of water holds.  The amount of dissolved salts present in the water (salinity) tells us a lot about DO concentrations. For example, freshwater absorbs more oxygen than salt water.

How Does Salinity Affect The Solubility Of Oxygen?

Salt is a valuable resource; salt intake helps to reduce blood pressure, cardiovascular disease, stroke, and coronary heart attack, but if salinity levels are too high, the taste of drinking water is negatively impacted, and DO levels decrease below what is considered “healthy water”.

According to the Environmental Protection Agency (EPA), dissolved oxygen levels measured at 3 mg/L are in the danger zone for supporting common aquatic life, and levels below 1 mg/L cannot support any aquatic life. When dissolved oxygen levels decrease because of salinity increases, water conditions soon become lethal for aquatic life. So, increasing the salt concentration will lead to a decrease in oxygen solubility, negatively affecting any aquatic life inside your aquarium.

However, high DO levels cause issues to drinking water pipes, despite the benefits of improving the taste, therefore, a happy medium must be met. Dissolved oxygen levels in healthy water should generally be above 6.5-8 mg/L and between about 80-120 %.

The ability of water to dissolve gases like carbon dioxide and oxygen decreases when electrolytes are added. When ionic salts enter the water, ions from the salts naturally attract water molecules because the ions want to bond, leaving hydrogen and oxygen ions available to be captured or released. This bond formation from the salt ions usually decreases the non-polar oxygen molecules, driving DO out of the water and decreasing the solubility of dissolved oxygen molecules. 

Even though oxygen solubility decreases as salinity increases, the decline is much less significant than the effect temperature has on dissolved oxygen. As the water temperature increases, dissolved water decreases more, regardless of the salinity level. This is why during the summer months when water temperatures increase, many fish die from oxygen starvation.  

To understand the , below are two graphs showing the results comparing the relationship between dissolved oxygen, salinity, and temperature. Note that generally, seawater has a salinity level of 35 ppt (parts per thousand), brackish water (for example, estuaries that mix with freshwater and seawater) 15 ppt, and freshwater 0 ppt. 

From the graphs above, a conclusion that as salinity increases, the amount of oxygen dissolved into the water decreases can be made. In seawater that has a higher salinity (35 ppt), the amount of dissolved oxygen is higher than in freshwater (0 ppt). Therefore, saltwater has a lower capacity (saturation level) to hold oxygen than freshwater does.

Compensating Salinity When Measuring Dissolved Oxygen

When measuring dissolved oxygen that has salinity levels greater than 1ppt (parts per thousand), it is necessary to compensate for salinity. To do so, you will need to adjust the calibration value using the following formula:

DO(salt) = DO – (k*S)

  • DO (salt):  The dissolved oxygen concentration (mg/L) of air-saturated salt water.
  • DO: The dissolved oxygen concentration (in mg/L) of air-saturated distilled water.
  • S: The salinity value (ppt) of the sample.
  • K: The constant, which varies according to the sample temperature, and can be determined using salinity correction constant values.

Measuring Salinity And Dissolved Oxygen

Whether you have a home aquarium or work with wastewater treatments, measuring salinity and dissolved oxygen are important factors when determining water quality. 

To accurately measure dissolved oxygen in water, a dissolved oxygen meter is used. Dissolved oxygen probes work by measuring the amount of oxygen that diffuses across a membrane into a probe (sensor).

Dissolved oxygen is usually measured in milligrams per liter (mg/L) or percentage saturation (% sat), but sometimes it can be measured in parts per million (ppm), which allows measurement comparisons between sites that have different salinity and temperature values. 

There are three ways you can test the salinity of water

Whichever method you use to test salinity, the most crucial factor is understanding salinity measurements, depending on the type of water you are testing.

Summary

The solubility of oxygen (dissolved oxygen) is dependent on salinity; as salinity levels increase, the amount of oxygen dissolved in the water decreases. However, the partial pressure and the percentage saturation of oxygen are not affected by fluctuations in salinity. 

As temperature closely affects dissolved oxygen and salinity levels, all three water parameters must be measured together to determine water quality. 
If you have questions regarding dissolved oxygen or salinity or what probes would best suit your water testing needs, do not hesitate to contact our world-class team at Atlas Scientific.

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