Why Is Dissolved Oxygen Important?

Dissolved oxygen (DO) describes the amount of oxygen (O2) molecules that are dissolved in water. Oxygen is an essential element for all forms of life, therefore, dissolved oxygen plays an important role from the survival of aquatic organisms, to determining water quality for safe human use.

As dissolved oxygen (DO) is one of the most important indicators of water quality, it must be measured in a wide range of industries and applications.

Dissolved oxygen refers to the amount of oxygen (O2) dissolved in water or a given solution. Oxygen is dissolved into the water via diffusion from the surrounding atmosphere and as a byproduct of aquatic plant photosynthesis. When DO levels become too low, aquatic organisms cannot survive, and water quality is affected.

Why Is Dissolved Oxygen Important?

Earth consists of around 71% water, and all forms of life rely on oxygen that is dissolved in the water to survive. Therefore, there are many reasons why it is important to measure DO levels in different water systems. As humans, we rely on water for our everyday needs (drinking, washing, growing produce, etc.) and aquatic organisms rely on it for biological processes and survival.

Dissolved oxygen plays an important role in biological processes, both directly (organism physiology and survival), and indirectly (the effect on nitrogen and carbon cycles in water).

In water, dissolved oxygen uses up a specific amount of space. If DO levels are too high, water cannot hold onto other dissolved substances. If DO levels are too low, minerals from riverbeds and runoff will start to dissolve into the water, which affects water quality.

The Importance Of DO For Aquatic Organisms

Every living aquatic plant and animal relies on oxygen to survive in water. When DO levels become too low, hypoxic waters or dead zones are formed usually due to fertilizers fuelling the overproduction of algae and phytoplankton, creating algal blooms.

Hypoxic waters decrease the amount of life as oxygen is depleted dramatically. Mobile aquatic animals can usually survive hypoxic waters as they can move to other areas, however juvenile animals, eggs, and plants are restricted, so die or decay which contributes to algal blooms, often resulting in dead zones.

When levels become too low, it can also affect the life stages of aquatic organisms, causing a reduction in life, and in severe cases a trophic cascade (where the whole ecosystem is affected). Lethal DO levels for fish are between 1 and 3 mg/L, which is why it is important to test DO levels in different water bodies.

For animals to survive in water DO levels should be between 80 – 120% and contain a concentration above 6.5 – 8 mg/L; this is what we consider “healthy water”.

We can look at DO levels for specific aquatic organisms to understand why DO is so important:

Bottom organisms such as worms and clams are exposed to lower DO levels as oxygen cannot penetrate the sediment as easily as free-flowing water, however, they still require DO concentrations to be at least 1 mg/L to survive.
Bottom feeders that live above the sediment such as fish, crabs, and oysters require a higher DO concentration of 3 mg/L or more.
Fish that migrate, fish larvae and eggs, and plants require DO concentrations of at least 6 mg/L as they undergo different life stages to survive.

The Importance Of DO In Water Treatment Facilities

Dissolved oxygen is not only important for aquatic life, it also plays a vital role in industrial settings for many reasons. Water treatment facilities must measure DO levels to ensure they do not decrease, otherwise, the water can have a bad odor or taste.

Compounds such as activated carbon can be added to water if there are issues with bad odor or taste, and if the pH also becomes too low, sodium hydroxide is added to stabilize pH levels to neutral (7.0). Specialized DO probes and sensors are an easy and precise way to test dissolved oxygen content in water, and can be used in a wide variety of industries.

In wastewater and sewage treatments, it is important to test DO concentrations to measure the amount of decomposition of solids from bacteria in the water. If DO levels are too low, the bacteria is unable to break down solids and the bacteria dies, so the process is stopped until levels can be restored.

When DO levels are too high, energy is wasted within the sewage treatment plant. For the boilers to efficiently work, low DO levels are imperative to prevent corrosion and the buildup of scale inside the boiler. If scale and corrosion occurs inside the industrial boiler, the whole process becomes inefficient, hence the importance for testing DO levels.

What Does Dissolved Oxygen Tell Us About Water Quality?

Measuring DO levels tells us a lot about water quality and what animals can survive. While every aquatic organism has an optimal DO range, as briefly forementioned DO levels below 3 mg/L cause concern for organism distribution, and DO levels below 1 mg/L are considered hypoxic, usually resulting in animal and plant death.

Dissolved oxygen is additionally critical in water supplies for human consumption and everyday use. High DO levels are actually good for drinking water, as it makes it taste better, however, high DO levels are linked to rapid corrosion of water pipes. Therefore, water industry companies use the lowest DO levels that are safe for human consumption to be efficient.

Dissolved oxygen can be affected by many factors such as temperature, salinity, water flow, and human-induced runoff.

For more information on what affects dissolved oxygen levels in water, we have an article, which you can read here.

Summing Up Why Dissolved Oxygen Is Important

Dissolved oxygen in water plays an important role in many industries and applications, from the survival of aquatic organisms to determining water quality for drinking water.

As dissolved oxygen is so important, it can easily be measured using different DO probes or sensors for precise and reliable results.

If you would like to learn more about other water quality measurements, characteristics, or applications for DO, do not hesitate to contact our world-class team at Atlas Scientific.