Dissolved Oxygen Measurements¶
What is dissolved oxygen?¶
Dissolved oxygen (DO) is an indicator of how much oxygen is dissolved in water (obviously) and is available to living beings in water. The movement of water, and its collision with rocks or other elements, causes aeration and thus a higher concentration of oxygen. As a result, dissolved oxygen is higher in moving waters, while it decreases in stagnant waters.
Dissolved oxygen can also come from other sources, for example by exchange and diffusion with the atmosphere, aeration by wind and waves, or by the photosynthesis of aquatic plants or concentration of life. Likewise, temperature variations have an inversely proportional effect on dissolved oxygen: increases in temperature cause a decrease in dissolved oxygen and vice versa.
Dissolved oxygen is fundamental to aquatic life. The table below indicates the minimum amount of dissolved oxygen needed for the life of different organisms. Below that amount of oxygen in the water, what is known as hypoxia is caused, in other words, asphyxiation by lack of oxygen:
| Organism | DO (mg/l) |
|---|---|
| Salmon | 9-10 |
| Trout | 6.5 |
| Sea bass | 6.5 |
| Caddisfly larvae | 4.0 |
| Catfish | 2.5 |
| Carp | 2 |
| Mosquito larvae | 1 |
Why is it important?¶
Dissolved oxygen in water is fundamental for life. Bacteria in water consume oxygen by breaking down organic matter. An excess of decomposing organic matter (for example, due to plants or algae) can result in hypoxic conditions which can cause death in aquatic life due to a lack of oxygen. In addition, the presence of oxygen is very important for certain chemical reactions that occur in water. In general, water contamination tends to decrease the amount of oxygen in the water, usually caused by the reaction of substances that provoke high oxygen consumption.
When a lack of oxygen is caused by a high demand for it, due to pollution or biological activity, the process does not usually occur directly where the spill or dumping takes place, but later where the decomposition occurs. In this case, it is often very difficult to establish relationships between waste dumping or pollution with high oxygen demands and a decrease in oxygen quantities.
Changes in the chemistry of water (at the level of oxygen and pH), may exceed the ability of aquatic organisms to acclimatize and survive. This can lead to chain reactions, where habitats are further degraded due to the loss of biodiversity and biomass.
In the sea
Organisms that live in the sea are generally acclimatized to a specific percentage of oxygen. Fluctuations in this parameter can therefore have devastating effects: if dissolved oxygen decreases, some species will be able to adapt, but it is quite possible that if the decrease is not gradual enough, at least some will become extinct. In addition, the ocean is a major producer of oxygen globally, through the photosynthesis of algae.
Impact
Dead zones (also known as hypoxic zones) where oxygen is lacking are multiplying in the ocean. Global warming (which limits exchanges between different water layers), increasing water temperatures (which consequently contain less oxygen), and the increasing presence of fertilizers create fatal zones for marine animals.
Today, there are more than 245,000 km² spread over 400 dead zones. The fish that survive are underweight and their reproductive system appears to be permanently damaged. Other slower-moving marine animals (lobsters or crabs, for example), crustaceans, and mollusks are doomed in areas with very little oxygen. The decomposition of corpses then accentuates the phenomenon. These anoxic episodes (during which the amount of oxygen is insufficient) may last a few hours or a few months.
How is dissolved oxygen measured?¶
The concentration of dissolved oxygen changes dramatically depending on the depth and distance from the shore. If the objective is to measure differences between different points, it is necessary to take samples systematically, if possible continuously. If this is not possible, the sample should be taken as far offshore as possible, while sampling is safe and at a depth of one arm below the surface.
If multiple samples or continuous measurements are compared, they must have similar agitation conditions (not taking some near the shore with many waves and others in stagnant areas), unless, again, the goal is to measure these differences. The best areas for sampling are those where the water flows smoothly and is not at the extremes of agitation or stillness.
It is important to take into account the daily variations in dissolved oxygen caused by temperature and plant activity. Note the time at which you take the samples, in addition to the temperature of the sample (both air and water, if you can). To compare different areas, always take samples at the same time, unless your goal is to understand how dissolved oxygen changes over the course of a day.
Measuring dissolved oxygen is not easy. It is important not to aerate the samples before taking the measurement, and to ensure that there are no air bubbles in your sample container.
