In this article, we will talk specifically about dissolved oxygen in water in an aquaponic system. Indeed, it is in this form that it is assimilated by fish, plants and bacteria. Of course, oxygen is a very important part of your system because it is the very basis of life. We will therefore see together the oxygen needs for your aquaponic system. How is dissolved oxygen measured? What will be the effects of lack of oxygen? And finally how to properly oxygenate your system?

Oxygen requirements for aquaponics.

First of all, remember that aquaponics is fish, plants and bacteria living in symbiosis. One of the things that these three elements have in common is that they all need oxygen to live, each at different concentrations. However, as it is a fundamental element of the system, there are minimums to respect.

Regarding fish, the majority of them will survive up to levels of 4 to 5 milligrams per litre. But that is a generality. It will mainly depend on each species. For example, two fish, especially trout, will require much more oxygen, namely eight milligrams per litre. The reason is because they live in streams, rivers, with very oxygenated water, whereas pond fish will tolerate much lower dissolved oxygen levels.

The second thing to know about dissolved oxygen in water is that its rate depends on many things, but in particular on salinity.

Take some water, measure the dissolved oxygen level then add salt. This level of dissolved oxygen will decrease, and therefore there will be less oxygen that can be absorbed by the fish. In aquaponics, we usually have no or very little salt in our systems, so that's not a concern. On the other hand, the second parameter, which is the temperature of the water, will have a huge impact on us. So for the same water, if it is ten degrees or if it is 30 degrees, it will not have the same level of dissolved oxygen.

On the plant side, they breathe with the aerial part, with the leaves. But you should also know that the roots absorb oxygen. So dissolved oxygen is going to be very important for root growth, but also for nutrient uptake. If there is not enough dissolved oxygen in the water, the plant will not be able to recover all the available nutrients that are necessary for its growth: nitrogen, potassium, phosphorus, magnesium, etc. Overall, the plants will need a minimum of three milligrams of oxygen per liter of water.

As for bacteria, they are generally all aerobic, basically they need oxygen. To function well, they will need to have 4 to 8 milligrams of oxygen per liter of water.

How is dissolved oxygen measured?

To test your water , you can use more or less expensive devices on the market. Basically, you have a probe that you place in the water and that will give you the oxygen concentration directly. These are probes that take about thirty seconds to obtain the measurement.

You can also take a measurement at different points in the system.

What are the effects of lack of oxygen?

In summer it is the most dangerous time for the aquaponics system because, as we have just said, the water will rise in temperature and therefore the dissolved oxygen available for fish, plants and bacteria will decrease. .

Generally speaking, plants and bacteria will get away with it anyway. What is going to be most critical is for the fish. So if you see your fish trying to get air from the surface, trying to gulp air, there could be plenty of reasons for that. It may also be due to the fact that there is too much ammonia or nitrites in your water which will clog their gills and therefore they will not be able to absorb oxygen.

Then, on the plant side, we saw that it was important for root growth and for nutrient uptake. So a lack of oxygen will cause the roots to rot. As with fish, there may be other reasons for root rot. However, lack of oxygen is one such reason. So at the air level, you'll see that's not happening. The leaves are not pretty.

And finally, bacteria. Obviously, if there is not enough oxygen in the system, all biological processes will slow down or even stop, and in particular that of the nitrogen cycle. So the ammonia will no longer be degraded into nitrite and then into nitrates. It will poison your fish and it won't give your plants any more nitrogen either, and they won't grow anymore.

How to properly oxygenate your system?

The first thing to do is natural ventilation by water circulation. So oxygen enters the water when it comes into contact with air bubbles. We see for example that when emptying a bell siphon, there is a lot of water that will come from the culture tank to the sump or in the fish tank. And so all that whirlpool and all those air bubbles are going to be in contact with the water. The more contact surface there is, the more the water will be oxygenated. Thus, as soon as the water circulates, as soon as the water cascades, you will naturally add oxygen to your system.

Another way to add oxygen is to put a venturi effect , that is to say put a small tube that will suck in the air when the water circulates. This venturi effect, which is therefore a small mechanical system, will add dissolved oxygen to your system.

Finally, to finish, the simplest, most effective but not free way to add oxygen to the water in your aquaponics system is the burner. As in your aquarium, you are going to have to buy a burner, which is between five and twelve watts in terms of electrical power. On this burner, you will put hoses, so tubes with possibly non-return valves and at the end of the porous stones which will let the air bubbles escape.

It is therefore the most efficient system because it creates a lot of air bubbles and we know that the amount of oxygen added depends directly on the contact surface. So thousands of bubbles equals thousands of square centimeters of surface area between air and water, and so water will pick up oxygen easily. You can put one to X porous stones according to your needs.

To conclude, oxygen is a fundamental element that we often neglect because globally, there is always a little of it. Yet if we forget to test our water, especially in the summer, we run the risk of losing all our fish.