The water is not just a mix of nutrients, minerals, and water. There are a number of organisms and compounds that your fish, plants, and bacteria need in an aquaponics system. One of the important aquaponic water parameters is dissolved oxygen (DO), which is required by
- Fish for respiration
- Plants for respiration, health, and strength of the root system and nutrient uptake
- Bacteria for nitrification and respiration
Let’s get into the details about the importance of DO for fish, plants, and bacteria
Different fish species need a varied range of dissolved oxygen. Normally, fish require 4–5 mg/liter of dissolved oxygen. Some species such as salmon and trout need higher levels of oxygen around 8mg/Lt. This is one of the important factors to consider while choosing fish for aquaponics.
Fish gasping for air at the surface is a clear sign of lack of oxygen. This behavior of fish swimming close to the water surface to take air into their mouths is called piping. This is an emergency situation that needs immediate attention.
When the DO drops below 3 mg/l, even the hardy fish die. Keep in mind that even though there may be enough DO to keep an adult alive, reproduction may be hampered by the need for higher DO for eggs and immature stages.
Depletion in DO can also cause major shifts in aquatic organisms composition in water bodies. Pollution-tolerant organisms, such as worms and fly larvae will replace the species that cannot tolerate low levels of DO (mayfly nymphs, stonefly nymphs, and beetle larvae). Nuisance algae and anaerobic organisms (that live without oxygen) may also become abundant in waters with low levels of DO.
Lowest DO at which fish survived for 24 hours (Summer)
- Black Bass – 5.5 mg/L
- Northern Pike – 6.0 mg/L
- Yellow Perch – 4.2 mg/L
- Common Sunfish – 4.2 mg/L
- Black Bullhead – 3.3 mg/L
Plants use their stems and leaves to absorb oxygen during respiration, but the roots also need to have oxygen. Most plants need high levels of DO (> 3 mg/liter) within the water.
Plant root systems require oxygen for aerobic respiration. It is an essential plant process that releases metabolic energy for root growth and nutrient uptake.
Without oxygen, the plants can experience
- Root-rot, a situation where the roots die and fungus grows.
- The permeability of the roots to water and there will be an accumulation of toxins so that both water and minerals are not absorbed in sufficient amounts to support plant growth.
- Anaerobic conditions in which there can be a net loss of nutrient ions from a plant’s root system.
- Plants are unable to take up mineral nutrients in the concentrations required for maximum growth and development
- A shortage of calcium in the shoot will occur. This shortage of calcium makes the development of a calcium disorder such as tip burn and severe under oxygen starvation conditions.
Oxygen requirements for plants in flower tend to be more demanding in comparison to vegetative states. This is due to the size of the root system, temperature, and nutrient uptake rates, not the specific stage of growth.
Healthy roots supplied with sufficient oxygen are able to
- Absorb nutrient ions selectively from the surrounding solution as required.
- Ensure healthy root tips are able to take the levels of calcium required for new tissue growth and development.
- Boosts nutrient uptake by ensuring roots have the energy required to rapidly take up and transport water and mineral ions.
Some water plants, such as water chestnut, lotus or taro, do not need high levels of DO and can withstand low-oxygen waters such as those in stagnant ponds.
Bacteria need an adequate level of dissolved oxygen (DO) in the water in order to maintain high levels of productivity. Nitrification is an oxidative reaction, where the bacteria derive the energy to live when oxygen is combined with the nitrogen. Optimum levels of DO are 4–8 mg/liter. Nitrification does not occur if the DO concentration drops below 2 mg/liter.
Moreover, without sufficient DO concentrations, another type of bacteria can grow. The one that will convert the valuable nitrates back into unusable molecular nitrogen in an anaerobic process known as denitrification.
Check more about nitrifying bacteria here.
Effects of Other Parameters on Dissolved Oxygen
- Water temperature and Dissolved Oxygen have a unique relationship that can affect aquaponic food production. As the water temperature increases, the ability of water to hold DO decreases. Colder water holds more oxygen than does warm water. This is due to the increased molecular activity of the warm water pushes the oxygen molecules out of the spaces between the moving water molecules.
- Water with low PH holds less oxygen.
- Dissolved oxygen decreases exponentially as salt levels increase. That is why, at the same pressure and temperature, salt water holds about 20% less dissolved oxygen than fresh water.
- Dissolved oxygen will increase as pressure increases. This means that if the concentration of dissolved oxygen is at 100% air saturation at the surface, it would only be at 70% air saturation three meters below the surface.
Oxygen, Temperature, and Metabolism
The temperature rise also increases the oxygen requirement of all organisms in aquaponics:
- There will be an increase in bacteria (biological) demand for DO at higher stocking rates and higher temperatures. (Rate of metabolism doubles every 10 degrees C, 18 degrees F.)
- As the temperature of the root system warms, the rate of respiration of the root tissue also increases and more oxygen is required by the plant. The respiration rate of the roots will double for each 10 C rise in temperature up to 86 F (30 C).
- A trout needs five to six times more DO when the water temperature is 24 degrees C (75 degrees F) as compared to when the water temperature is 4 degrees C (41 degrees F). There is a need of increased DO to support the increase in metabolic rates – phenomenon shared by other cold-blooded aquatic animals.
However, With an increase in temperature, DO itself decreases e.g. The oxygen content of an aerated solution at 50 F (10 C) is about 13 ppm. but as the solution warms up to 68 F (20 C) the ability of the liquid to ‘hold’ oxygen drops to 9-10 ppm. By the time the solution has reached 86 F (30 C) it is only 7.5 ppm.
This means that the DO in water will be much more rapidly depleted. This will result in stress in the overall system.
SEE ALSO: Role of Temperature for Aquaponics
Recommendations to ensure adequate DO levels
Higher levels of oxygenation leave the system more robust, less prone to failure, and better for fish welfare. So overall, you should add as much DO as possible to your aquaponic system. You can do this by following these recommendations.
- Do not overstock fish, and refrain from adding more than 20 kg of fish per 1000 liters of total water.
- Dynamic water flow, with cascading water falling back into the system, helps to aerate the water and add DO.
- Use Air pumps, if at all feasible. The suggested rate is 5–8 liters of air per minute for each cubic meter of water, coming from at least 2 air stones in different locations in the fish tank.
- Flood-and-drain cycles in media beds, air stones in external biofilters, or cascading water return lines to the canals and sump tanks help in aeration.
- The falling action of water provides aeration. So, make changes in your system design accordingly.
- In warm locations or during the hottest times of the year, increase aeration using air pumps or try to lower the temperature. (especially if raising delicate fish)
- Backup (redundant) aeration systems are a valuable asset to an aquaponic system. You can use them during power outages and equipment failures. Simple battery backups for air pumps have saved countless fish throughout the industry.