The Importance of Dissolved Oxygen in Aquaponics

The water in aquaponics systems contains nutrients and minerals, and dissolved oxygen that the plants, fish, and bacteria require to carry out their functions. Dissolved oxygen is a critical indicator of water quality. Just like humans need oxygen to breathe, aquatic animals need a sufficient amount of oxygen dissolved in the water to survive. Fish, plants, and bacteria required dissolved oxygen for the following:

• Fish for respiration.
• Plants for respiration, health, the strength of their roots, and nutrient uptake.
• Bacteria for nitrification and respiration.

Dissolved oxygen (DO) is one of the essential parameters for growing fish and plants and is critical to the bacteria that convert fish waste into nutrients for the plants. Let's get into details about the importance of DO in aquaponics systems in this article. 

The Importance of Dissolved Oxygen on Fish, Plants, and Bacteria

Below discusses the effects of dissolved oxygen on fish, plants, and bacteria in aquaponics.  

Fish

Different fish species need different ranges of dissolved oxygen. In general, most fish species grow and thrive within a DO range of 4-5 mg/liter. If the DO level drops below 4 mg/liter, fish may swim at the surface, breathe rapidly, and lose appetite. Warm water fish like bluegill, catfish, and bass require about 5 mg/liter of DO, while coldwater fish like trout require about 6.5 mg/liter of DO to maintain good health and growth.

Some fish like tilapia are tolerant of lower DO, but it will affect their growth rates. This is one of the important factors you should consider in choosing the best fish for your aquaponics system. Oxygen depletion often occurs during the summer months because warm water holds less oxygen than cool water. High temperatures also increase the metabolic rate of fish, resulting in the need for more oxygen. 

Plants

Plant roots need oxygen for respiration, a critical plant process that releases energy for root growth and nutrient uptake. During respiration, plants absorb oxygen through stems and leaves and roots. Most plants need high DO levels, which may exceed >3 mg/liter of dissolved oxygen. Healthy roots with enough oxygen supply can absorb the required nutrients from the water and ensure proper plant growth. 

Effects of low DO on the Plants:

• Root rot
• Poor ability to absorb suitable amounts of water and nutrients because of the accumulation of toxins.
• The plant roots system loses a net amount of nutrients in anaerobic conditions.
• Calcium deficiency in the shoot.

Plants need a lot of oxygen when they are blooming with flowers compared to their vegetative stage. This is because of the temperature, nutrient uptakes, and the size of the plant's roots. To learn the best plants for aquaponics systems, click here. 

Signs of inadequate oxygen supply to the plant roots:

• Wilting of the plants under warm conditions.
• Browning and dying roots
• Slimy to touch roots.

Bacteria

The bacteria need enough dissolved oxygen in the water to function effectively and do their work. The optimum dissolved oxygen levels for the bacteria are 4-8 mg/liter. The nitrification process will stop if the DO level is below 2 mg/liter. Nitrification is a biochemical reaction that happens inside the bacteria, which converts ammonia from the fish waste into other forms of nitrogen, like nitrites and nitrates. Nitrification occurs when the DO level is 3.0 mg/liter. 

How Other Water Quality Parameters Affect the Dissolved Oxygen

• The temperature has a significant effect on the oxygen level of an aquaponics system. High water temperature holds less DO. Colder water can carry more oxygen than warmer water. This is because of the enhanced molecular activity in warm water that pushes oxygen out of the spaces between molecules of the moving water. 
• Water with low PH levels holds lower dissolved oxygen. 
• Dissolved oxygen increases when there is pressure in the water. 

Factors that affect Dissolved Oxygen in the Water

• Temperature establishes the maximum oxygen-holding capacity of the water, so it plays a prominent role in DO levels in an aquaponics system. 
• The weather also contributes to the DO levels. Still and windless days do not allow water circulation, limiting surface diffusion of the oxygen.
• Excessive algae growth can affect the DO level in aquaponics systems because algae also consume oxygen needed by plants, fish, and bacteria.
• Uneaten fish food and other wastes in the fish tank. 

How to Increase Dissolved Oxygen in Aquaponics

Low dissolved oxygen levels are not usually a problem with small home aquaponics growers with low fish stocking rates. The problem tends to be more present in commercial aquaponics systems with high fish stocking rates. If the DO levels in your aquaponics system are too low, you can increase them by doing the following:

1. Using Air Pump

Air pumps supply oxygen to the water by pumping air into the water in the form of bubbles. Air pumps work well with air stones, which help break up the bubbles into smaller bubbles for oxygen to be dissolved in the water. To learn more about aquaponics air pumps, read our article on choosing the right air pump.

2. Creating Turbulence in the Water

The water in your aquaponics system can stay oxygenated without an air pump if it has enough turbulence to provide oxygen absorption. This method is usually applied in flood and drain aquaponics systems, where the pump carries there to the grow bed and lets it drain into the fish tank below the grow bed. The cascading effect of the water will create turbulence in the water in the fish tank.

3. Increase Oxygen in the Root Zone 

Having oxygen around plant roots is essential for plant growth. Here are some methods of adding oxygen around plant roots:

• Media Based System: By flooding and draining the water in the grow bed (a cycle of nutrient-rich water followed by drawing to allow air in and around plant roots.
• NFT: By providing a constant stream of nutrient-rich water in the channel while keeping space for free oxygen absorption between plant roots and stems.
• Raft: Growing plants on top of the nutrient-rich water and keeping an air space between the water surface and plant stem.

Tips in Ensuring Enough DO Levels in Your Aquaponics System

Higher DO levels in aquaponics systems make the system more robust, less prone to problems and failure, and better welfare for the fish and plants. So it is important to add as much DO to your system as possible. You can do these by following the tips listed below.

• Avoid overstocking your fish and follow the ideal fish ratio.
• Ensure that your system has a continuous flow of water. The cascading flow of water falling back into the fish tank aerates the water very well and adds dissolved oxygen.
• Use air pumps and air stones to add more air.
• Design your aquaponics system where there is a constant flow of water. 
• In warmer climates, increase aeration by using air pumps and lowering the water temperature.
• Keep extra batteries, water pumps, and aeration systems back up to use during a power interruption or equipment failure.