In aquaculture, dissolved oxygen (DO) is the foundation of water quality, animal health, and growth performance. Fish and shrimp farmers must not evaluate water quality or growth rates in isolation from DO levels, as DO directly impacts microbial balance, algal development, feed conversion ratio (FCR), and farming success.
1. Sources of Dissolved Oxygen in Aquaculture and Related Challenges
A. Natural Diffusion and Mechanical Agitation
From a theoretical perspective:
As long as the DO level in aquaculture ponds is lower than the oxygen concentration in the atmosphere (air contains approximately 20.947% oxygen by volume), atmospheric oxygen will continuously dissolve into the water until equilibrium is reached or the water becomes saturated (8.4 mg/L at 25°C).
This raises common questions among farmers:
- Should oxygen-releasing agents work slowly for 24 hours to be effective?
- Will rapid oxygen release escape into the atmosphere too quickly?
The answer is no. Since pond water, especially the bottom layer, is far from saturated, oxygen continues to move from air into water, not the other way around. Therefore, fast-releasing oxygen agents are preferable to quickly elevate and maintain sufficient DO levels.
However, natural diffusion is extremely slow due to the surface tension of water. Without mechanical agitation from aerators or wind, oxygen penetration mainly affects the upper water layer. Deep pond zones remain oxygen-poor without vertical circulation.
The Role of Mechanical Aerators
- 1. Increase the Air-Water Interface: Splashing breaks the surface tension and expands oxygen exchange.
- 2. Promote Vertical Mixing: Surface water, rich in photosynthesis-produced oxygen, is pushed to the bottom.
While aeration contributes some direct oxygen, its main function is vertical mixing, ensuring oxygen-rich water reaches deeper zones. Therefore, optimal timing for aerator operation should follow this principle. Contrary to common belief, sunny midday is the best time to run aerators due to peak surface DO.
B. Photosynthesis by Phytoplankton
Photosynthesis is the main natural source of oxygen in aquaculture systems. During sunny conditions, DO levels in the photic zone can reach 200% to 300% of saturation. Afternoon hours are the best time to operate aerators to:
- Push oversaturated oxygen from the surface to the oxygen-poor bottom
- Bring bottom water to the surface for re-oxygenation
Some farmers mistakenly believe that aeration is unnecessary during sunny days. In fact, this is when aerator efficiency is highest. Also, from a DO perspective, algae-rich water is preferable over crystal-clear water, as algae significantly contribute to oxygen production.
2. Dissolved Oxygen Consumption in Aquaculture
A. Surface Degassing
Though rare, oxygen may escape into the atmosphere when DO levels exceed saturation by more than fivefold. This may occur during intense sunlight, especially in surface layers. Still, if strong water movement is present (e.g., via aerators), surface degassing may accelerate.
Strategy Tip: Avoid excessive aeration in the evening. Consider reducing aerator use after sunset and instead maximize usage during midday when surface DO is highest.
B. Microbial Decomposition
Microbial breakdown of organic matter (waste, dead algae, feces) consumes over 80% of pond DO. The most effective way to preserve DO is to reduce organic load:
- Clean sludge
- Mid-cycle water exchange
- Apply bottom conditioners to reduce microbial metabolism
C. Respiration of Cultured Animals and Algae
The DO consumption by fish/shrimp depends on stocking density. Higher density = higher DO demand. Moreover, algae consume oxygen at night, contributing to oxygen depletion. Therefore, managing algae density and maintaining water transparency are crucial.
3. Strategies to Improve Dissolved Oxygen
A. Mechanical Aeration (Most Effective)
Mechanical aerators are the most effective and widely adopted tools to improve DO. They enhance both surface exchange and water column mixing. Common types include:
| Aerator Type | Oxygen Efficiency | Flow Type | Ideal Use | Power Need |
|---|---|---|---|---|
| Paddle Wheel Aerator | ★★★★☆ | Surface flow | Shrimp/fish ponds, daily use | Medium |
| Impeller Aerator | ★★★★★ | Vertical circulation | Deep ponds, intensive farming | High |
| Air Jet Aerator | ★★★☆☆ | Moderate turbulence | Emergency use, mobile units | Low-Med |
| Wave Maker Aerator | ★★★★☆ | Gentle circulation | Nursery tanks, uniform flow | Low |
| Roots Blower | ★★★★★ | Air injection (pipes) | Raceways, biofilters, large pond systems | High |
B. Chemical Oxygen Releasers
Common types:
- Sodium Percarbonate: Fast release, emergency use
- Calcium Peroxide: Slow and long-lasting
- Sodium Peroxide: High oxygen, but sensitive to moisture
- Hydrogen Peroxide: Immediate effect, needs careful dosing
- Surfactants: Improves gas exchange, indirectly boosts oxygenation
Note: These products require dry storage and low metal content in media.
C. Biological Oxygenation
Promote controlled algae growth through light management and fertilization. Algae generate abundant oxygen via photosynthesis.
D. Reducing Oxygen Consumption
- Remove excess organics: via sludge cleaning, water exchange
- Apply water conditioners: to inactivate nitrogen-based compounds
- Control stocking density and algae levels
4. Final Thoughts: Dissolved Oxygen = Productivity
DO isn’t just another parameter — it’s a determinant of profitability. Low DO leads to floating shrimp, feed refusal, poor protein assimilation, high FCR, and disease susceptibility.
Studies show that DO >5 mg/L supports nearly 50% better feed efficiency than conditions with long-term DO <2.7 mg/L.
Therefore, every measure taken to improve DO — especially through mechanical aerators — is a direct investment in yield, survival, and production cost reduction.
