A
Comprehensive Guide to Understanding Dissolved Oxygen in the context of
Aquaculture farms?
OR
What Is
Dissolved Oxygen and Why Is It So Important in Fish and Shrimp Farming?
Updated
for 2026
Fish and
shrimp live in water, but they still need oxygen just as land animals do. They
cannot normally use the oxygen chemically bound inside a water molecule, H₂O.
Instead, they depend on free molecular oxygen, O₂, dissolved in the water.
This
dissolved oxygen, commonly called DO, is one of the most important
water-quality parameters in aquaculture. It directly affects respiration,
feeding, growth, stress, survival and the overall health of fish and shrimp.
Quick Summary
Dissolved
oxygen is molecular oxygen, O₂, physically dissolved in water. Fish and shrimp absorb this
oxygen through their gills. DO enters ponds naturally from the atmosphere and
photosynthesis, or artificially through aeration. It is continuously consumed
by fish, shrimp, microorganisms and decomposition, so maintaining adequate
DO—particularly at night and before sunrise—is essential in aquaculture.
What exactly is dissolved oxygen
in water?
Dissolved
oxygen is ordinary oxygen gas, O₂, present as individual molecules within
water.
This is
an important distinction.
A
molecule of water is:
H₂O
But fish
and shrimp do not normally separate the oxygen atom from H₂O to breathe.
The
oxygen they need is:
O₂
—the same
molecular oxygen present in the atmosphere.
When air
comes into contact with water, some oxygen molecules from the air move into the
water and become dispersed among the water molecules.
The
oxygen is not converted into another chemical compound. It remains O₂, but is
now physically dissolved in water.
A useful
comparison is carbon dioxide dissolved in a soft drink. You may not be able to
see the dissolved gas while the bottle is closed, but it is present throughout
the liquid.
The same
principle applies to oxygen dissolved in pond water, although at much lower
concentrations.
IMAGE 1 –
WHAT DOES “DISSOLVED OXYGEN” ACTUALLY MEAN?
Why is dissolved oxygen measured
in mg/L?
DO is
normally expressed as milligrams of oxygen in one litre of water—mg/L.
For
example:
5 mg/L DO
means that approximately 5 milligrams of molecular oxygen are dissolved in
every litre of water.
That
sounds like a very small quantity—and it is.
One litre
of water weighs approximately one kilogram, yet only a few milligrams of
dissolved oxygen may be available to the aquatic animals living in it.
This is
one reason fish and shrimp can become oxygen-stressed relatively quickly when
oxygen demand suddenly increases or an aeration system fails.
DO can
also be expressed as percentage saturation.
Percentage
saturation tells us how much oxygen the water contains compared with the
maximum equilibrium amount it could hold under the existing temperature,
salinity and atmospheric pressure.
For
normal farm management, however, mg/L is usually the most practical number
for farmers to understand and monitor.
How do shrimp actually breathe
dissolved oxygen?
Shrimp
absorb dissolved oxygen from the surrounding water primarily through their
gills.
Water
passes across the gill surfaces. Oxygen dissolved in that water moves across
the thin gill membrane into the shrimp's circulatory system.
This
movement takes place largely by diffusion—oxygen moves from an area
where its concentration or partial pressure is higher toward an area where it
is lower.
Once
absorbed, oxygen is carried to the tissues and used by cells to release energy
from nutrients.
Shrimp
require this energy for:
- Movement
- Feeding
- Digestion
- Growth
- Moulting
- Osmoregulation
- Immune function
- Normal cellular activity
When DO
becomes too low, the shrimp cannot absorb oxygen quickly enough to meet its metabolic
demand.
This is
why low DO can affect feeding, growth and stress resistance even before the
oxygen level becomes low enough to cause mortality.
IMAGE 2 –
HOW DOES A SHRIMP BREATHE UNDERWATER?
How do fish use dissolved oxygen?
Most fish
obtain oxygen by continuously moving water across their gills.
Fish
gills contain a very large surface area and a very thin separation between the
surrounding water and the blood.
As
oxygen-rich water passes over the gills, oxygen diffuses from the water into the
blood and is carried throughout the body.
At the
same time, carbon dioxide produced by metabolism moves in the opposite
direction—from the fish into the surrounding water.
Different
fish species have different oxygen requirements and different tolerances to low
DO.
The
important point is that fish and shrimp depend on oxygen already dissolved
in the water around them.
They
cannot simply breathe the air above the pond when dissolved oxygen becomes
inadequate.
Where does oxygen naturally come from
in ponds, lakes and rivers?
There are
two major natural sources of dissolved oxygen.
1. Oxygen from the atmosphere
Whenever
air and water meet, oxygen can move across the water surface.
If the
water contains less oxygen than its equilibrium concentration with the
atmosphere, oxygen tends to move:
Air →
Water
Wind,
waves, rain, currents and turbulence continuously renew the air-water surface
and can increase this transfer.
In
completely still water, oxygen enters relatively slowly because only the upper
surface is directly exposed to the atmosphere.
This is
one reason natural water movement helps maintain oxygen levels.
2. Oxygen from photosynthesis
Algae,
phytoplankton and aquatic plants release oxygen during photosynthesis.
During
daylight they use sunlight to convert carbon dioxide and water into organic
matter, releasing oxygen in the process.
In
productive aquaculture ponds, photosynthesis can produce a significant quantity
of oxygen during the day.
But there
is an important limitation:
Photosynthesis
stops when there is insufficient light.
After
sunset, oxygen production from photosynthesis falls dramatically or stops.
Respiration,
however, continues.
Why is DO usually high in the
afternoon and low before sunrise?
DO
generally increases during daylight because photosynthesis produces oxygen and
then falls throughout the night because oxygen consumption continues after
photosynthesis stops.
A
simplified daily pattern looks like this:
|
Time |
Typical DO Situation |
|
Sunrise |
Often
near the daily minimum |
|
Morning |
Photosynthesis
starts and DO begins rising |
|
Afternoon |
DO may
reach the daily maximum |
|
Sunset |
Photosynthetic
oxygen production stops |
|
Midnight |
Fish,
shrimp, algae and bacteria continue consuming oxygen |
|
3–6 AM |
Often
the most critical DO period |
|
Sunrise |
Photosynthesis
gradually begins again |
A pond
might show 6, 7 or even 8 mg/L DO in the afternoon, yet fall toward much
lower levels before sunrise.
This
leads to a very practical rule for farmers:
Don’t judge aeration at 4 PM. Judge it at 4 AM.
The real
test of an aeration system is not how much oxygen happens to be present during
a sunny afternoon.
It is
whether the system can maintain sufficient DO during the hours when oxygen
demand continues but natural oxygen production has largely stopped.
IMAGE 3 –
THE DAILY DO CYCLE: WHY 4 AM MATTERS
Why does dissolved oxygen reduce in pond water?
Dissolved
oxygen is continuously being consumed by biological and chemical processes in
the pond.
Several
different processes contribute to oxygen demand.
Fish and shrimp consume oxygen
Every
fish and shrimp consumes oxygen continuously.
As the
animals grow, the total biomass in the pond increases.
A pond
immediately after stocking does not have the same oxygen demand as the same
pond near harvest carrying several tonnes of fish or shrimp.
Feeding increases oxygen demand
Feed
affects oxygen demand in several ways.
Fish and
shrimp use more oxygen during digestion and metabolism after feeding.
Uneaten
feed and faecal matter also become organic material that microorganisms must
decompose.
That
decomposition consumes additional oxygen.
Bacteria consume oxygen
Bacteria
perform essential functions in aquaculture ponds.
They
break down organic matter and participate in processes involving ammonia and
other nitrogen compounds.
Many of
these processes require oxygen.
Therefore,
higher organic loading and increased bacterial activity can significantly
increase total pond oxygen demand.
Algae also consume oxygen
Algae
produce oxygen during daylight through photosynthesis.
But algae
are living organisms and they also respire.
During
the night they consume oxygen instead of producing it.
A dense
algal bloom can therefore produce very high afternoon DO while simultaneously
creating substantial oxygen demand overnight.
Pond-bottom material consumes oxygen
Organic
matter accumulates in pond sediments.
Bacteria
and other organisms decompose this material, creating what is often called sediment
oxygen demand.
In older,
heavily fed or intensively stocked ponds, this can become an important part of
total oxygen consumption.
Can dissolved oxygen escape from
the water again?
Yes.
Oxygen transfer between air and water works in both directions.
Water can
only maintain a certain equilibrium concentration of oxygen under particular
environmental conditions.
If the
water contains less oxygen than the equilibrium level, oxygen tends to move:
Air →
Water
If the
water becomes supersaturated with oxygen—for example after very strong
afternoon photosynthesis—oxygen can move:
Water →
Air
Therefore,
dissolved oxygen is not permanently stored in the pond.
There is
a continuous exchange:
Air ⇄ Water
This is
why a very high afternoon DO level cannot simply be considered an oxygen
reserve that will remain available throughout the entire night.
Some
oxygen will be consumed, and some may also escape back into the atmosphere.
What determines whether pond DO
rises or falls?
A simple
way of understanding DO is as a balance:
Change in DO = Oxygen entering the water − Oxygen
consumed or lost
Oxygen enters
through:
- Photosynthesis
- Natural atmospheric transfer
- Artificial aeration
Oxygen is
consumed or lost through:
- Fish respiration
- Shrimp respiration
- Plankton respiration
- Bacterial activity
- Decomposition of organic
material
- Pond-bottom oxygen demand
- Chemical oxidation
- Escape to the atmosphere
when water is supersaturated
If oxygen
supply is greater than oxygen consumption, DO rises.
If
consumption becomes greater than supply, DO falls.
IMAGE 4 –
THE POND OXYGEN BALANCE
How does artificial aeration put
oxygen into water?
Aeration
equipment increases oxygen transfer by increasing contact between atmospheric
air and pond water.
Different
types of aerators achieve this in different ways.
A
paddlewheel aerator splashes and agitates water at the surface.
Other
mechanical aerators may spray water through air or draw atmospheric air into
moving water.
Diffused
aeration works differently.
A blower
supplies air through submerged pipes, tubes or diffusers. The air is released
underwater as bubbles.
As each
bubble rises through the water:
Air
inside the bubble → Bubble surface → Dissolved oxygen in water
Oxygen
moves from the bubble into the surrounding water.
The
amount transferred depends on factors such as:
- Bubble size
- Total bubble surface area
- Water depth
- Bubble contact time
- Airflow
- Existing DO concentration
- Temperature
- Salinity
Why do fine bubbles help oxygen
transfer?
For the
same volume of air, many fine bubbles provide a much greater total air-water
surface area than a few large bubbles.
This additional
surface area gives oxygen more opportunity to move from the air into the water.
Fine
bubbles also tend to rise more slowly than large bubbles, increasing the amount
of time they remain underwater.
Both
factors can improve oxygen-transfer efficiency.
However,
the objective should not simply be to produce the smallest possible bubble.
If pores
become too restrictive, excessive blower pressure may be required.
A
practical fine-bubble aeration system therefore needs to balance:
Fine Bubbles + Good Airflow + Low Pressure +
Adequate Contact Time
The
engineering objective is efficient oxygen transfer—not simply
impressive-looking bubbles.
What is SOTR and why is it used
in aeration?
SOTR, or
Standard Oxygen Transfer Rate, is a technical measure of how much oxygen an
aeration system can transfer into water under defined standard conditions.
SOTR is
useful to engineers and equipment designers because it helps compare aeration
technologies under controlled conditions.
For the
farmer, however, the practical result is much simpler:
DO measured in mg/L
The
farmer does not normally need to calculate SOTR every morning.
The
farmer needs to know whether the pond is maintaining adequate DO, particularly
when oxygen demand is highest.
How does water temperature affect
dissolved oxygen?
Warm
water can hold less dissolved oxygen than cool water.
As water
temperature rises, oxygen becomes less soluble.
At the
same time, warmer temperatures often increase the metabolism of fish, shrimp
and microorganisms.
This can create
an important double effect:
Higher temperature = Lower oxygen-holding capacity
+ Higher oxygen demand
This is
why aeration becomes particularly important during hot weather and in intensive
tropical aquaculture.
How does salinity affect
dissolved oxygen?
As
salinity increases, the amount of oxygen that water can hold decreases.
Dissolved
salts reduce oxygen solubility.
This is
especially relevant in shrimp farming because shrimp may be cultured in
freshwater, brackish water or highly saline environments.
At the
same temperature, saline water normally holds less dissolved oxygen than
freshwater.
Therefore,
DO conditions should always be interpreted together with salinity and
temperature.
What other factors affect oxygen dissolution?
Several physical
conditions influence how much oxygen water can hold and how rapidly oxygen
transfers into it.
Temperature
Colder
water generally holds more oxygen than warmer water.
Salinity
Freshwater
can generally hold more dissolved oxygen than saline water at the same
temperature.
Atmospheric pressure
Higher
atmospheric pressure increases the amount of oxygen available to dissolve.
Altitude
At higher
altitude, atmospheric pressure is lower, so the equilibrium concentration of
dissolved oxygen is also lower.
Existing DO level
If DO is
far below saturation, there is a stronger driving force for oxygen to move into
the water.
As DO
approaches saturation, the driving force becomes smaller.
Water movement
Turbulence,
mixing and circulation continuously create fresh air-water contact and can
increase oxygen transfer.
Water depth
For
fine-bubble systems, greater depth can increase bubble contact time, although
the blower must also overcome greater hydrostatic pressure.
What does oxygen saturation mean?
Oxygen
saturation describes how much oxygen the water contains compared with the
equilibrium amount it could hold under the existing conditions.
If water
is at approximately 100% saturation, it contains roughly the equilibrium
amount of oxygen expected at that temperature, salinity and atmospheric
pressure.
If it is
at 50% saturation, it contains roughly half that equilibrium amount.
Water can
sometimes exceed 100% saturation, particularly during periods of intense
photosynthesis.
This is
called supersaturation.
The concept
is useful because the same DO concentration can represent different saturation
conditions at different temperatures and salinities.
Why can a well-aerated pond still
have low DO?
Because
oxygen supply is only one side of the equation. Oxygen demand can sometimes
increase even faster.
A pond
may have substantial aeration but still experience low DO because of:
- Increased fish or shrimp
biomass
- Heavy feeding
- High water temperature
- High bacterial activity
- Large quantities of organic
material
- Dense phytoplankton
respiration at night
- Biofloc oxygen demand
- Poor circulation
- Dead zones in the pond
- Sudden algal die-off
- Aeration equipment operating
below its intended performance
This is
why farmers should ultimately measure the water rather than judge aeration
simply from visible bubbles.
Is 4 mg/L DO enough for shrimp
and fish?
Around 4
mg/L is a useful practical reference point, but it should not be treated as a
universal minimum for every species or farming condition.
Many
aquaculture farmers aim to maintain DO above approximately 4 mg/L, with
higher levels providing a better safety margin in intensive culture.
Requirements
vary according to:
- Species
- Animal size
- Biomass
- Water temperature
- Salinity
- Feeding rate
- Stocking density
- Culture system
A farm
should therefore avoid operating permanently close to the lowest tolerable DO
level.
The
better objective is to maintain sufficient oxygen with a reasonable safety
margin, particularly before sunrise and during periods of maximum oxygen
demand.
How can dissolved oxygen be measured?
Several
methods are available.
Portable DO meters
These are
commonly used for routine pond management.
Modern DO
meters may use:
- Electrochemical sensors
- Optical or luminescent
sensors
They
normally display DO directly in mg/L and may also show percentage saturation
and temperature.
Chemical measurement
DO can
also be measured using chemical methods such as Winkler titration.
These
methods can be accurate but are less convenient for frequent pond-side
measurements.
Continuous DO monitoring
Larger or
more intensive farms may use permanently installed probes that continuously
monitor DO.
These
systems can be connected to:
- Data loggers
- Alarms
- Mobile monitoring systems
- Automatic aerator controls
For most farmers,
however, the important question is not the brand or technology of the DO meter.
It is:
When and where are you measuring DO?
When should farmers measure DO?
Useful
measurement times include:
- Before sunrise
- During the late-night or
early-morning period
- After heavy feeding
- During very hot weather
- After several cloudy days
- Following heavy rain
- During sudden changes in an
algal bloom
- When stocking biomass
becomes high
- Whenever fish or shrimp
behaviour suggests oxygen stress
Measurements
at different locations and depths can also reveal areas of poor circulation or
local oxygen shortage.
A single
afternoon measurement may provide useful information, but it does not
necessarily tell the farmer whether oxygen supply will remain adequate
throughout the night.
How does diffused aeration compare with paddlewheel
aeration?
Diffused
aeration and paddlewheel aeration both transfer oxygen into aquaculture water,
but they work in fundamentally different ways.
A
paddlewheel brings water to the air.
The
paddles lift, splash and agitate pond water at the surface. This creates
turbulence and exposes more water to atmospheric oxygen.
Paddlewheels
are also particularly effective at producing directional water circulation.
Diffused
aeration takes the opposite approach.
It brings
air into the water.
A blower
sends atmospheric air through submerged aeration tubes or diffusers. The air is
released underwater as bubbles and transfers oxygen while rising through the
water column.
With
fine-bubble aeration, the same air is divided into many smaller bubbles,
creating a large total air-water interface and increased contact time.
|
Feature |
Fine-Bubble Diffused Aeration |
Paddlewheel Aeration |
|
Basic
principle |
Takes
air into the water |
Brings
water into contact with air |
|
Oxygen-transfer
area |
Along
the rising bubble path |
Mainly
at the agitated surface |
|
Air-water
contact |
Large
with fine bubbles |
Created
through splashing and turbulence |
|
Water
movement |
Vertical
circulation from bubble plume |
Strong
horizontal/surface circulation |
|
Equipment
inside pond |
Mainly
diffuser/tubing |
Motor-driven
paddle assembly |
|
Mechanical
moving parts in pond |
Few |
More |
|
Particularly
useful for |
Distributed
oxygen transfer |
Strong
directional circulation |
Neither
technology needs to perform every function better than the other.
In some
aquaculture farms, a combination can be useful—fine-bubble diffusion for
distributed oxygen transfer and paddlewheels where strong directional
circulation is required.
The more
important question is:
How much useful oxygen is being transferred into
the water for the energy being consumed?
IMAGE 5 –
TWO DIFFERENT WAYS TO PUT OXYGEN INTO POND WATER
Why does AirOxi use fine-bubble
diffused aeration?
AirOxi
aeration systems are designed around the principle of creating distributed
air-water contact below the pond surface.
Air from
a blower passes through the porous AirOxi aeration tube and is released
underwater as a large number of bubbles.
The
objective is to combine:
Fine Bubbles + Easy Airflow + Low Operating
Pressure + Good Air-Water Contact
The
new-generation AirOxi aeration tube has been further developed by benchmarking
against leading international aeration tubes, with particular attention to:
- Fine bubble formation
- Low airflow resistance
- Operation at relatively low
air pressure
- Consistent airflow
- Reduced tendency toward pore
blockage
- Longer intervals between
cleaning
But
regardless of aerator design, AirOxi recommends judging an aeration system by
its practical result:
What dissolved oxygen is the pond maintaining when
oxygen demand is highest?
How much aeration does a fish or
shrimp farm need?
Aeration
requirement cannot be determined from pond area alone.
Two ponds
of exactly the same size can have very different oxygen requirements.
Aeration
demand depends on factors including:
- Species
- Fish or shrimp biomass
- Stocking density
- Feed rate
- Pond volume
- Water depth
- Temperature
- Salinity
- Culture method
- Biofloc or microbial load
- Target DO
- Existing aeration and
circulation equipment
The
requirement also changes during the crop.
A newly
stocked shrimp pond has a much lower biomass than the same pond close to
harvest.
As
biomass and feeding increase, oxygen demand usually increases substantially.
Calculate Your Approximate Aeration Requirement
Farmers
planning a new aeration system or reviewing an existing installation can use
the AirOxi Aeration Calculator:
https://www.airoxi.com/calculator
The
calculator provides a practical starting point for estimating aeration
requirements according to the aquaculture application.
Farmers
can also explore:
AirOxi
Aeration Products:
https://www.airoxi.com/products
AirOxi
Aquaculture Knowledge Centre:
https://www.airoxi.com/blog
What is the easiest way to
understand dissolved oxygen?
Think of
the pond as having an oxygen bank account.
Oxygen is continuously deposited by:
Atmospheric
Transfer + Photosynthesis + Artificial Aeration
Oxygen is continuously withdrawn by:
Fish +
Shrimp + Algae + Bacteria + Decomposition + Pond Bottom
And the
amount of oxygen that the water can hold is affected by:
Temperature
+ Salinity + Atmospheric Pressure
During a
sunny afternoon, oxygen deposits may exceed withdrawals and DO rises.
After
sunset, photosynthesis stops while nearly all the withdrawals continue.
The
balance therefore begins to fall.
This is
why dissolved oxygen is not something that a farmer adds once and then stores
permanently in the pond.
It is a continuously
changing balance between oxygen supply and oxygen demand.
So when
checking an aquaculture pond, the most useful question is not:
“How many
bubbles can I see?”
It is:
“What is my dissolved oxygen when the pond needs
oxygen the most?”
And that
leads to one simple rule worth remembering:
Don’t
judge aeration at 4 PM. Judge it at 4 AM.
