Dissolved Oxygen in the Aquatic Environment
Sufficient dissolved oxygen is necessary for good water quality.
Our atmosphere is 21 percent oxygen or 210,000 part per million (ppm), but seldom will a pond have more than 10 ppm oxygen dissolved in its' water.
Numerous scientific studies suggest that 4-5 ppm of DO is the minimum amount that will support a large, diverse fish population, and DO above 5 in a water body is healthy.
Just as we need air to breathe, aquatic organisms need DO to live.
Fish, invertebrates, bacteria, and plants all need DO for respiration.
Oxygen levels that remain below 1-2 mg/l for a few hours can result in large fish kills.
Photosynthesis and aeration are two common methods for oxygenating water bodies.
Aeration from a stream or by mechanical means and photosynthesis by phytoplankton and algae allow pathways for oxygen to get into the water.
The amount of dissolved oxygen is a measure of the biological activity of the water masses.
The difference between the physical concentration of oxygen in the water and the amount of oxygen used by aquatic organisms is called biological oxygen demand (BOD).
When BOD levels are high, dissolved oxygen levels decrease because the oxygen that is available in the water is being consumed.
Most oxygen deletions occur in the summer months because 1) warm water holds less dissolved oxygen than cool or cold water, and 2) because the waters oxygen demand is greater in warm water than in cold water.
Most aquatic organisms become more active during warm water periods and less active during cold water periods thus using more oxygen.
Additionally, when water visibility is limited to less than 12 inches due to phytoplankton density oxygen depletion becomes a concern.
These heavy or dense blooms use large amounts of DO at night and on very cloudy/overcast, windless days causing an oxygen depletion and possible fish kill.
Soon the plankton die-offs increase BOD because the remaining dissolved oxygen is consumed by aerobic bacteria and fungi in the process of decaying the dead algae and few live phytoplanktons remain to produce more oxygen.
When these plants die, they become food for bacteria, that in turn multiply and use large amounts of oxygen.
Raw sewage, non-point source pollution, agrichemicals, organic chemicals, and other debris emptied into the water can create increase in BOD by the increase action of bacteria and oxygen use while feeding on and decomposing the matter.
These actions thus lower the amount of DO in the water for other organisms and creating an increase of stress into the ecosystem.
A natural change in DO levels occur nearly every year and are known as temperature inversion or typically referred as "pond turnover".
This temperature stratification leads to oxygen stratification, with the warm surface water containing dissolved oxygen (and fish) while the deeper, cool water becomes depleted of oxygen because of decomposition and lack of sunlight for photosynthesis.
The inversion can create some concern, but will quickly balance out.
Treating of aquatic weeds with an herbicide during the summer can cause oxygen depletion.
The risk of oxygen depletion can be lowered by treating only part of the pond at a time.
Mechanical aeration usually can save fish during oxygen depletion.
Conclusion: Adequate dissolved oxygen is necessary for good water quality.
Dissolved oxygen is entirely essential for the survival of all aquatic organisms.
Dissolved oxygen above 5 mg/L (ppm) promotes a healthy aquatic ecosystem.
Our atmosphere is 21 percent oxygen or 210,000 part per million (ppm), but seldom will a pond have more than 10 ppm oxygen dissolved in its' water.
Numerous scientific studies suggest that 4-5 ppm of DO is the minimum amount that will support a large, diverse fish population, and DO above 5 in a water body is healthy.
Just as we need air to breathe, aquatic organisms need DO to live.
Fish, invertebrates, bacteria, and plants all need DO for respiration.
Oxygen levels that remain below 1-2 mg/l for a few hours can result in large fish kills.
Photosynthesis and aeration are two common methods for oxygenating water bodies.
Aeration from a stream or by mechanical means and photosynthesis by phytoplankton and algae allow pathways for oxygen to get into the water.
The amount of dissolved oxygen is a measure of the biological activity of the water masses.
The difference between the physical concentration of oxygen in the water and the amount of oxygen used by aquatic organisms is called biological oxygen demand (BOD).
When BOD levels are high, dissolved oxygen levels decrease because the oxygen that is available in the water is being consumed.
Most oxygen deletions occur in the summer months because 1) warm water holds less dissolved oxygen than cool or cold water, and 2) because the waters oxygen demand is greater in warm water than in cold water.
Most aquatic organisms become more active during warm water periods and less active during cold water periods thus using more oxygen.
Additionally, when water visibility is limited to less than 12 inches due to phytoplankton density oxygen depletion becomes a concern.
These heavy or dense blooms use large amounts of DO at night and on very cloudy/overcast, windless days causing an oxygen depletion and possible fish kill.
Soon the plankton die-offs increase BOD because the remaining dissolved oxygen is consumed by aerobic bacteria and fungi in the process of decaying the dead algae and few live phytoplanktons remain to produce more oxygen.
When these plants die, they become food for bacteria, that in turn multiply and use large amounts of oxygen.
Raw sewage, non-point source pollution, agrichemicals, organic chemicals, and other debris emptied into the water can create increase in BOD by the increase action of bacteria and oxygen use while feeding on and decomposing the matter.
These actions thus lower the amount of DO in the water for other organisms and creating an increase of stress into the ecosystem.
A natural change in DO levels occur nearly every year and are known as temperature inversion or typically referred as "pond turnover".
This temperature stratification leads to oxygen stratification, with the warm surface water containing dissolved oxygen (and fish) while the deeper, cool water becomes depleted of oxygen because of decomposition and lack of sunlight for photosynthesis.
The inversion can create some concern, but will quickly balance out.
Treating of aquatic weeds with an herbicide during the summer can cause oxygen depletion.
The risk of oxygen depletion can be lowered by treating only part of the pond at a time.
Mechanical aeration usually can save fish during oxygen depletion.
Conclusion: Adequate dissolved oxygen is necessary for good water quality.
Dissolved oxygen is entirely essential for the survival of all aquatic organisms.
Dissolved oxygen above 5 mg/L (ppm) promotes a healthy aquatic ecosystem.
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