Animal Science

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Unit

• Animal Science

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Problem Area

• Aquaculture

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Lesson

• Factors Affecting Dissolved Oxygen in Water

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Student Learning Objectives

• 1. Explain the daily cycle of dissolved oxygen
  present in ponds.
• 2. Identify factors which affect the level of
  dissolved oxygen present in water.
• 3. Explain how aquatic animals extract oxygen
  from water.
• 4. Describe management practices for maintaining
  desirable oxygen levels in pond culture.
• 5. Describe management practices for maintaining
  desirable oxygen levels in recirculating tank
  systems used for aquaculture.

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Terms

• Atmospheric pressure
• Biochemical oxygen demand
• Cellular respiration
• Diffusion
• Gills
• Mechanical aeration
• Photosynthesis
• Phytoplankton
• Recirculating tank system
• Turn-over

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What is the daily cycle of dissolved oxygen in
ponds?

• Generally ponds show depleted oxygen levels at
  dawn and increased levels in the afternoon.
• A. Dissolved oxygen in water comes from two
  sources photosynthesis or diffusion.
• 1. Photosynthesis by aquatic plants is the more
  important source of dissolved oxygen in ponds.
  They consume carbon dioxide and water to produce
  carbohydrate and release oxygen into the water.
• 2. Factors that influence plant growthlight
  intensity, herbicide use, temperature,
  etc.ultimately affect the level of dissolved
  oxygen in the water. Active photosynthetic
  activity during daylight hours accounts for the
  peak levels of dissolved oxygen in the afternoon.

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• B. Cellular respiration is the primary factor for
  the low levels of dissolved oxygen at dawn.
  During cellular respiration organisms use oxygen
  to metabolize food energy. At night
  photosynthesis ceases yielding more oxygen being
  used than is being produced.
• C. Under typical conditions photosynthesis and
  diffusion is sufficient to meet the bio-chemical
  oxygen demandthe amount of oxygen needed by
  aquacrops, plants and microorganisms. When oxygen
  demand exceeds the dissolved oxygen available
  aquacrops become stressed and growth slows or
  worse yet a kill-off occurs.

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What factors affect the level of dissolved oxygen
present in water?

• A. Water at higher temperatures is less able to
  hold dissolved oxygen. Therefore in summer months
  when water temperature increases the natural
  ability for water to hold dissolved oxygen is
  decreased.
• B. Excessive organic matter as a result of
  uneaten food, runoff or other sources consumes
  oxygen during decomposition. This consumption of
  oxygen increases demands for oxygen limiting the
  availability for aquacrops.
• C. Freshwater is able to dissolve more oxygen
  than salt water.

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• D. Atmospheric pressure is defined as the force
  exerted by the weight of air. As atmospheric
  pressure increases more oxygen is diffused into
  the water.
• E. Seasonal temperature changes affect the oxygen
  level in ponds as well. A phenomenon known as
  turn-over causes oxygen levels to change through
  the stratified temperature zones of a pond. As
  surface water warms in spring its density becomes
  greatest at 39 F. When this happens the surface
  water mixes into the lower level diluting the
  oxygen that was in the surface water and
  increasing the need for oxygen due to increased
  decomposition of organic compounds due to the
  warmer water. This occurs twice annually usually
  in the late spring and late fall.

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How do aquatic animals extract oxygen from water?

• Gills are blood-filled structures where gas
  exchange occurs in fish.
• A. Large quantities of oxygenated water flow
  across the gills in fish as it swims. Through
  diffusion, the movement of a substance from high
  concentration to low concentration, oxygen is
  taken into the gills. As water passes over the
  gills the oxygen level of the blood in the gills
  is low due to the increased levels of CO2 while
  the oxygen level in the water is greater causing
  diffusion to occur. Oxygen is then entered into
  the bloodstream via capillaries.

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What management techniques are used to maintain
desirable dissolvedoxygen levels in aquaculture
ponds?

• A. Pond design is the initial step in providing
  desired levels of dissolved oxygen. Ponds should
  not be built on land that has naturally high
  levels of decomposing organic matter. This would
  consume oxygen that is needed by the aquacrops.
  Also the location of the pond in relation to
  deciduous trees that could shed leaves into the
  water causing an increase in the demand for
  oxygen is important to consider.

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• B. Aquatic weeds can cause catastrophic problems
  in ponds. They may cover the surface of a pond
  prohibiting light to penetrate, reducing the
  photosynthesis carried out by phytoplankton,
  small floating plants. Also as the weeds die,
  decomposition of the organic matter consumes
  oxygen.
• C. Mechanical aeration is an artificial means of
  using mechanical energy to add oxygen to water.
  Mechanical aeration can include the use of air
  pumps, water pumps, or paddles.

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What practices are used for maintaining desirable
oxygen levels in recirculating tank systems used
for aquaculture?

• A recirculating tank system is one in which fish
  are grown in tanks or vats while water is
  constantly recycled.
• A. To manage dissolved oxygen in recirculating
  aquaculture systems the first step is provide
  mechanical aeration. This is most often achieved
  through a bubbler system. Atmospheric air is
  pumped through a series of hoses into an air
  diffuser which causes tiny air bubbles to form.

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• B. Managing the amount of feed provided is
  another important practice. Producers try to
  eliminate feeding excessive amounts of feed that
  use oxygen when microbial action is necessary to
  decompose them.
• C. Careful monitoring of dissolved oxygen levels
  is important to diagnose problems. Chemical
  oxygen tests are an affordable way to test. A
  more costly method is the use of an automated
  oxygen probe. When a problem is observed the
  producer can then determine the best course of
  action to correct the situation.

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Review/Summary

• What is the daily cycle of dissolved oxygen in
  ponds?
• What factors affect the level of dissolved oxygen
  present in water?
• How do aquatic animals extract oxygen from water?
• What management techniques are used to maintain
  desirable dissolved oxygen levels in aquaculture
  ponds?
• What practices are used for maintaining desirable
  oxygen levels in recirculating tank systems used
  for aquaculture?