Lesson Factors Affecting Dissolved Oxygen in Water
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.
Terms Atmospheric pressure Biochemical oxygen demand Cellular respiration Diffusion Gills Mechanical aeration Photosynthesis Phytoplankton Recirculating tank system Turn-over
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.
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.
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.
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.
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.
What management techniques are used to maintain desirable dissolved oxygen 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.
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.
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.
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.
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?