8. Open Aquarium (OPENAQ)

The ecosystem in a sealed aquarium illustrates basic principles of ecosystem operation. When light is shining, there is production of oxygen and organic matter that are used by consumers which in turn regenerate the nutrient raw materials N (including carbon-dioxide) that are recycled back to plant use. Section II-7 (Figure II-8) showed the way organic matter and nutrients go up and down with alternating periods of light and dark.

The model OPENAQ diagrammed in Figure III-9 has more detail showing the behavior of oxygen. Most sealed aquariums have an air space above the water. Oxygen diffuses back and forth between the gas phase and the dissolved oxygen state within the water. When oxygen is supersaturated it diffuses or bubbles into the gas phase. When respiratory consumption uses up dissolved oxygen at night, it is somewhat replenished by diffusion in from the air space.

The computer simulation graphs in Figure III-9 include the rate of production P, rate of respiration R and the net production Pnet that is the difference between the two. Net production is positive in the daytime and negative at night. As a result the oxygen goes up and down. This is a good model to use while measuring the dissolved oxygen at the course of a day.

Examples of Sealed Microcosms

Closed aquariums may be found in the school rooms of the world by the millions. Some grow in bright light; some have animals; some contain salt water ecosystems. Some have been observed working for decades although permanently sealed. Because the small contained ecosystems have many of the features of larger ecosystems they are sometimes called it microcosms.

"What if" Experiments

  1. What would be the effect of adding a large quantity of organic matter. Set initial value of Q=10,000.
    What will be the effect of this on the relative balance of production and respiration?

  2. What would happen if light were turned on continuously?
    Why would production of organic matter and oxygen only increase temporarily?

  3. What would be the effect of adding a large quantity of inorganic nutrients N? Set TN=50. Does the excess of one kind of metabolism continue? When metabolic processes are regulated by ecosystem feedbacks so as to restore a balance of processes, the process is called homeostasis.

  4. The rate of diffusion exchange between air and water depends on the diffusion coefficient K7 in Figure III-9a. This diffusion depends on how fast the waters are being stirred. What is the effect of reducing the diffusion coefficient to one tenth of its present value? This might occur if plastic was floated on the water surface.


Howard T. Odum* and Elisabeth C. Odum+
* Dept. of Environmental Engineering Sciences, UF
+ Santa Fe Community College, Gainesville

Center for Environmental Policy, 424 Black Hall
University of Florida, Gainesville, FL, 32611
Copyright 1994

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