index Drain Logistic Compete Coop Exclus Interact Oscillat Preypred Netprod
Openaq Pulse Saltmrsh Prairie Fire Lagoon Climax Species Specarea

Open Aquarium


The system is an aquarium open to air. This model illustrates basic principles of ecosystem operation. When light is shining, there is production of oxigen and organic matter that are used by consumers which in turn regenerate the nutrient materials N (including carbon dioxide) that are recycled back to to plant use.

Oxygen diffuses back and forth between the gas phase in the air and the dissolved oxygen state within the water.

When respiratory consumption uses up dissolved oxygen at night, it is somewhat replenished by diffusion in from the air space.


  • Net production is positive in the daytime and negative at night. Asresult the oxygen goes up and down
  • This is a good model to use while mesuring the dissolved oxygen during the course of the day.
  • Diagram
    S = Solar energy
    N = Nutrients
    X = Oxygen
    Q = Organic matter
    P = Production
    R = Respiration
    R = residual solar energy
    NT = Total nutrients
    F = proportion of nutrients in Q
    XS= Oxygen saturation concentration 
    Z = depth of aquarium
    K = interaction coefficients 
    DQ, DX, DT = increments
    T = time
      S = SIN(T / 3.78)
      N = ((NT - F * Q) / Z)
      R = S / (1 + K0 * N)
      DQ = K2 * R * N - K4 * X * Q
      DX = K1 * R * N - K3 * X * Q + K5 * (1 - X / XS)
      Q = Q + DQ * DT 
      X = X + DX * DT / Z
      P = K1 * R * N
      R = K3 * X * Q
      T = T + DT
    The graph shows the changes of S (sun), N (Nutrients), X (Oxygen), DQ(net production), P (Productio), R (Respiration) over a period of time of seven days. 
    Source code:
    "What if" Experiments:
  • What would be the effect of adding a large quantity of organic matter. Set initial value of Q =10000. What will be the effect of this on the relative balance production and respiration?
  • What would happen if light were turned on continuously. Why would production of organic matter and oxygen will increase temporarily?
  • 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 feedback so as to restore a balance of processes, the process is called homeostasis.
  • The rate of diffusion exchange between air and water depends on the diffusion coeficient K5 in Figure. This diffusion depends on how fast the waters are being stirred. What is the effect pof reducing the diffusion coefficient to one tenth of its present value? This might occur if plastic was floated on the water surface.