Eutrophication: Introductory Exercises

Six exercises are provided for the student with an introductory interest in limnology. They may be completed with the model in Figure 1.

1. Total Carbon

Total carbon in lake is 55.25 mg per liter at the start of the simulation. Draw a time graph of how this variable would behave over a 100 year simulation if the nutrients flowing into the lake is held constant at 0.001 mg/yr. How much carbon would be in the lake at the end of 100 years?

2. Mix of Carbon

Around 94% of the carbon in the lake is stored in the nutrient pool at the start of the simulation. Do you expect this same percentage at the end of a 100 year simulation with a constant inflow of nutrients at 0.001 mg/yr?

3. Build and Verify:

Build the model in Figure 1 and verify the initial conditions in Figure 1.

4. Time Constants and DT

Review the Chapter 11 discussion of "time constants" and the selection of DT. What is the shortest time constant in Figure 1? How small should you set DT to ensure numerical accuracy? Anderson set his DT at 0.05 years. Do you think his value is sufficiently small to guarantee accurate results?

5. Simulate Natural Eutrophication

Run the model for 100 years to simulate the change in lake conditions due to the constant inflow of 0.001 mg/yr of nutrient. Does the simulation match your expectation from the 1st exercise? from the 2nd exercise? How does the simulation compare with Anderson's (1973, 131) ) results?

6. Two Feedback Loops

This diagram shows two feedback loops from the model in Figure 1. Complete the descripton of these loops by labeling each arrow as + or - and each loop as (+) or (-).