Learning target: I can use a trophic pyramid model to predict how competing solutions would change a green, algae-choked reservoir, evaluate them against criteria and constraints, choose one, and plan how to test and improve it.
You are advising the town of Alder Springs. Read the problem, the survey data and the criteria and constraints. Study the four options and the reservoir's trophic pyramid. Use the pyramid to predict what each option would do, score each option in the decision matrix, choose one option or a combination, and justify your choice with the model. Then plan the test that would show whether your fix is working and say what you would change if it is not. All numbers in this scenario are teaching estimates, not measurements from a real lake.
You will need: This worksheet, Pencil, Calculator
Alder Reservoir is a 40-hectare lake that supplies drinking water to the town of Alder Springs and is used for swimming and fishing. For years a leaking sewer line fed nutrients into the lake. Algae grew so thickly that the water turned pea-green, and each late summer dead algae sinking to the bottom were broken down by decomposers that used up so much oxygen that fish died. The town repaired the sewer line in 2024, cutting the nutrients flowing in by about 70 percent. One year later the water is still green. Nutrients already stored in the bottom mud keep feeding the algae, and the lake's food web has not changed: it is full of small plankton-eating fish called shiners and has very few predatory fish.
| Measurement | 2024 (sewer leaking) | 2025 (sewer repaired) |
|---|---|---|
| Nutrient input from land (kg of phosphorus per year) | 900 | 270 |
| Algae cells per drop of water | 1800 | 1500 |
| Zooplankton per liter | 12 | 15 |
| Shiners (plankton-eating fish) per net haul | 140 | 150 |
| Largemouth bass (predatory fish) per net haul | 2 | 2 |
| Secchi depth in August (m) | 0.3 | 0.4 |
| Late-summer fish kill | yes | yes, smaller |
| Option | What is done | Cost over 3 years | How fast it acts | Known side effects |
|---|---|---|---|---|
| A: Stock predatory fish | Release 20,000 young largemouth bass in year 1 and 10,000 in year 2 | $12,000 | Bass need 2 to 3 years to grow large enough to eat many shiners | Fishing improves; bass also eat some zooplankton while small |
| B: Net out shiners | Hire a crew to net and remove about half the shiners each spring for three years | $27,000 ($9,000 per year) | Shiner numbers drop within weeks of each netting | Shiners breed back quickly if nothing eats them; nutrients leave the lake in the removed fish |
| C: Algaecide | Spray a copper-based algaecide twice each summer | $18,000 ($6,000 per year) | Algae die within days | Also kills zooplankton; dead algae sink and use up oxygen as they decay; copper is a chemical |
| D: Add zooplankton | Buy and release water fleas each spring | $12,000 ($4,000 per year) | Zooplankton rise for a few weeks | Shiners eat most of the added zooplankton within a month |
| Shiners | Zooplankton | Algae | Lasting or temporary? |
|---|---|---|---|
| Option A | |||
| Option B | |||
| Option C | |||
| Option D |
| Meets cost limit? (yes/no) | No chemicals? (yes/no) | Likely clear water by year 3? (0-2) | Avoids fish kills? (0-2) | Lasting effect? (0-2) | Total score |
|---|---|---|---|---|---|
| Option A | |||||
| Option B | |||||
| Option C | |||||
| Option D |
To predict an option, write the four levels in a column: bass, shiners, zooplankton, algae. Mark the level the option changes first with a big arrow. Then move one level at a time: if a level goes down, the level it eats goes up (less eating) and the level that eats it goes down (less food). The first step is a direct effect; every step after that is indirect. Stop when you reach the algae.
Write a one-page proposal to the town council for your recommendation. Include a labeled pyramid showing the lake now and the lake after your fix, a three-year budget, the August measurements you will report, and the single result that would make you change the plan. Then add a paragraph on one risk the model cannot predict, such as a warm summer, a new fish species arriving, or nutrients still stored in the mud, and how you would watch for it.