The Green Pond Problem
A school’s retention pond measures 40 m by 25 m with an average depth of 1.5 m. In July, fertilizer from a nearby lawn drains into the pond. The water turns bright green with algae. Students measure dissolved oxygen: 8 mg/L at 6:00 a.m. and 11 mg/L at 3:00 p.m. On the same morning, fish are gulping at the surface. The school considers three proposals: (A) spread 20 kg of barley straw, which releases compounds that slow algae growth; (B) plant a 30 m² stand of cattails where the runoff enters; (C) install a $4,000 fountain aerator that mixes air into the water. The principal wants a solution that works within one school year and costs less than $1,500. A student says, “Just add more fertilizer-plants need nutrients.” The class must use the data to judge each proposal and explain the morning oxygen drop.
1. Explain why dissolved oxygen is lower at 6:00 a.m. than at 3:00 p.m., using the roles of photosynthesis and cellular respiration in the pond.
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At 3:00 p.m., sunlight is strong, so algae and pond plants carry out photosynthesis and release oxygen faster than the pond organisms use it. That raises dissolved oxygen to 11 mg/L. At 6:00 a.m., there has been no sunlight for hours, so photosynthesis has stopped, but algae, plants, fish, and decomposers continue cellular respiration and consume oxygen. The result is the lower 8 mg/L reading. The fish gulping at the surface are responding to this low morning oxygen. The problem is worsened by the algae bloom: when algae die, bacteria decompose them and also use oxygen. Thus the pond has a daily oxygen cycle, and the early-morning low is a sign that the pond’s oxygen supply is under stress, especially as the bloom grows and then decays.
2. Evaluate proposals A, B, and C against the principal’s constraints and the pond data. Which is the strongest choice, and what is one weakness of that choice?
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Proposal C is outside the budget at $4,000, so it cannot be selected as stated even though aeration would directly raise oxygen. Proposal A costs less and may slow algae, but barley straw is a preventive treatment that works slowly and does not stop fertilizer from entering the pond; it also does not address the morning oxygen drop directly. Proposal B, planting cattails at the runoff entrance, fits the budget and targets the cause: cattails take up nutrients and slow runoff, so less fertilizer reaches the pond. That should reduce future blooms and the oxygen lows they cause. Its weakness is that plants need time to establish and may not fully block nutrients in the first season, and the cattails themselves must be harvested or nutrients may return when they decay. B is the strongest choice because it is affordable and treats the source, but it should be combined with reducing lawn fertilizer.
3. A classmate proposes adding fertilizer to “help the pond plants.” Predict the likely effect on dissolved oxygen and explain your prediction using the pond system.
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Adding fertilizer would likely lower dissolved oxygen, especially in early morning. Fertilizer adds nitrogen and phosphorus, which are limiting nutrients for algae. More nutrients would cause a larger algae bloom. During the day, photosynthesis might raise oxygen, but at night the extra algae respire and consume oxygen. When the bloom dies, decomposers break down the dead algae and use even more oxygen. The pond already shows a morning low of 8 mg/L and fish gulping, so the system has little margin. A larger bloom would increase the daily oxygen swing and could cause a fish kill. The classmate is focusing on the plants’ need for nutrients but ignoring that the pond is already receiving too many nutrients from lawn runoff.