The Fertilizer Runoff Puzzle
Two streams feed a lake. Stream A drains farmland and carries high nitrate, while Stream B drains forest and carries low nitrate. Over three summers, the lake's algae bloom earlier and larger each year. A monitoring team samples the lake weekly and finds that nitrate in the lake rises sharply after heavy rain in May but falls by July, even though fertilizer application continues. Dissolved oxygen in the deep water drops to 3 mg/L by August. The team must decide whether to focus management on fertilizer timing, stream B, or in-lake treatment, and wants a testable plan.
1. Formulate a hypothesis that explains why lake nitrate peaks after May rain but declines by July, and describe the evidence that would support it.
Read the worked answer
A strong hypothesis is that spring rain flushes nitrate from farmland into the lake, producing the May peak, and that summer algae growth then consumes that nitrate, drawing it down by July. This fits the timing: the peak follows heavy rain in the agricultural stream, and the bloom grows as nitrate is taken up. Supporting evidence would include a sharp rise in Stream A nitrate and flow right after May storms, a lag of days to weeks before the lake peak, and a decline in lake nitrate that tracks rising algal biomass. Sampling Stream B as a control should show little change. The hypothesis is testable because it predicts a specific sequence: rain, then stream nitrate spike, then lake spike, then bloom, then nitrate drawdown. If lake nitrate fell without a bloom, the consumption explanation would weaken.
2. The team must choose a management focus. Evaluate the three options and justify a defensible choice.
Read the worked answer
The evidence points to fertilizer timing and runoff from Stream A as the root cause, because the nitrate peak follows rain on farmland and the bloom follows the nitrate. Managing Stream B is unlikely to help because it carries low nitrate and is not the source. In-lake treatment could reduce symptoms but would not stop the annual loading and would need repeating. The most defensible focus is reducing nitrate delivery from farmland, for example by timing application away from heavy-rain months, planting cover crops, or adding buffer strips along Stream A. The team should pair this with monitoring: measure Stream A nitrate and flow before and after the change and track the May peak and August oxygen. Success would look like a smaller May nitrate spike and higher deep-water oxygen. In-lake treatment can be a backup if loading falls but blooms persist.