The Fertiliser Runoff Problem
A farmer applies ammonium nitrate fertiliser to a field beside a small lake. Within two weeks, the lake shows a dense green algal bloom; later the algae die and the water becomes cloudy with a foul smell, and dissolved oxygen drops sharply. A nearby stream also shows elevated nitrate levels. The farmer argues that 'nitrate is a plant nutrient, so it cannot be a pollutant'. A local official proposes adding lime (calcium hydroxide) to the lake to fix the problem.
1. Explain the sequence of events in the lake using the nitrogen and oxygen cycles.
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Runoff carries nitrate and ammonium into the lake. These are limiting nutrients, so algae grow rapidly (eutrophication). When the algae die, aerobic bacteria decompose them, consuming dissolved oxygen. Oxygen depletion kills fish and other aerobic organisms, and anaerobic decomposition produces foul-smelling compounds such as hydrogen sulfide and ammonia. So the farmer's statement is wrong: a nutrient becomes a pollutant when it is present in excess in the wrong place. The key judgement is that the harm comes from the ecosystem imbalance, not from nitrate's toxicity to plants.
2. Evaluate the proposal to add lime to the lake.
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Adding Ca(OH)₂ raises pH and can precipitate some phosphate, but it does not remove the nitrate already present, and it may harm organisms adapted to the existing pH. It treats a symptom, not the cause. A large pH change can also release ammonia as NH₃, which is toxic to fish. The correct priority is to reduce nutrient input: buffer strips, controlled fertiliser timing and dosage, and constructed wetlands. Lime might be a temporary measure in an acidified lake, but not here.
3. Suggest two quantitative checks the official could make to confirm the diagnosis.
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(1) Measure nitrate concentration in the lake and stream before and after fertiliser application; a clear rise confirms runoff. (2) Measure dissolved oxygen and biochemical oxygen demand (BOD) over time; a falling oxygen level with rising BOD confirms organic loading from decomposing algae. Phosphate measurement is also useful because phosphate is often the limiting nutrient in freshwater, so controlling it may be more effective than controlling nitrate alone.