The Pond That Died Overnight
A small pond next to a vegetable farm receives runoff after heavy rain. Before the rain, the pond had 8 mg/dm³ dissolved oxygen, 0.5 mg/dm³ nitrate, and 25 species of invertebrates. Three days after the rain, the surface is covered with a thick green mat of algae. The nitrate level is 12 mg/dm³. Dissolved oxygen at 06:00 is 2 mg/dm³, but at 15:00 it is 9 mg/dm³. Water temperature is 24 °C at 06:00 and 29 °C at 15:00; the pond is shallow, with an average depth of 0.8 m. Dead fish are found in the early morning. A farmer says the algae “are good because they make oxygen”, and plans to spray a herbicide to kill the algae. A nearby stream feeds the pond.
1. Explain how the heavy rain led to the algal bloom and to the very low oxygen at 06:00. Use the nitrate and oxygen figures in your answer.
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Rain washes nitrate fertiliser from the farm into the pond. Nitrate rises from 0.5 to 12 mg/dm³. This nutrient enrichment is eutrophication. Algae use the nitrate and grow rapidly, forming the green mat on the surface.
During the day, photosynthesis by the algae produces oxygen, so at 15:00 the oxygen is 9 mg/dm³. At night, photosynthesis stops, but the algae and other organisms continue to respire and consume oxygen. Dead algae also sink and are decomposed by aerobic bacteria, which use oxygen. Before dawn, respiration has used oxygen for many hours without replacement, so at 06:00 oxygen falls to 2 mg/dm³. Fish generally need more than about 4–5 mg/dm³, so they suffocate, especially in the early morning. The shallow pond and warmer water at 15:00 make oxygen less soluble and worsen the effect.
2. Predict the effect of spraying herbicide to kill all the algae. Explain why the farmer’s plan could make the oxygen problem worse in the days after spraying.
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Killing the algae stops daytime oxygen production, but the dead algae remain in the pond. Bacteria decompose the dead algae and use oxygen, so the biological oxygen demand rises sharply. The mat also blocks light from submerged plants, which then die and are decomposed. Oxygen at night and early morning could fall below 2 mg/dm³, killing more fish and invertebrates. Nutrients released from the dead algae can feed another bloom later. The herbicide may also be toxic to other pond life.
The plan treats the symptom rather than the cause. It can cause a second oxygen crash instead of solving the eutrophication.
3. The farmer says the algae are beneficial because the water has 9 mg/dm³ oxygen at 15:00. Evaluate this claim using the data. Suggest one practical action for the farm and explain how it reduces the problem.
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The 9 mg/dm³ is only a daytime reading during active photosynthesis. The same data show 2 mg/dm³ at 06:00, which is below the level needed by most fish. A healthy pond should not have such a large daily swing. The bloom also blocks light and produces a large amount of dead organic matter. Over 24 hours the net effect is oxygen depletion, not a benefit, so the farmer’s claim is not supported.
Practical action: leave a vegetated buffer strip between the vegetable plots and the stream or pond, and apply fertiliser at the recommended rate, avoiding application just before heavy rain. The buffer strip traps runoff and takes up nitrate, so less nutrient reaches the pond. This reduces algal growth at the source rather than killing algae after the bloom.