The Pond That Turned Green: Nitrogen, Algae and Oxygen
A village pond of about 2 000 m² receives runoff from a farmyard where cattle are kept. In May the water is clear and oxygen concentration near the surface is 9 mg/L. By late July the water is pea-green, the surface oxygen has risen to 14 mg/L at noon, but at 4 a.m. it drops to 2 mg/L. On the third morning dozens of small fish are found floating. Local residents report the pond had no fish kills in previous years, when the farmyard was further from the shore. A student claims: 'The algae make extra oxygen, so the pond should be healthier than before.'
1. Using your knowledge of photosynthesis, respiration and the carbon and oxygen cycles, explain why surface oxygen is high at noon but falls sharply before dawn.
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Reasoning chain:
- Farm runoff supplies nitrate and phosphate → mineral nutrition limits algal growth is removed → algal bloom.
- In daylight the dense algae photosynthesise, releasing O₂ faster than the whole community respires → dissolved O₂ rises (14 mg/L at noon).
- At night photosynthesis stops but respiration continues - in the algae themselves, in the bacteria decomposing dead algae, and in animals.
- So oxygen is consumed all night with no replacement; the minimum is reached just before dawn (2 mg/L).
Conclusion: the daily oxygen curve in a bloom is a large oscillation, not a permanent gain. The student's claim is wrong because it counts only the daytime half of the cycle. Fish die when the night-time minimum falls below their tolerance.
2. Explain why dead fish appear a few days after the bloom peaks, and predict what happens to oxygen at the bottom of the pond.
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Reasoning chain:
- Algae are short-lived; when they die they sink and become dead organic matter.
- Decomposers (bacteria and fungi) break this matter down by aerobic respiration, consuming dissolved O₂.
- Near the bottom there is no light, so no photosynthesis can compensate - the oxygen deficit is greatest at the bottom (hypoxia/anoxia).
- Fish avoid the surface heat and seek deeper, cooler water, so they meet the lowest oxygen and suffocate; the kill peaks after the bloom collapses, matching the observed delay.
Prediction: bottom oxygen approaches zero, sediments turn anoxic, anaerobic decay produces H₂S and methane, and nutrients are released back into the water - a self-reinforcing cycle (eutrophication).
3. Suggest two management measures and justify each in terms of the nutrient cycle.
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Measure 1 - Intercept the runoff (buffer strip of vegetation, relocated manure store, fencing the shore). Justification: the bloom is driven by limiting nutrient input, chiefly nitrates and phosphates. Cutting the input removes the cause rather than the symptom.
Measure 2 - Remove biomass / avoid further loading (harvest floating algae, do not add fertiliser or detergent near the pond, maintain oxygenating plants and shade at the margins). Justification: every kilogram of algae removed also removes the nitrogen and phosphorus locked in it, shortening the decomposition load that consumes oxygen.
What not to do: adding algicide alone simply creates a large pulse of dead organic matter and makes the oxygen crash worse - it treats the symptom and accelerates the decomposition stage.