A Pond, Poultry Runoff and a Fish Kill
A village pond receives runoff from a poultry farm. In week 1 the water is clear, with dissolved oxygen (DO) 8 mg/L, low algal count, and biochemical oxygen demand (BOD) 2 mg/L. By week 3 the pond is green; daytime DO is 12 mg/L, algal count is high, and BOD is 3 mg/L. By week 5 the algae have declined, DO at dawn is 1 mg/L, BOD is 18 mg/L, and dead fish are found. A local officer suggests adding more nitrate fertiliser to 'feed the algae and restore oxygen'. A student argues instead that the pond has undergone eutrophication and that the fish died of oxygen depletion, not of a toxin released by the algae. The panchayat has limited funds and asks for the most defensible explanation and a low-cost intervention.
1. Arrange the events from week 1 to week 5 in a causal sequence. Explain why DO first rose and then fell.
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Sequence: poultry runoff adds nitrogen and phosphorus → week 3 algal bloom → algal photosynthesis raises daytime DO to 12 mg/L → algae die and sink → aerobic decomposers (bacteria and fungi) oxidise the dead organic matter → BOD rises to 18 mg/L → oxygen is consumed faster than it is replaced → week 5 dawn DO falls to 1 mg/L → fish suffocate. The early DO rise is not a sign of health; it is a daytime photosynthetic effect in a nutrient-enriched system. The later fall occurs because the organic load from dead algae fuels decomposition, and at night respiration continues without photosynthesis. The data fit eutrophication, not a simple algal toxin, because BOD rises sharply as DO collapses.
2. At week 5, why is the DO lowest at dawn? Explain using photosynthesis and respiration, and state why BOD is a better indicator of organic pollution than a single daytime DO reading.
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DO is lowest at dawn because oxygen is consumed by respiration of algae, animals and decomposers throughout the night while photosynthesis is zero. During the day, photosynthesis by algae can raise DO above atmospheric equilibrium, so a single daytime reading may look normal or even high in a polluted pond. BOD measures the oxygen consumed by microbial decomposition of organic matter over a standard incubation period; it integrates the organic load rather than a moment's balance. A high BOD (18 mg/L) with low dawn DO shows that oxygen demand is high and that the pond cannot sustain fish, whereas a daytime DO of 12 mg/L at week 3 could have been misleading.
3. Evaluate the officer's proposal and the student's claim. Which is supported by the data, and what low-cost intervention would you recommend?
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The officer's proposal is not supported. Adding nitrate would further fertilise the pond, prolong the algal bloom and increase the eventual organic load, making oxygen depletion worse. The student's claim is supported by the BOD rise from 2 to 18 mg/L, the dawn DO of 1 mg/L, and the timing of the fish kill after the algal decline. The primary cause is eutrophication-driven oxygen depletion, although algal toxins could contribute and cannot be excluded without assays. A low-cost intervention is to stop or divert the poultry runoff, for example with a vegetated buffer strip and better manure storage, and to aerate or remove dead algal mats mechanically. Adding more nutrients or herbicides would be counterproductive.