A Pond, a Fertilizer Spill, and an Algal Bloom
A farm pond receives runoff after a fertilizer spill. Within two weeks, the pond surface is covered in green algae. Measurements show dissolved oxygen at the surface is 14 mg/L at midday but drops to 2 mg/L before dawn. Deeper water stays near 1 mg/L. Fish are gulping at the surface in early morning. Nitrate is 40 mg/L, ten times the upstream reference pond. Water temperature is 26 °C. A student proposes adding herbicide to kill the algae and 'restore oxygen.' A second student proposes planting a vegetated buffer strip between the field and pond.
1. Explain the daily oxygen pattern and why fish struggle most before dawn.
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Algae photosynthesize in daylight, releasing O₂ and raising surface oxygen to 14 mg/L. At night, photosynthesis stops but respiration (algae, bacteria, fish) continues consuming O₂, so oxygen falls to 2 mg/L by dawn. Decomposers breaking down dead algae also consume oxygen, keeping deep water near 1 mg/L. Fish struggle before dawn because oxygen is at its daily minimum, and warm water (26 °C) holds less dissolved oxygen than cold water, worsening the stress.
2. Evaluate the herbicide proposal and the buffer-strip proposal.
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Herbicide would kill algae quickly, but dead algae become food for decomposers, whose respiration can crash oxygen further, possibly causing a fish kill. It treats the symptom, not the cause, and may harm other organisms. The buffer strip addresses the cause: vegetation intercepts and absorbs nutrients before runoff reaches the pond, reducing future blooms. It is slower but sustainable and safer for the ecosystem. The buffer strip is the better long-term choice; herbicide might be a last resort under expert supervision.
3. The pond's nitrate is 40 mg/L versus 10 mg/L upstream. Explain how excess nutrients lead to oxygen depletion despite algae producing oxygen.
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Excess nitrate and phosphorus fuel rapid algal growth. When algae die, bacteria decompose them and consume large amounts of dissolved oxygen. The short-term oxygen produced by photosynthesis is outweighed by respiration from the large biomass of algae and decomposers, especially at night and in deep water. This process, eutrophication, can create hypoxic zones that suffocate fish and other animals.