Pond Fertilizer and the Oxygen Crash
A class studied a pond near school. In April, the water was clear. They found algae, duckweed, water fleas that eat algae, minnows that eat water fleas, and herons that eat minnows. Bacteria and fungi lived in the mud. In June, fertilizer from a nearby lawn washed into the pond. Algae grew so thick that the water turned green. By July, the algae began to die. Dissolved oxygen fell from 8 mg/L in April to 2 mg/L in July. Minnows were gasping at the surface, and fewer water fleas were found. The fertilizer did not contain any material that minnows can eat. Students wondered why animals suffered when the algae had grown so much.
1. Explain how fertilizer runoff could lead to low dissolved oxygen and stress on minnows. Trace matter and energy through the pond.
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Fertilizer adds nutrients such as nitrates and phosphates that algae use to grow. The algae population increases rapidly, making the water green. When those algae die, bacteria and fungi decompose them. Decomposition is a form of cellular respiration and uses dissolved oxygen from the water. The oxygen level falls from 8 mg/L to 2 mg/L. Minnows need oxygen dissolved in water, so they gasp at the surface where oxygen mixes in from the air. While algae are alive, they release oxygen during photosynthesis, but at night they also respire and use oxygen. The large mass of dead algae fuels decomposers, whose oxygen use exceeds the oxygen produced. The fertilizer feeds producers, not minnows directly.
2. Predict how the heron population might be affected and explain using the food chain.
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Herons eat minnows, so if minnows decline or die, herons have less food in this pond. Some herons may fly to another pond or river, and others may raise fewer young. The effect may not be immediate because herons can move and may eat other prey. Energy flows from algae to water fleas to minnows to herons. A large algae bloom does not help herons because herons cannot eat algae, and the middle links in the chain are harmed by low oxygen. The best prediction is fewer heron visits and lower nesting success unless the pond recovers.
3. A student says, 'More algae means more food, so the pond should support more life.' Evaluate this claim using the evidence.
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The claim is not supported. More algae is not automatically more food for all animals. Minnows eat water fleas, not algae. Water fleas might increase briefly, but low oxygen and decaying algae harm them. Thick algae also block sunlight from underwater plants. The evidence is the oxygen drop from 8 mg/L to 2 mg/L, the gasping minnows, and the decline in water fleas. The pond cannot support more animal life when oxygen is a limiting resource. The fertilizer increased producer growth, but decomposition used up oxygen, so the food web was disrupted rather than enriched.