The Pond Food Web and Pollution
A pond contains algae, water fleas (Daphnia) that eat algae, small fish that eat water fleas, and herons that eat small fish. A factory releases warm water into the pond, raising the temperature by 5°C. The warm water also contains a small amount of a toxic chemical that accumulates in animal tissues. After several months, the number of water fleas decreases sharply, while the algae increase. The small fish population also drops, and herons are seen less often. A student says the herons left because they were poisoned directly by the chemical. A teacher suggests the herons left because their food supply decreased.
1. Explain how the warm water and the toxic chemical could cause the observed changes in the pond. Include the effect on each organism in the food chain.
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Warm water holds less dissolved oxygen, so respiration of aquatic animals is harder. The toxic chemical may kill water fleas directly or reduce their reproduction. With fewer water fleas, algae are eaten less, so algae increase. The small fish depend on water fleas for food, so fewer water fleas means less food for fish, causing fish numbers to drop. The chemical may also accumulate in fish tissues (bioaccumulation), harming them. Herons eat small fish; with fewer fish, herons have less food, so they leave or decrease. The student's idea that herons were poisoned directly is possible but less likely because the chemical is in small amounts and herons are at the top of the food chain, where accumulation could occur, but the main effect is through the food supply. The teacher's suggestion is supported by the drop in fish.
2. Suggest two ways the factory could reduce its impact on the pond, and explain how each would help.
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First, the factory could cool the water before releasing it, for example by using a cooling tower or a holding pond. This would prevent the temperature rise, so the water would hold more oxygen and the water fleas and fish would not be stressed. Second, the factory could treat the water to remove the toxic chemical before release, for example by filtering or chemical treatment. This would prevent the chemical from killing water fleas and from accumulating in fish. Both changes would help restore the food chain: water fleas would increase, algae would decrease, fish would have food, and herons would return. Another option is to release the water at a different site or dilute it, but cooling and treatment are direct solutions.