The Green Pond After the Fish Arrived
Students built a model pond in a 40 L tub in the courtyard, with algae growing on the walls, about 60 water fleas, 8 pond snails, 3 dragonfly nymphs, and 2 small minnows. In this food web, water fleas eat algae, snails scrape algae off surfaces, nymphs eat water fleas and small snails, and minnows eat water fleas and nymphs. For four weeks the water stayed clear and the class counted organisms every Friday. Then a custodian added 20 mosquito fish to clean up mosquito larvae. Three weeks later the water was pea-green and algae coated the tub. A dip-net count found about 10 water fleas and 1 nymph; the snails were still about 8 but had moved onto the water plants. Students disagree about whether the fish or the warmer weather caused the bloom.
1. Explain, using the food web, how adding the mosquito fish could have led to more algae.
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The algae eaters in this tub are water fleas and snails, and the water fleas are eaten by nymphs and minnows. When 20 mosquito fish went in, they most likely ate water fleas and nymphs: the counts dropped from about 60 fleas and 3 nymphs to about 10 fleas and 1 nymph. With far fewer grazers feeding on it, the algae could grow faster than it was being eaten, so it filled the water and coated the walls. This is a change that travels down the food web, not simply more food for algae. The snails barely changed and had climbed onto the plants, so they were not controlling the algae on the tub walls. The key evidence is the order: fish added, grazers crashed, algae bloomed.
2. What evidence would help the class decide between the fish explanation and the warmer-weather explanation?
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Two kinds of evidence would separate them. First, a written record of water temperature and daily weather for the four clear weeks and the three green weeks. If the bloom began before the warm spell, warmth is unlikely to be the main cause; if the tub heated up first, the fish explanation weakens. Second, a control tub set up the same way, with the same water, plants, and light but no mosquito fish, counted on the same days. If the control stays clear while the fish tub turns green, the fish are strongly implicated. As a check, examining the stomach contents of a few fish under a teacher's supervision would show directly whether they had been eating water fleas and nymphs.
3. Predict what is likely to happen to the two minnows and to the pond as a whole over the next month, and explain your reasoning.
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The minnows are likely to struggle. Mosquito fish compete with them for the same prey, water fleas and nymphs, and the nymph count was already down to about 1, so little food remains in the tub. A large school of new fish may also crowd or harass them. Over the next month the pond would probably lose most of its animal consumers and stay algae-dominated, because the grazers that kept the water clear are gone and nothing in the tub replaces them quickly. Water fleas can rebound if the fish are removed, but the tub is small and the food web is simple, so recovery would be slow. This models what happens when a species is added without checking what it eats.