After heavy rain, fertilizer from a farm ran into a river; algae grew, sunlight was blocked, plants died, and fish numbers dropped. Which solution addresses the cause of the problem?
- Put up warning signs near the river so people know the water looks green.
- Remove algae by hand each week, even though it grows back after more rain.
- Plant buffer strips and use less fertilizer so less runoff reaches the river. (correct answer)
- Add more fish to replace the fish that died from low oxygen.
Explanation: This question tests 3rd grade ability to evaluate solutions to environmental problems and impacts on organisms (NGSS 3-LS4-4: make claim about merit of solution to problem caused when environment changes). When environmental problems occur—like pollution, habitat destruction, invasive species—scientists and communities evaluate different solutions to reduce impacts on organisms. The best solutions address the root cause of the problem, not just treat symptoms. For nutrient pollution, preventing runoff at the source is most effective. In this scenario, the environmental problem is fertilizer runoff causing algae blooms that block sunlight, kill underwater plants, and reduce oxygen, causing fish deaths. Solution options presented are: removing algae weekly, planting buffer strips and reducing fertilizer use, adding more fish, or putting up warning signs. Choice B is correct because planting buffer strips and using less fertilizer addresses the root cause—excess nutrients entering the river. Buffer strips are plants along the riverbank that absorb nutrients before they reach water, while using less fertilizer means less can run off. This solution would help organisms by preventing future algae blooms, allowing sunlight to reach underwater plants so they can produce oxygen, enabling fish to breathe and survive. Choice A is incorrect because removing algae by hand only treats the symptom—algae will keep growing back after each rain as long as fertilizer keeps running into the river. This is a common error where students choose visible action over prevention, not recognizing that the algae is a symptom of too many nutrients. Help students trace cause-and-effect chains: fertilizer → runoff → nutrients in water → algae growth → blocked sunlight → plant death → low oxygen → fish death. Practice identifying root causes by asking "But why?" repeatedly. Use demonstrations with food coloring in water to show how runoff moves. Emphasize that preventing pollution from entering water is more effective than trying to clean it up repeatedly.