After nutrient enrichment from sewage contamination, a lake often becomes inhospitable to fish. Why does this occur?

• A. Fish are outcompeted by bacteria for food resources.
• B. Sewage increases the water temperature, making it too warm for fish to survive.
• C. Increased nutrients promote algal blooms, which lead to oxygen depletion as algae decompose.
• D. The added nutrients directly poison the fish.

Answer: (C)

Verified step by step guidance

1. Understand that nutrient enrichment from sewage contamination primarily introduces excess nutrients like nitrogen and phosphorus into the lake environment.
2. Recognize that these nutrients stimulate rapid growth of algae, a phenomenon known as an algal bloom.
3. Know that when the algae die, they are decomposed by aerobic bacteria, which consume dissolved oxygen in the water during this process.
4. Realize that this increased bacterial activity depletes the oxygen levels in the water, creating hypoxic (low oxygen) or anoxic (no oxygen) conditions.
5. Conclude that fish and other aerobic aquatic organisms cannot survive in these low oxygen conditions, making the lake inhospitable to fish.