Question

how does eutrophication lead to fish kills?
a eutrophication causes algae blooms, which disrupt the food web in the ecosystem, lead-ing to fish kills.
b eutrophication causes algae blooms, which fish then ingest thinking it is food, causing their deaths.
c eutrophication causes algae blooms on the surface of water. when the algae dies, de-composition uses up dissolved oxygen in the water, causing fish death.

Explanation:

Eutrophication occurs when excessive nutrients (like phosphorus and nitrogen) enter a water body. This leads to rapid growth of algae (algae bloom). When the algae die, decomposers (bacteria) break them down. The process of decomposition (aerobic respiration by bacteria) consumes dissolved oxygen in the water. Fish need dissolved oxygen to survive. When oxygen levels drop too low (hypoxia or anoxia), fish suffocate and die.

• Option A: While algae blooms can disrupt the food web, the main direct cause of fish kills in eutrophication is oxygen depletion, not just food - web disruption.
• Option B: Algae blooms themselves (unless they are toxic algae, which is not the general case of eutrophication - induced fish kills described here) are not ingested in a way that directly causes fish deaths. The key is oxygen depletion.
• Option C: This correctly describes the process of algae bloom due to eutrophication, algae death, decomposition consuming dissolved oxygen, and subsequent fish death due to lack of oxygen.

Answer:

C. Eutrophication causes algae blooms on the surface of water. When the algae dies, decomposition uses up dissolved oxygen in the water, causing fish death.