An agricultural watershed contributes nitrogen and phosphorus to a slow-turnover lake for many years. Algal growth increases, sunlight penetration decreases, dead organic material accumulates, and microbial decomposition intensifies. Fish kills later occur even though the original pollutants were nutrients rather than directly toxic chemicals. Which chain of events best explains the fish mortality?

Question

Answer

Nutrients reduce plant growth, which eliminates decomposition and causes oxygen to accumulate.

Answer

Nutrient enrichment stimulates excessive plant and algal growth; decomposition of the resulting organic matter increases oxygen demand and lowers dissolved oxygen to levels harmful to aquatic organisms.

Answer

Nutrients prevent microorganisms from using organic matter, causing carbon dioxide to disappear.

Answer

Nutrients immediately poison all fish by direct chemical toxicity.

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An agricultural watershed contributes nitrogen and phosphorus to a slow-turnover...

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