Eutrophication
Over-enrichment of water bodies with nutrients causing excessive algal growth and oxygen depletion.
Real World
Lake Erie experienced a massive toxic algal bloom in 2014 that contaminated Toledo, Ohio's drinking water for three days, forcing 500,000 residents to use bottled water — caused by agricultural phosphorus runoff.
Exam Focus
Explain the full causal chain: nutrient input → algal bloom → decomposition → oxygen depletion → fish kill for full marks.
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