8.5 Eutrophication

What is eutrophication?

Eutrophication is the process by which a body of water becomes enriched with nutrients, especially nitrogen and phosphorus, leading to excessive plant and algae growth. It can be natural over long time scales, but cultural (anthropogenic) eutrophication is accelerated by human inputs like fertilizer runoff, sewage, and detergents.

The algal bloom cascade

Excess nutrients trigger algal blooms that cover the water's surface, blocking sunlight from reaching submerged plants. When algae die, decomposers (bacteria) break them down, consuming dissolved oxygen. This creates a hypoxic/anoxic zone (dead zone), killing fish and other aquatic organisms that need oxygen to survive.

Sources and solutions

Major nutrient sources include agricultural runoff (fertilizers, manure), wastewater treatment discharge, and urban stormwater. Solutions include riparian buffer strips, wetland restoration, reduced fertilizer application, improved sewage treatment, and no-till farming to reduce runoff into waterways.

Key terms

Eutrophication

Nutrient enrichment of water, often nitrogen/phosphorus, causing excessive algae and plant growth.

Cultural eutrophication

Human-accelerated eutrophication caused by fertilizer runoff, sewage, and other nutrient pollution.

Algal bloom

Rapid, dense growth of algae on the water surface, often triggered by excess nutrients.

Dead zone (hypoxic zone)

Area of water with very low dissolved oxygen, caused by decomposers consuming oxygen while breaking down dead algae.

Riparian buffer strip

Vegetated area along a waterway that filters nutrients and sediment from runoff before it enters the water.

Try an AP-style question

What starts the process of cultural eutrophication?

1. Cold, clean mountain runoff
2. Excess nitrogen and phosphorus
3. Too little sunlight
4. A decrease in nutrients