How Does Eutrophication Work? Causes, Process and Examples

By: Earth How · Last Updated: May 17, 2020

Eutrophication occurs in 4 simple steps:

EXCESS NUTRIENTS: First, farmers apply fertilizer to the soil. Then, excess nutrients run off from the field into the water.
ALGAE BLOOM: Next, the fertilizer rich in nitrate and phosphate spark the overgrowth of algae in water bodies.
OXYGEN DEPLETION: When algae forms, it blocks sunlight from entering water and uses up oxygen. Eventually, water becomes oxygen-depleted.
DEAD ZONES: Finally, water that is completely depleted of oxygen becomes a dead zone and can no longer support life.

Now that you have the basics of the eutrophication process, let’s detail the causes and examples of eutrophication in lakes.

In Greek, eutrophication means “well-nourished”. But eutrophication in the sense of water science, it’s more like an “over-nourished” water body.

Table of Contents show

1. Over-fertilization

Basically, over-fertilization of water causes algae to grow on the surface. When fertilizer enters into the water, this becomes food for algae.

Because eutrophication stimulates algae growth, it’s common to see thick green blooms in the water. But the issue with algae is that it absorbs sunlight preventing it from reaching the bottom.

Especially, blue-green algae or “cyanobacteria” can be harmful to plants and humans. For example, it can be toxic if consumed. This type of algae is becoming a major environmental issue in most parts of the world.

When algae grows to such an extreme level, it entirely stops light reaching plants in the water. Eventually, plants that need sunlight cannot photosynthesize and die.

2. Algal blooms and oxygen depletion

As algae begins to form, it blocks sunlight from entering the bottom of ponds, lakes and rivers. As more nutrients drain into the water, eutrophication repeats in a vicious algal bloom cycle and releases more nutrients in the water.

When algae receive enough sunlight, they produce oxygen through photosynthesis and release it in the water. But without light, algae stop generating oxygen and consume it instead.

When algae die, bacteria begin to decompose the remains, using up oxygen for respiration. Eventually, the decomposition causes the water to become depleted of oxygen. Over time, this causes the water to carry less oxygen than before.

It can reach a certain point when fish cannot swim and suffocate to death in the water. Overall, a eutrophic lake can no longer support life. Finally, water without oxygen is anoxic and over time becomes a dead zone. When a water body reaches this point, it can no longer support fish and aquatic life like amphibians.

3. Dead zones are worst-case scenarios

Dead zones are more concentrated where we have industrialized nations. Especially, industrial farming practices that contains nitrogen and phosphate or animal waste.

In the northern Gulf of Mexico adjacent to the Mississippi River, this is the largest hypoxic zone in the United States (and the second largest worldwide).

What the map shows below are dead zones worldwide. You can see areas like the Caspian Sea completely filled with algal blooms.

Eutrophication disturbs the aquatic life through nitrogen-enriched fertilizer. Over time, this imbalance can cause aquatic life to start dying and in the worst-case scenario a complete dead zone.

Ecosystem threats in the world

Eutrophication can end in disaster for fisheries, tourism and local economies.

We rely on clean, healthy water for aquatic life and animals that count on it in the food chain.

In fact, blue-green algae in itself is harmful to pets and for water consumption. As we reshape the land, a clean water supply becomes a serious threat to people.

How can we fix dead zones? It’s costly to reduce. The best answer is preventative techniques to reduce fertilizer or completely retiring cultivation near eutrophic risk zones.