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.
Basically, over-fertilization of water causes algae to grow on the surface. When fertilizer enters 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.
2. Algal blooms and oxygen depletion
As algae begin 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.
3. Dead zones are worst-case scenarios
Dead zones are more concentrated where we have industrialized nations. Especially, industrial farming practices that contain 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.
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 are 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.