IB Biology Topic D4.2.8: Eutrophication of Aquatic and Marine Ecosystems Due to Leaching Notes

Eutrophication: How Leaching Nutrients Suffocate Aquatic Ecosystems

Stand beside a lake smothered in a thick green mat of algae and you see the water turn murky, the fish grow scarce, and a once-lively ecosystem fall silent.

1. This is eutrophication, the over-enrichment of water with nutrients such as nitrogen and phosphorus.
2. It begins when these nutrients leach from farmland and drain into rivers, lakes, and coastal seas.

Eutrophication Unfolds as a Chain of Seven Stages

1. Nutrient leaching: rain dissolves water-soluble nitrogen and phosphorus from fertilisers.
2. Runoff: the nutrient-rich water flows into rivers and lakes, overloading them.
3. Nutrient enrichment: the excess nutrients fuel rapid growth of algae and aquatic plants.
4. Algal bloom: a dense layer of algae covers the surface and blocks sunlight, so submerged plants can no longer photosynthesise.
5. Decomposition: the algae die and bacteria decompose them, raising the biochemical oxygen demand (BOD).
6. Oxygen depletion: oxygen falls, creating hypoxic (low oxygen) or anoxic (no oxygen) conditions.
7. Aquatic life declines: fish and other aerobic organisms die or leave the water body.

Rising BOD Is What Drives Oxygen Down

1. The dead algae add huge amounts of organic material for bacteria to break down.
2. These decomposers consume oxygen as they work, so a high BOD strips oxygen from the water faster than it can be replaced.
3. The greater the BOD, the deeper the hypoxia and the more aquatic life suffocates.

Eutrophication Damages Biodiversity, Food Webs, and Economies

1. Biodiversity loss: oxygen depletion kills fish and aerobic organisms, while shaded submerged plants die from lack of light.
   1. In Lake Victoria, eutrophication has driven the decline of native fish species.
2. Food web disruption: losing key species starves predators higher up the food chain.
   1. Some algal blooms also release toxins that poison fish, birds, and mammals.
3. Economic consequences: fisheries collapse as fish populations crash.
   1. Tourism falls as green, foul water drives visitors away.
   2. Water treatment costs rise to strip the excess nutrients before use.

Prevention Targets Nutrients at the Source and in the Water

1. Sustainable agriculture: apply fertiliser efficiently and plant buffer zones between farmland and water to absorb nutrients.
2. Improved wastewater management: remove nutrients from wastewater before release and switch to phosphate-free detergents.
3. Ecosystem restoration: aerate affected water to raise oxygen and reintroduce native plants to soak up excess nutrients.