Eutrophication

In the syllabus and the exam

Eutrophication is examined as a specific chain of cause and effect that candidates must reproduce in the correct order, not just as a single-word definition: excess nutrients, usually nitrates and phosphates from agricultural fertiliser runoff, sewage or detergents, enter a body of water; this causes rapid, excessive growth of algae and other aquatic plants, called an algal bloom, on the water's surface; the bloom blocks sunlight from reaching submerged plants, which then die from lack of light for photosynthesis; decomposers (mainly bacteria) then break down the dead algae and plants, and this decomposition process itself consumes large amounts of dissolved oxygen through the decomposers' own respiration; the resulting drop in dissolved oxygen then causes fish and other aquatic animals to die from suffocation. A frequent error is stopping the explanation at 'algae grow and block sunlight' without continuing the chain to decomposition and oxygen depletion, which is where most of the marks for a full explanation are allocated, since fish death is caused by lack of oxygen, not directly by the algae themselves. Candidates should also be able to name specific human sources of the excess nutrients (fertiliser runoff, untreated sewage discharge, detergents containing phosphates) and suggest a specific prevention measure for each, such as buffer strips of vegetation along riverbanks or treating sewage before discharge. Candidates should further note that eutrophication can occur naturally over very long timescales in some lakes, but the process examined at SPM level is almost always the accelerated, human-caused version driven by nutrient pollution.

Example

Fertiliser washed from farmland into a nearby river causes algae to grow rapidly on the surface, blocking sunlight from reaching plants below and, after the algae die, the bacteria that decompose them use up dissolved oxygen, killing fish in the eutrophication process.

Related terms

SustainabilityEcosystemGlobal warming

Source:SRC-DSKP-EN