Cambridge O Level Biology · Syllabus 5090 · Relationships of Organisms with One Another and with the Environment
Eutrophication
What is Eutrophication?
The process in which nutrient enrichment of water leads to a fall in dissolved oxygen and the death of aerobic organisms. Nitrogen-containing fertiliser or untreated sewage entering a river or lake raises the concentration of nitrate and other nutrient ions, so algae and other producers grow rapidly. The dense growth reduces the light reaching plants below and producers die, decomposer populations increase as they feed on the dead material, and their aerobic respiration uses up the dissolved oxygen. Fish and other organisms that need dissolved oxygen then die. The fertiliser does not remove oxygen itself; the decomposers do.
This definition is part of the Relationships of Organisms with One Another and with the Environment chapter in Cambridge O Level Biology.
Eutrophication in context
Ecosystems run on one input and two very different accounting systems. Light energy from the Sun enters through photosynthesis, passes along food chains by feeding, and is transferred to the environment as thermal energy at every step — so energy flows through an ecosystem and is never recycled. The atoms carried with it are different: carbon and nitrogen move from air to producer to consumer to decomposer and back to the air, so matter cycles and is used again. Everything else in this topic — short food chains, ecological pyramids, the carbon and nitrogen cycles, eutrophication, conservation — is a consequence of those two sentences.
Questions students ask about Eutrophication
What exactly kills the fish in eutrophication?
A shortage of dissolved oxygen. Nutrient enrichment makes producers grow rapidly; the dense growth reduces light and producers die; decomposer populations increase as they feed on the dead material; and those decomposers respire aerobically, using up the dissolved oxygen. The fertiliser itself removes no oxygen, and shading alone does not explain the fish deaths.