Eutrophication

Core idea

Eutrophicationis the process by which an excess of nutrients — mineral ions such as nitrate — entering water triggers a chain of biological events that depletes dissolved oxygen and can kill organisms that require it. It isn’t one event; it’s a sequence, and each step causes the next.

The required sequence

1. Nitrate ions, or other mineral ions, increase in the water — typically from fertiliser washed or leached off nearby land.
2. Aquatic producers use the extra mineral ions and their growth increases.
3. That extra producer biomass eventually dies.
4. Decomposition of the dead biomass increases.
5. Decomposers carrying out this decomposition respire aerobically, and more decomposers means more aerobic respiration.
6. Aerobic respiration uses dissolved oxygen, so more of it is used up.
7. Dissolved oxygen concentration in the water decreases.
8. Organisms that require dissolved oxygen — fish among them — may die.

Written as one chain: excess mineral ions lead to more producer growth, which leads to more dead organic material, which leads to more decomposition, which leads to more aerobic respiration by decomposers, which leads to less dissolved oxygen, which leads to the death of oxygen-dependent organisms.

The step that actually matters most

It’s tempting to shorten all of this to “too much algae blocks the light,” but that isn’t the mechanism this syllabus point is built around. The major biological consequence comes from the increased decomposer respiration that follows the death of the enlarged producer population — a dense algal layer reducing light for submerged plants can contribute to the picture, but it isn’t a compulsory step in the required sequence above.

Don’t confuse cause and consequence

Fertiliser enrichment is the upstream cause; oxygen depletion is the downstream consequence, several steps later. “Fertiliser kills fish” skips straight past the biology a Supplement-level answer is supposed to demonstrate. It also isn’t the fertiliser molecule itself doing any direct harm — the actual killing mechanism is decomposers using up dissolved oxygen during their own aerobic respiration, which is the same underlying biological process taught back in Chapter 12, just occurring at a much larger scale than usual because of the nutrient surplus.

Where this fits in the wider picture

The Core-level version of this same story — excess fertiliser lowers dissolved oxygen in aquatic ecosystems — is covered alongside sewage and plastic pollution on water pollution. The nitrate ions that start the whole sequence trace back to the nitrogen cycle from Chapter 19, and the aerobic respiration that ends it is the same process decomposers use throughout the carbon and nitrogen cycles — eutrophication is what happens when that ordinarily useful process is pushed to a much larger scale by a sudden nutrient surplus.