One change leads to the next: An algal bloom does not stay a bloom. The algae die, bacteria break them down, and the bacteria use up the oxygen that fish need. Learn it as a chain: each step causes the next.
The points to remember
- Excessive growth of phytoplankton: an algal bloom.
- The bloom shades the water, so rooted water plants die; algae live only days, so they die in huge numbers too.
- High rates of decomposition: bacteria break down the dead matter.
- The bacteria cause rapid consumption of dissolved oxygen.
- Oxygen falls to hypoxia (very low) and then anoxia (none).
- Aquatic life that depends on dissolved oxygen dies: fish, crabs, worms.
Remember it as: Bloom, doom, decay, no air, no life.
Example (SL): the Gulf of Mexico. Each spring the Mississippi floods carry fertiliser into the Gulf. Blooms follow in May, the dead algae sink, and by July bacteria have used up the oxygen in the bottom water over thousands of square kilometres.
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The end of the chain has its own words. Hypoxia comes first; anoxia can follow. A dead zone is the result.
The points to remember
- Hypoxia: dissolved oxygen below about 2 mg per litre.
- Fish swim away if they can; crabs, clams and worms on the bottom cannot, so they die.
- With no oxygen, bacteria decompose without air and make hydrogen sulphide (rotten-egg smell).
- Many dead zones are seasonal: worst in summer, when warm water holds less oxygen.
- Tolerant species such as some jellyfish survive, so biodiversity falls.
Reading the graph (SL). The dead zone covered a record 22 700 km² in 2017, about the size of Wales. It was smallest in 2020 (5500 km²), when a hurricane stirred oxygen back into the water. The target of 5000 km² has almost never been met.
Reading oxygen data: In a table of oxygen by month, the lowest value is usually in July or August. Link it to the chain: spring nutrients, bloom, decomposition, oxygen falls.
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A systems model shows the same chain, plus the arrows that loop back. Those loops are positive feedback.
The points to remember
- Loop 1: more nutrients > more death > more decomposition > more nutrients released.
- Loop 2: less oxygen > more animals die > more decomposition > even less oxygen.
- Positive feedback amplifies the change, pushing the system away from its equilibrium.
- Past a tipping point, the lake settles in a new equilibrium: algae dominate, few species.
- Also: dead water plants mean less oxygen made, less food for herbivores, less shelter.
Close the loop: A list of boxes in a line is a chain, not feedback. For full marks, an arrow must lead back to an earlier box, so a change promotes more of the same change.
The papers ask you to draw this model. Build it in five moves. Here it is built for Lake Erie's western basin in August.
The input
Draw a box at the top: phosphate from farms enters along the Maumee River.
The bloom
Add an arrow down to a box: the bloom, a green scum seen from space.
The chain
Keep going down, one box per change: shade, death, decomposition, low oxygen, dead fish.
Loop back
Arrow from decomposition up to the bloom: it releases phosphate. Arrow from dead fish up to dead matter.
Label it
Put + on each loop and write: positive feedback, towards a tipping point.
Boxes and arrows: Write a change in each box ('oxygen falls'), not a single word ('oxygen'). Every arrow means 'leads to'. Two loops, both marked +, are better than one.
Practice with exam-style questions
Answer exam-style questions and get AI feedback that shows you exactly what examiners want to see in a full-marks response.
How this comes up: Paper 2 Section A asks how nutrient pollution affects food production from a named sea [4]; Section B can ask you to draw and explain the model [7].
Nutrients carried by the Mississippi River into the Gulf of Mexico have created a dead zone where brown shrimp and fish are caught.
Explain how nutrient pollution could impact aquatic food production in the Gulf of Mexico.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.