1.2.12—Tipping point effects
- Syllabus
- First assessment 2026
- Objective
- 1.2.12
- Level
- HL
Near a Threshold, a Small Input Can Trigger a Shift
In a nutrient-loaded lake near a threshold, a small extra nitrate or phosphate input can trigger a large regime shift through reinforcing algal growth and oxygen loss.
The final input is a trigger, not necessarily the sole cause. Algal shading lowers plant growth; decomposition consumes oxygen; low oxygen kills organisms and reinforces the turbid state.
A lake already receiving farm runoff tips after one storm adds more nutrient, while a low-nutrient lake may absorb the same pulse without collapse.
The system was already pressured; the storm crossed a threshold in an altered system.
The final small change is a trigger, not necessarily the whole cause.