Case study example: Freshwater lake ecosystem (eutrophication)
A freshwater lake is usually a stable system with clear water, aquatic plants, and healthy fish populations. Its resilience depends on balanced nutrient levels and sufficient oxygen.
Human activities such as agriculture and sewage discharge can add excess nutrients (nitrates and phosphates) to the lake.
- Extra nutrients enter the lake
- Algae grow rapidly (algal bloom)
- Algae block sunlight so plants die
- Decomposition uses up dissolved oxygen
- Fish and invertebrates die from low oxygen
If oxygen levels fall too low, the lake can cross a tipping point and shift from a clear, healthy state to a polluted, low-oxygen state.
After this shift, recovery is difficult because nutrients stored in sediments continue to feed algal growth even if pollution inputs are reduced.
This is a positive (reinforcing) feedback loop: more nutrients → more algae → less oxygen → ecosystem damage → conditions that favour more algae.
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