Lake stratification
In summer, a deep lake in a temperate region forms layers by temperature (thermal stratification):
- Epilimnion: warm, light upper layer, well mixed by wind, rich in oxygen.
- Metalimnion (thermocline): middle layer where temperature drops quickly with depth.
- Hypolimnion: cold, dense bottom layer; little light; oxygen may run out as dead matter decays.
Water is densest at about 4 °C. In autumn the surface cools, sinks, and the lake turns over, mixing oxygen down and nutrients up. A spring turnover also happens in lakes that freeze in winter. Ice floats, so lakes freeze from the top and life survives below.
Trophic status
- Oligotrophic lakes: low nutrients, clear water, high oxygen, few algae (e.g. many mountain lakes).
- Mesotrophic: in between.
- Eutrophic lakes: rich in nutrients, green and murky, many algae, low oxygen at depth.
Eutrophication
Eutrophication is nutrient enrichment of a water body, mainly by nitrogen and phosphorus. Natural eutrophication takes centuries as a lake ages. Cultural (accelerated) eutrophication is caused by humans: sewage, phosphate detergents and fertiliser runoff.
The sequence:
- Nutrients enter → algal bloom (and weeds like water hyacinth).
- Algae block light → submerged plants die.
- Algae die → bacteria decompose them → BOD rises, DO falls.
- Fish kills; foul smell from anaerobic decay (H₂S); some algal blooms are toxic.
- The lake fills with sediment and may become a marsh.
Water hyacinth was nicknamed “the terror of Bengal” for choking water bodies.
Control: treat sewage before discharge, use phosphate-free detergents, reduce fertiliser runoff with buffer strips, remove weeds, dredge sediments and aerate water.
Key points
- Layers: epilimnion (top), thermocline, hypolimnion (bottom).
- Water densest at 4 °C.
- Key nutrients: N and P (phosphorus often limiting in freshwater).
- Eutrophication → algal bloom → low DO → fish kill.
Exam tips
Questions ask for the correct order of events, the nutrients responsible, or the layer names. Remember that it is the decay of algae, not the algae while alive, that uses up oxygen.
Common mistakes
- Thinking eutrophication adds oxygen because algae photosynthesise; overall it lowers DO.
- Mixing up epilimnion (top) and hypolimnion (bottom).
- Thinking eutrophication is caused by heavy metals; it is caused by nutrients.