IB Environmental systems and societies 4.1D Stratification Notes

Stratification: density differences lock water bodies into stable layers that resist mixing

1. In most lakes and oceans temperature falls with depth, so warm, less dense water sits above colder, denser water.
2. This density difference produces stratification, distinct layers that resist vertical mixing.
3. Because layers stay separate, stratification controls the distribution of oxygen, nutrients, and organisms, shaping productivity and stability.
4. The cause is water's maximum density at 4°C: water above or below 4°C is less dense, so it floats (covered fully in 4.1B).

The three thermal layers of a deep lake

1. Epilimnion: the warm, wind-mixed surface layer, rich in oxygen from air exchange and photosynthesis, where phytoplankton grow.
2. Metalimnion (thermocline): the transition zone where temperature drops rapidly, forming a barrier that blocks the exchange of oxygen and nutrients between layers.
3. Hypolimnion: the cold, dark, dense bottom layer where decomposition dominates, so oxygen is low but decomposition-released nutrients are high.

The seasonal cycle in temperate lakes

1. Spring turnover: surface and deep water reach a similar density near 4°C, so wind mixes the whole column and redistributes oxygen and nutrients.
2. Summer stratification: the epilimnion warms and a thermocline isolates the cold hypolimnion, depleting surface nutrients and deep-water oxygen.
3. Autumn turnover: the surface cools back toward uniform density and wind mixing reoxygenates the depths and returns nutrients to the surface.
4. Winter inversion: 0°C ice floats on top while the densest 4°C water lies at the bottom, letting freshwater life survive beneath the ice.
5. Turnover is the key ecological event: it mixes oxygen down and nutrients up, triggering phytoplankton blooms that feed the food web.

Climate warming makes stratification stronger and longer, starving the deep water of oxygen

1. Warming heats the surface faster than the deep water, so the density gradient steepens and vertical mixing weakens.
2. Stratification therefore becomes stronger, longer, and more stable, and turnover is delayed or suppressed.
3. With less mixing, deep water is no longer reoxygenated, driving anoxia and dead zones that threaten aquatic life.
4. Nutrients stay trapped at depth, cutting the nutrient return to the surface and lowering productivity (this reduced nutrient upwelling links to thermohaline circulation and upwelling in 4.1E).
5. The North Pacific and Southern Oceans already show declining nutrient mixing, falling plankton populations, and reduced carbon uptake.

A warmer, more stratified surface exchanges less heat and CO2 with the deep water, so more heat and carbon stay near the surface, which strengthens stratification further: a positive feedback loop.