4.1D Stratification (HL) Notes

Temperature, Density, and Stratification in Water

1. In most aquatic ecosystems, temperature decreases with depth, resulting in warmer, less dense water above and colder, denser water below.
2. This layering creates stratification, the formation of distinct layers that resist vertical mixing.
3. Stratification affects the distribution of oxygen, nutrients, and organisms, influencing the productivity and stability of aquatic ecosystems.

Relationship Between Temperature and Density

1. Water behaves uniquely compared to most substances because it reaches maximum density at 4°C.
2. Above or below this temperature, water becomes less dense.
3. As a result:
   1. At temperatures above 4°C, heating decreases density, causing warm water to float.
   2. At temperatures below 4°C, cooling decreases density, allowing the colder water to float above denser water.
4. When the air temperature drops below 0°C, the surface water freezes while deeper water remains liquid at about 4°C.

Thermal Layers in Deep Lakes

1. Epilimnion

1. Warm, well-mixed surface layer exposed to sunlight.
2. Rich in oxygen (due to air exchange and photosynthesis).
3. Supports phytoplankton and aquatic vegetation.

2. Metalimnion (Thermocline)

1. Transition zone with rapid temperature decrease.
2. Acts as a barrier to nutrient and gas exchange.
3. Limits mixing of oxygen and nutrients between upper and lower layers.

3. Hypolimnion

1. Cold, dark, and dense bottom layer.
2. Poor in oxygen but rich in nutrients from decomposition.
3. Decomposition and respiration dominate; photosynthesis is absent.

Seasonal Stratification in Temperate Lakes

Seasonal Stratification in Lakes

Spring Turnover

1. Surface and deep waters are both near 4°C and of similar density.
2. Wind action mixes the entire water column, redistributing oxygen and nutrients evenly.

Summer Stratification

1. The epilimnion (surface layer) warms and becomes less dense.
2. Beneath it lies the metalimnion (thermocline), where temperature drops rapidly with depth.
3. The hypolimnion remains cold, dense, and isolated from the surface.
4. Stratification prevents nutrient mixing, causing surface nutrient depletion and oxygen depletion below.

Autumn Turnover

1. Cooling surface temperatures restore uniform density.
2. Wind mixing reoxygenates the deeper layers, replenishing nutrient availability.

Winter Inversion

1. Surface water cools below 4°C and becomes less dense, forming ice at the surface.
2. Beneath, water remains at ~4°C, allowing freshwater ecosystems to survive under ice.

Ecological Importance of Surface Ice

1. Ice acts as an insulating layer, preventing the complete freezing of water bodies.
2. Beneath the ice, water remains around 4°C, allowing fish, amphibians, and microorganisms to survive.
3. This insulation maintains oxygen exchange and metabolic activity at minimal levels.

Stratification and Thermoclines in Lakes and Oceans

1. A thermocline is a distinct layer where water temperature drops rapidly with depth, separating the warm, mixed surface layer (epilimnion) from the cold, dense deep layer (hypolimnion).