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).
2. The metalimnion is the transition zone containing the thermocline.
3. In the ocean, the thermocline depth and strength vary with latitude:
   1. Tropical oceans: Strong, permanent thermocline due to constant solar heating.
   2. Temperate regions: Seasonal thermocline forms in summer and breaks down in winter.
   3. Polar oceans: Weak or absent thermocline due to uniformly low temperatures and frequent mixing by storms.

Mixing and Nutrient Cycling

1. Seasonal or wind-driven turnover events mix surface and deep layers, replenishing oxygen and bringing nutrients to the surface.
2. This process triggers phytoplankton blooms, supporting food webs in both lakes and marine ecosystems.
3. Meromictic lakes (e.g., Lac Pavin, France) exhibit permanent stratification, where deep layers (monimolimnion) never mix, remaining anoxic and high in dissolved minerals.

Oxygen and Nutrient Gradients

1. Surface waters are oxygenated due to wind action and photosynthesis.
2. Deeper waters become oxygen-poor as decomposition consumes oxygen faster than it can be replenished.
3. Nutrients (nitrogen, phosphorus, iron) accumulate in the hypolimnion, fueling surface productivity during upwelling events.

Stratification in Different Water Bodies

1. Deep lakes: Seasonal stratification driven by temperature changes; mixing during spring and autumn.
2. Coastal areas: Influenced by freshwater inflow and tides; salinity and temperature interact to produce stratification.
3. Enclosed seas (e.g., the Baltic Sea): Stable stratification caused by low-salinity surface waters overlying dense, salty bottom waters.
4. Open ocean: Global-scale stratification with permanent thermoclines, affecting ocean circulation and nutrient cycling.

Effects of Global Warming and Salinity Changes on Stratification

Increased Thermal Stratification

1. Global warming has intensified stratification, particularly within the upper 200 meters of ocean water.
2. As atmospheric temperatures rise:
   1. Surface water becomes warmer and less dense, while deep water remains cold and dense.
   2. The density gradient between layers increases, reducing vertical mixing.
3. This stronger stratification traps heat and CO₂ near the surface, disrupting heat exchange with deeper waters.

Role of Salinity in Stratification

1. Salinity also influences density:
   1. Higher salinity → denser water.
   2. Lower salinity → less dense water.
2. Melting ice caps in polar regions (e.g., Antarctica, Arctic) introduce freshwater into the ocean, reducing surface salinity and enhancing stratification stability.
3. In tropical regions, increased evaporation leaves salt behind, increasing salinity and density of surface waters.

Consequences of Increased Stratification

Reduced Gas Exchange

1. Warmer surface water holds less oxygen and carbon dioxide due to lower solubility.
2. Deeper layers become oxygen-depleted, causing hypoxia and threatening marine biodiversity.

Decreased Nutrient Upwelling

1. Stratification prevents nutrient-rich deep water from reaching the surface.
2. This limits phytoplankton productivity, weakening the marine food web.

Positive Climate Feedback Loop

1. Reduced oceanic CO₂ uptake increases atmospheric greenhouse gas levels.
2. This intensifies warming, further strengthening stratification—a self-reinforcing cycle.