Big picture: Water bodies develop distinct layers (stratification) based on temperature and density differences. This affects nutrient distribution, oxygen levels, and aquatic ecosystems.
- Thermocline
- The layer of water where temperature changes rapidly with depth, separating warm surface water from cold deep water.
- Pycnocline
- The layer where water density changes rapidly with depth, often coinciding with the thermocline.
- Epilimnion
- The warm, well-mixed surface layer of a lake above the thermocline.
- Hypolimnion
- The cold, dense bottom layer of a lake below the thermocline.
Why stratification matters
- Prevents mixing of surface and deep water
- Traps nutrients in deep water, limiting surface productivity
- Oxygen may be depleted in the hypolimnion
- Affects species distribution — different organisms at different depths
- Seasonal changes in stratification drive ecological cycles
Seasonal turnover in temperate lakes
- Summer: strong stratification — warm epilimnion, cold hypolimnion
- Autumn: surface cooling reduces temperature difference — overturn occurs
- Winter: inverse stratification — ice on surface, 4°C water sinks to bottom
- Spring: ice melts, surface warms to 4°C — spring overturn mixes entire lake
Key concept: Turnover events are critical for lake productivity — they bring nutrients from the bottom to the surface and replenish oxygen in deep water.
- Upwelling
- The movement of cold, nutrient-rich water from the deep ocean to the surface, driven by wind patterns and the Coriolis effect.
- Downwelling
- The sinking of surface water, often where warm and cold currents meet or where evaporation increases surface density.
Upwelling significance
- Brings nutrients to the photic zone, supporting high productivity
- Major upwelling zones are among the most productive fisheries
- Examples: Peruvian coast, California coast, West Africa
- El Nino disrupts upwelling along the South American coast, causing fishery collapse
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Key concept: The thermohaline circulation (global ocean conveyor belt) is a system of deep ocean currents driven by differences in water temperature (thermo) and salinity (haline). It redistributes heat around the planet.
How thermohaline circulation works
- Cold, salty water in the North Atlantic becomes very dense and sinks (deep water formation)
- This dense water flows south along the ocean floor toward Antarctica
- It then flows into the Indian and Pacific Oceans
- Warm surface currents (including the Gulf Stream) flow back toward the North Atlantic
- The entire cycle takes approximately 1,000 years to complete
Importance of thermohaline circulation
- Distributes heat from tropics to higher latitudes — warms Europe
- Carries dissolved nutrients and oxygen to the deep ocean
- Drives carbon absorption — cold water absorbs more CO2
- Influences global weather patterns and climate zones
- Supports deep-sea ecosystems through nutrient delivery
How climate change affects ocean systems
- Warming oceans increase stratification — reduced mixing and nutrient supply
- Melting Arctic ice reduces salinity in North Atlantic — may weaken thermohaline circulation
- Weaker circulation could cool Europe while the rest of the world warms
- Reduced deep water formation decreases ocean carbon uptake
- Warmer water holds less dissolved oxygen — expanding ocean dead zones
- Changes in upwelling patterns affect fisheries and food security
Key concept: The Atlantic Meridional Overturning Circulation (AMOC), a key component of the thermohaline system, has weakened by approximately 15% since the mid-20th century. A complete shutdown is considered a climate tipping point.
IB exam tip: Be able to explain the potential consequences of weakened thermohaline circulation as a positive feedback loop and climate tipping point.