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First teaching 2024

First exams 2026

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Water Stratification (HL) (DP IB Environmental Systems & Societies (ESS))

Revision Note

Alistair Marjot

Written by: Alistair Marjot

Reviewed by: Jacque Cartwright

Water Stratification

What is water stratification?

  • Water stratification refers to the layering of water that occurs in a body of water due differences in temperature and density

  • In most large bodies of water:

    • Warmer, less dense water stays on the surface

    • Colder, denser water sinks to the bottom

  • This separation into layers restricts the mixing of water between these different layers

    • This leads to a stable and persistent stratification in deep lakes, oceans, and seas

Temperature and density in water

  • Water temperature varies with depth

    • Typically, water near the surface is warmer due to sunlight, while deeper water is cooler.

  • Water is most dense at 4°C

  • This means colder water (below 4°C) will float above it

  • As a result, a lake or pond can freeze from the top downwards

    • This allows aquatic life to survive beneath a layer of ice during winter

    • This phenomenon is crucial for freshwater ecosystems

    • The insulating ice layer protects life below from freezing temperatures

Formation of the thermocline

  • A thermocline is a transition layer between the warmer, mixed water at the surface and the cooler, denser water below

    • In this layer, temperature drops rapidly with increasing depth

    • The thermocline creates a barrier that restricts the vertical movement of nutrients, oxygen, and organisms between the upper and lower layers

  • Stratification occurs in deeper lakes, coastal regions, enclosed seas, and the open ocean

    • Lakes typically show greater stratification during summer and winter, with mixing occurring in spring and autumn

    • Oceans experience more stable stratification throughout the year due to their depth and larger thermal gradients

Diagram of water temperature layers showing warmer, less dense surface water, cooler, denser bottom water, and a thermocline preventing mixing.
Stratification and thermocline formation in a deep lake

Effects on oxygen and nutrients

  • Stratified water layers have different amounts of dissolved oxygen and nutrients:

    • The relatively warmer surface water is usually oxygen-rich because of contact with the atmosphere and photosynthesis by aquatic plants and phytoplankton

      • The direct contact with the air leads to oxygen replenishment

    • Normally, colder water can hold more oxygen

      • However, in stratified water systems, the deep colder layer may be oxygen-poor due to the lack of mixing with the oxygen-rich surface waters and absence of light for photosynthesis

      • The lack of mixing means that oxygen levels can become very low (hypoxic) in deeper layers, which can sometimes harm aquatic organisms living at those depths

    • Cold, deep water often contains higher concentrations of nutrients

      • This is because dead organisms sink and decompose, releasing nutrients that become trapped in lower layers

Impact of global warming on stratification

  • Global warming has intensified ocean stratification, particularly in the upper 200 metres of water

    • Warming surface waters are becoming less and less dense than colder, deeper water

    • This is increasing the separation between layers and reducing vertical mixing

    • This is trapping heat at the surface and preventing the mixing of oxygen and nutrients between layers

    • As a result, deeper waters are becoming increasingly oxygen-depleted, and fewer nutrients are reaching the surface, where marine organisms need them for growth

  • Salinity also affects stratification, particularly in polar regions

    • Salinity refers to the concentration of salt in the water, which affects water density

    • Higher salinity makes water denser, while lower salinity makes water less dense

    • In places like Antarctica, melting ice caps reduce the salinity of surface waters

    • This makes them less and less dense compared to the deeper, saltier waters below

    • This intensifies ocean stratification because the difference in density between the layers becomes more pronounced

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Alistair Marjot

Author: Alistair Marjot

Expertise: Biology & Environmental Systems and Societies

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

Jacque Cartwright

Author: Jacque Cartwright

Expertise: Geography Content Creator

Jacque graduated from the Open University with a BSc in Environmental Science and Geography before doing her PGCE with the University of St David’s, Swansea. Teaching is her passion and has taught across a wide range of specifications – GCSE/IGCSE and IB but particularly loves teaching the A-level Geography. For the past 5 years Jacque has been teaching online for international schools, and she knows what is needed to get the top scores on those pesky geography exams.