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

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

Awaiting image: Lake stratification

Image caption: 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