Syllabus Edition
First teaching 2024
First exams 2026
Water Stratification (HL) (DP IB Environmental Systems & Societies (ESS))
Revision Note
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
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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