Lake Stratification
Stratification is the division of a water column into strata, or layers, of water with different properties
Thermal image of Ice Lake’s stratification over a 22 month period. The lake mixes every spring and fall, realigning the temperature throughout the lake. The thermocline exists at different depths depending on the season.
These divisions are usually defined by temperature and density, though other parameters such as salinity and chemical distinctions can also be used
When referring to temperature and density strata within a lake, the layers are usually called the epilimnion, metalimnion and hypolimnion from top to bottom
The depth of the epilimnion is dependent on the temperature exchange, usually determined by water clarity and depth of mixing (usually initiated by wind)
The hypolimnion is separated from the upper layers by the chemocline or halocline. These clines mark the boundary between oxic and anoxic water and salinity gradients, respectively
The boundary between the epilimnion and metalimnion is called the thermocline – the point at which water temperature begins to steadily drop off
Thermal stratification is usually seasonal, with clear delineations between layers during the summer, narrower layers in winter, a “turnover” in the spring and fall when temperature is fairly uniform throughout the water column
In lakes, this thermal stratification is responsible for the density differences of the water layers. Seasonal changes in temperature allow for turnover within the lake due to changes in density that cause mixing of these layers
This turnover redistributes dissolved oxygen throughout all of the layers.
Scientists monitor this issue by deploying data buoys that contain dissolved oxygen sensors, temperature strings, photosynthetically active radiation (PAR) sensors, and chlorophyll sensors
The PAR sensors are placed at specified depths to measure the amount of sunlight that the phytoplankton are receiving to conduct photosynthesis
The dissolved oxygen sensors allow scientists to track oxygen levels at different layers to monitor anoxic conditions
The temperature string provides a temperature profile of the lake that allows scientists to follow changes in thermal stratification
Graphical depiction of a data buoy and a temperature string used to create a temperature profile of a water body
The chlorophyll sensor uses light frequencies to determine the chlorophyll concentration within the lake, which can estimate the phytoplankton biomass
For more detailed information on how these sensors work, view our other Sensor Whiteboards on the Nexsens Blog
A unique characteristic of water is the density differences between its solid and liquid states. Contrary to most substances, water is less dense in its solid state
This density difference occurs due to the reshaping of the hydrogen bonds that allow for greater space between the water molecules
If water did not have this unique property, bodies of water would freeze at the top; the ice would sink to the bottom, freezing the body of water from the bottom up and killing all of the organisms within
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