2. Liminology Flashcards
What is Retention Time?
The average time spent by a water parcel within the lake.
Tr=V/Q
Longer retention times indicate significant stratification.
If Tr>1 year the system can be considered stratified and SEASONAL
What are the 3 layers in a lake?
Epilimnion: Surface layer, experiences sol heating, aeration, and wind.
Thermocline: Tranisitional regions supporting strong gradients (sun doesn’t penetrate)
Hypolimnion: Cold and stable at 4 degrees C. Exchanges with sediments and groundwater. Lack of aeration leads to bio-decomposition.
What is the potential energy of a lake?
The difference in PE between its current, stratified state and its fully mixed state.
What sources may disrupt stratification?
Radiation: Short wave and Long wave
Inflows/Outflows: Precipitation, groundwater or streams/
Evaporation & Condensation
Wind
How can radiation affect stratification?
Short-term radiation heats surface water, increasing gradient, enhancing stratification.
Penetration depth depends on wavelength.
Can create localised heating & instabilities causing mixing,
Long wave radiation emitted at night cools the surface creating an unstable gradient with fluid below.
How does condensation/evaporation affect stratification?
Evaporation extracts latent heat from the surface to permit phase change - cools the fluid.
Condensation releases latent heat and warms the surface.
How does wind affect stratification?
Most significant source of kinetic energy.
Wind shear drags surface downwind, leading to set up. The tilting leads to basin-scale circulation (bottom currents offset surface motion).
If wind stops, downwind potential energy is released as SEICHES (basin-scale waves)
What is mixing regime A?
Ri > (L/(2H))^2
Tilting of isopicnals is very slow.
Interface is small but internal seiches can still persist for a long time.
Any longitudinal gradients are gradients are quickly obliterated.
What is mixing regime B?
L/2H
Internal seiching is the predominant feature of this regime.
The amplitude of the seiches is large.
As Ri approaches L/2H thermocline may start to surface.
What is mixing regime C?
1
Thermocline is steeply inclined, deepening epilimnion rapidly to reach the bottom.
Thermocline is diffused by mixing. Shear vorticity e.g. Kelvin Helmhotz.
Forms longitudinal temperature gradient. Final state is a totally mixed lake.
What is mixing regime D?
Ri<1
Deepening is so rapid and chaotic that the interface is not clearly defined.
What is the summer stratification like?
Solar radiation heats the epilimnion establishing a concentration gradient and strong stratification.
What is the autumn stratification like?
Solar radiation is reduced and the epilimnion cools.
With typical autumn winds, the lake will turnover to completely mix.
Wipes out stratification and aerates the bottom water.
What is the winter stratification like?
INVERSE STRATIFICATION
Surface water cools to below 4 degrees so is lighter than the water below it.
If ice forms, wind mixing is not possible and stratification is established.
No discernible layer between epilimnion and hypolimnion.
What is a DIMITIC lake?
A lake that undergoes the full cycle of seasons.
