Heat and stratification

1
and
Stratification
Lake Mixing

• Key Questions
• What are the principal vertical zones in a lake?
• What are the different mixing regimes in lakes,
  and what causes them to vary?
• What are possible causes of meromixis in lakes?
• What controls lake stability, and how do we
  quantify stability?

2
Light and energy distribution
Source of heat for most lakes is
atmosphere/solar. Water conducts heat
poorly. What might temperature profile be?
Temperature
0 10 20
z
z
3
Common temperature profiles
Observed temperature profiles in lakes deviate
significantly from distribution of energy
input. Why? - Wind stress does work on
lake surface - Diel/weekly density-driven
currents
t
Temperature 4 25
0 10 20
Light
Winter
z
z
Summer
4
Density characteristics of water
Ice
From Denny 1993
5
Vertical Zonation in Lakes
t
Temperature
0 10 20
Inverse/ Normal
Epilimnion Metalimnion/
Thermocline Hypolimnion
z
z
6
Seasonal Patterns
What determines hypolimnetic temperature in
summer?
Winter Spring Summer Fall
t
t
t
tv
Temperature
Temperature
Temperature
Temperature
0 10 20
0 10 20
0 10 20
0 10 20
z
z
z
z
z
z
z
z
Inverse Isothermal Normal
7
Annual temperature changes in a dimictic
temperate lake
8
Lake Mixing Types Climate and latitude
Amictic ice-covered Cold monomictic Cold
polymictic Dimictic Warm polymictic Warm
monomictic Oligomictic
Hutchinson 1957
9
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10
Mixed Layer Depth and Productivity
11
Factors determining the depth of the thermocline
Wind and Morphology

• Mixing depth ? Area0.25
• Importance of fetch

12
Factors determining the depth of the thermocline
Morphology and Transparency/radiation
Fee, EJ et al. 1996. Effects of lake size, water
clarity, and climatic variability on mixing
depths in Canadian Shield lakes. LO 41 912-920
13
Salinity
Salt increases density
????999.8425940.06793952(T)-0.00909529(T)20.
0001001685(T)3-0.000001120083(T)40.0000000065
36332(T)50.824493(S)-0.0040899(T)(S)0.00007
6438(T)2(S)-0.00000082467(T)3(S)0.000000005
3875(T)4(S)-0.00572466(S)1.50.00010227(T)(
S)1.5-0.0000016546(T)2(S)1.50.00048314(S)2
14
More mixing terms

• Holomictic Meromictic

Mixolimnion
Monimo- limnion
15
Salinity and Meromixis
Salinity gradients can overcome thermal
gradients E.g. Lake Nyos, Lake Kivu
16
Salt sources and types of Meromixis
Crenogenic Meromixis inputs of salt from saline
springs Biogenic meromixis Decomposition of
organic matter Ectogenic external inputs of
salt water Cryogenic meromixis Salt exclusion
from ice
17
Resistance to mixing and stability

• (?1- ?2) / (z2-z1)

18
Conclusions

• Heat inputs, outputs, and wind mixing cause most
  lakes to thermally stratify into an epilimnion,
  metalimnion (thermocline) and hypolimnion
• Wind speeds and exposure (fetch), and
  transparency affect the depth of a lakes
  epilimnion
• Temperature and seasonality regulate lake mixing
  regime
• Salinity strongly affects stratification and
  density, and is very important to meromixis
• Lake stability is related to the strength buoyant
  forces (the density gradeint) relative to kinetic
  energy (surface waves and currents)