Announcements

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Announcements

• Jon Cole seminar 100, Engineering 101
• Field trip, Friday 230 530
• Warm cloths
• Notebook
• Warm, waterproof boots (i.e. pac boots)
• Homework Wednesday

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Thermal Stratificationin Lakes

• 22 Jan 2007
• AWER 4500

http//waterontheweb.org/curricula
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A reminder Temperature water density
Means warmer water (gt4C) FLOATS on top of colder
water Water lt 4C floats on top of 4C water
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Thermal Stratification in Lakes A Seasonal Cycle
Sunlight
Spring mixing
Wind
4º C
Isothermal Conditions
4º C
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• Wind continues to mix heat in gain of radiation
• Duration of mixing dictated by amount of wind and
  size of lake

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Spring?Summer Stratification
Sunlight
Wind
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• Hypolimnion temperature determined by length of
  spring mixing
• Hypolimnetic entrainment gradually over summer
  fall
• Thickness of layers

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Fetch (km)
Epilimnion Thickness (m)
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Temperature (m)
Depth (m)
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Depending on lake size and depth, only a small
portion may exhibit thermal tratification
Epilimnion (Mixed Layer)
Metalimnion
Hypolimnion
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Fall Cooling and turnover
Sunlight
Isothermal Conditions
4º C
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• Significance nutrients stored in hypolimnion
  brought to the surface

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Sunlight
Ice and Snow
0º C
Inverse Stratification
4º C
4º C
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• Significance
• Warmest water for organisms now at bottom of lake
• Bottom and surface water not mixingnutrients
  released from sediments cant mix to surface,
  even through intensity of stratification is weak
• Ice blocks transfer of gases at surface
• ??

Depth
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Significance of the layered system

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Multiple Thermal HabitatsBear
LakeUtah/IdahoCisco distribution
Thermocline
Fish
Fish
Source Luecke, C. W. Wurtsbaugh 1993. Trans.
Am. Fish Soc. 122 112-120
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Another way to diagram lake temperatures

• ISOTHERM a line designating the depth at which
  temperature was constant through time

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Vertical lines isothermal conditions Horizontal
lines stratification Tightly packed lines -
thermocline
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Geographic Morphometric Differences in Mixing
Patterns

• What would the annual mixing pattern be like in
• 5-m deep, 100 km2 lake in Utah?
• 50-m deep, 100 km2 lake in coastal California?
• 50-m deep, 100 km2 lake in the lowland tropics?
• 50-m deep, 100 km2 lake in Antarctica?

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Mixing patterns in lakes

• The pattern demonstrated here is called DIMICTIC
• Other types of lakes amictic, polymictic, warm
  monomictic, oligomictic, cold monomictic

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Polymictic

• Mixes many times a year
• Common in shallow lakes
• Leads to high productivity

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Warm Monomictic

• Mixes once each year
• Never gets cold enough to freeze, or too large!

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Meromictic Lakes

• All the lakes weve discussed so far are
  HOLOMICTIC
• Meromictic lakes
• Salt-stabilized
• Salinity

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A meromictic lake

• The Great Salt Lake

Temperature ( C)
Density
Density
Temperature (13 Dec. 1985)
Depth (meters)
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• Characteristics of monimolimnions
• High gradients in chemistry and microbiology
• Bottom temperatures can be higher
• Can have very different biota!