readings: snucins & gunn 2000
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Lec 2: Light and Heat. I. Light and Transparency II. Stratification:Vertical Temp. Gradients III. Circulation. Readings: Snucins & Gunn 2000. 1. All-important influence on in-lake conditions Solar Spectrum: Differing wavelengths and intensities. Solar Radiation. - PowerPoint PPT PresentationTRANSCRIPT
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Readings: Snucins & Gunn 2000
Lec 2: Light and Heat
I. Light and Transparency
II. Stratification:Vertical Temp. Gradients
III. Circulation
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Solar RadiationAll-important influence on in-lake conditions
• Solar Spectrum: Differing wavelengths and intensities
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• Variation in the solar spectrum• PAR: Photosynthetically Active Radiation• Infrared: Main heat sourcePAR
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Selective Absorption of the Solar Spectrum by 1 meter of Pure Water
100
50
0300 400 500 600 700
UV IRV B G Y O R
%Absorbed
Wave Length, nanometers
50% of remaining light is absorbed for each additional meter, yet:•30% blue light remains after 70m•6% yellow light remains after 70%•0% orange light remains after 17 m•0% of red light remains after 4 m 3
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Selective Light Transparency in Different Lakes
Tahoe, CA-NV
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2
3
4
5
Depth(m)
0.1 0.5 1.0 5 10 50 100Percent Incident Light
• Transparency of water depends on:– Wave length (water is
differential in its absorption of certain wave lengths)
– Suspended materials– Dissolved materials
• Different lakes tend to have different light absorption characteristics Long, MN
Crystal, WI
Montezuma Well, AZ
Itasca, MN
Little Triste, AZ
Secchi Disk
Saguaro, AZ
Seneca, NY
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Determination of Transparency
• Transparency– Secchi Disk (20cm diameter) - measures depth
of 95% light absorption – range <5 cm - >40 m (Crater Lake, Oregon, has the greatest transparency of any North American Lake)
– Light meter typically measures in photons or calories (lakes have light profiles just as they have oxygen & temperature profiles)
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S
Quantification of Transparency Extinction Coefficient
• Based on Bouguer’s (a.k.a. Lambert’s) & Beer’s laws, where:Io = intensity of entering lightId = intensity of light at depth Z
e = base of natural logarithms (approx. 2.7)
k = extinction coefficient
• The “Extinction Coefficient” (k) is the proportion of the original light absorbed at a depth
• The proportion of light transmitted through a depth is called the “Transmission Coefficient” (k is more commonly used)
kZod eII
kZ
o
d eI
I Z
IlnIlnk Zo
“Secchi Depth”
k = 1.7 / Zsd
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Turbidity• Turbidity is a measure of water’s cloudiness• Caused by suspended materials in water• Often varies seasonally, affected by:
– Water movements– Stream discharge– Plankton populations
• Settling time for suspended materials vary:– sand: 10 cm/second (still water)– colloids: <0.5 cm/year (still water) 7
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Light is Attenuated More Rapidly in Eutrophic Lakes
0 20 40 60 80 100
Light (% of incoming)
7
6
5
4
3
2
1
0
Dep
th (
m)
Olig
otro
phic
Mes
otro
phic
Eutrophic
A
1 10 100Light (% of incoming)
Olig
otro
phic
Mes
otro
phic
EutrophicB
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Photic Zone
Compensation
Depth
• Affected by water clarity• Important for system metabolism• Important habitat determinant
-Heat-Dissolved oxygen (DO)
Possible to calculate from Secchi Depth?
ddpz
k
6.4
k
100lnZ
99
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Thermal Characteristics of Lakes• Light and heat represent a continuum with wave lengths
e.g. >700 nm (infrared) = heat• Water selectively absorbs in the infrared
– at 820 nm 91% absorbed within the 1st meter– 99% absorbed within the 2nd meter
• Based on the absorption of light, you would expect the following temperature profile of a body of water at uniform temperature exposed to the sun:
Temperature
Depth
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• Lakes generally do not show heat distributions that directly reflect the relative absorption of light with depth
• Many lakes (esp. deep)stratify during part of the year
Thermal Characteristics of Lakes
• This results in a characteristic thermal profile:
Epilimnion (upper water)Metalimnion (middle water)Hypolimnion (lower water)
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Stratification• Layering based on differences in density
(temperature or salinity)
• Stratification alters biogeochemistry and ecology
• Lake with all same temperature called isothermal
• Thermal stratification into three layers
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A Thermally Stratified Lake
0 2 4 6 8 10 12 14 16 18
Temperature (0C)
25
20
15
10
5
0
Dep
th (
m)
Epilimnion
Metalimnion (thermocline)
Hypolimnion
Defined by at least1OC / m
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Principles governing thermal stratification1. Heat enters and leaves the lake (mostly) from the surface2. Temperature affects water density3. Warmer water has a much greater difference in density per degree change than cold water
Thermal Characteristics of Lakes
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• Amictic - no mixing. Applies only to lakes that permanently are ice covered. Arctic climates only
• Cold Monomictic - Temperature never exceeds the temperature of maximum density (4OC). Ice covered from late fall through late spring, mixes all summer. In very cold climates.
• Dimictic - Spring and Fall mixing periods. Lake surface freezes in winter, lake is thermally stratified in summer
• Warm Monomictic - Lake never freezes. Mixes over winter. Stratified from early spring through late fall
• Oligomictic - Circulates irregularly. Mostly in the tropics• Polymictic - Continually circulates at low temperatures
Lakes at high elevations near the equator
Classification of Lake Mixing RegimesFunction of latitude, elevation, morphometry
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Lake Thermal Profile - Time and Depth
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Diagrammatic Representation of Dimictic Mixing Regime
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Wind
Hypolimnion
MetalimnionEpilimnion
-Wind mixes surface heat down -Density differences cause resistance to vertical mixing-The work need to mix depends on the different desities of the strata-However, much more work is needed to mix 25o to 15o vs. 15o to 5o
Why? When is mixing most likely to occur? Effect of wind fetch?
Circulation Patterns in a Stratified Lake 18
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Annual Temperature Cycle of a Dimictic Lake Represented as Temperature-Depth Profiles
0O 4O 0O 4O 0O 4O 0O 4O
Depth
Temperature
SummerStratification
FallOverturn
WinterStratification
SpringOverturn
Ice
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0O4O 0O4O 0O4O 0O4O
Depth
Temperature
SummerStratification
Fall WinterMixing
Spring
20O 20O 20O 20O
Annual Temperature Cycle of a Warm Monomictic Lake
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Importance of Heat and it’s Distribution
• High heat retention – due to specific heat of water
• Most biological processes have Q10 values of 2-3
• Influence on DO concentrations (Important habitat variable)
• Determines who, when, & where re: community composition and ecosystem processes
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Horizontal Lake Zones and Biota
psammon
macrophytes
benthos
Shallow & deepwater emergents
Floatingplants
Submerged plants
Sublittoral zone
Profundal zone 22