Lake Morphometry Review Light in Lakes

1
Light Heat
UV
Infrared
2
The Electromagnetic Spectrum
3
Light Energy Units

• Einstein (mole of photons)
• µ E meter -2 sec-1
• Lux
• Foot-candles
• Energy units
• (watts, calories, joules)

4
Sample intensities
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Sunlight
Shading, Reflection, Absorption
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Sunlight
Shading, Reflection, Absorption
Surface Reflection
Scattering, Absorption (heating) ( attenuation)
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Different wavelengths of light are absorbed by
different things in water
Pure water transmits UV and visible
light, Absorbs IR
-Infrared
8
Light absorptionVariations in different lakes
Low DOC
Percentile Absorption Per Meter
Total absorption
DOC
Very High DOC
Particulates
Water
www.pca.state.mn.us/publications/mnenvironment

• Absorption by
• Water itself (IR)
• Particulates (algae, silt, etc)
• Colored dissolved organic
• matter (UV)

Modified from Wetzel 2001
9
Measuring light attenuation

• Simplest measure Secchi depth

Farmington Bay, Great Salt Lake
Bear Lake
10
Measuring light attenuation
Pettit Lake, Idaho
- Radiometer
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Calculating the extinction coefficient
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Calculating the extinction coefficient
The vertical extinction coefficient is - Slope
of this ln-transformed line Slope Rise
- (ln Iz ln I0) Run
? depth
- (ln 1.9 ln
35) 25 5 m
- ( 0.62
3.55) 20
m 0.146

• Only relevant for homogeneous layer of water
• Whats a better way to calculate the coefficient?

13
Secchi vs. Extinction coefficent
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Light attenuation in natural lakes
Light Intensity (uEinsteines m-2
sec-1)
50 100 150 200 250
300
Depth (meters )

Light intensity at different depths for lakes of
varying trans-parencies, in relation to the lower
light limits of selected biological processes.
Note the tremendous range over which many
biological process operate and how the depth
limits for various processes change in response
to transparency. Note that the figure is for
full daylight. Moonlight is about 6 orders of
magnitude less, and a cloudy night ski is about
10 orders of magnitude less than full sunlight.
15
EX Light and algae in lakes
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EX Feeding Rates of Fish
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Rules of Thumb
Compensation Depth
Photic zone area where phytoplankton primary
production exceeds respiration
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Rules of Thumb

• Photic zone extends to where approx. 1 of the
  surface light remains
• (called compensation depth)
• Photic Zone extends 2-3 Secchi depths
• Visual feeding for fish extends to 5-6 Secchi
  depths