Light in Lakes

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Light in Lakes
Why important? Energy for photosynthesis Heating
Light to see by
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Photic zoneSurface down to where there is lt 1
of surface light -- sufficient light for
photosynthesis
Photic zone
Aphotic zone
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figure 5-3
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As the light hits the water surface and travels
down through the water column, light gets
lost. Reflection 5-6 (depends on angle of
sun, waves, ) Scattering -- due to suspended
particles SESTON Tripton non-living, both
inorganic and detritus Plankton
living Absorption water itself dissolved
organic materials seston (Fig 5-10, light
absorption in 4 Wisconsin lakes)
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figure 5-17
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As a result of scattering and absorption, light
decreases exponentially with depth
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Light decreases exponentially with depth
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We can express this with a negative exponential
equation Iz I0e-?z Iz light at depth z I0
light at the surface ? (eta) light extinction
coefficient z depth
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Iz I0e-?z If we take the natural log of both
sides ln Iz ln I0 - ?z Straight line with ln
Iz a function of z. ln I0 is the intercept and ?
is the slope.
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How do we determine light extinction
coefficients? 1. Two point method solve ln
Iz ln I0 - ?z for ? ? ln I0 - ln Iz
z Measure light at the surface (just under)
and at a depth z. Substitute into the equation.
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2. Regression (or line fitting) ln Iz ln I0 -
?z Measure light at many depths. Plot ln Iz
versus depth. ? will be the slope of this line.
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3. Estimate from secchi depth ? ? 1.7 / zSD
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What are typical light extinction
coefficients? Lake Tahoe 0.002 m-1 99
would reach 5 m depth Lawrence Lake 0.39
m-1 14 would reach 5 m depth Wintergreen
Lake 1.00 m-1 0.7 would reach 5 m
depth Lake with a secchi depth of 7 m ? ? 1.7
/ 7 ? ? 0.24 m-1 30 of light would reach 5 m
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Different colors (wavelengths) of light are not
absorbed equally
Pure water
Fig 5-7
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PAR photosynthetically active radiation Light
adsorption by chla and chlb Light adsorption by
Ulva and Porphyra
Why arent all algae on the surface? UV
inhibition
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What color are things deep in lakes? Sort of
gray-blue So why do pictures of coral reefs have
all these nice colors?