BIO321
Sept. 20, 2000
Light under Water in Lakes
I: Review
A. Light reaching the surface of a lake is determined by:
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latitude
season
time of day
altitude
meteorological conditions
B. Reflective loss is about 50% of the light leaving the lakes surface
 Reflection(R)  1.18S A.77 (90˚=3%, 15˚=12%, 10˚=16%)
 SA = The sun's altitude or angular height in degrees
 Rs ≈ 6.5% in summer
 Rw ≈ 10% in winter
C. Scattering or internal reflection makes up the other 50% of reflective loss (about 5 - 10
percent of total light loss)
1. Causes
 Water molecules
 Solutes
 Particles
 Sediment (bottom)
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2. Impacts on scattering
increases with concentration of particulates
Scattering coefficient varies for different wavelengths()
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As in the sky, clear water scatters blue
wavelengths (clear lakes blue)
As the size of the molecule/particle
increases, the wavelength of light scattered
increases
 particulates make water blue-green
 organics make lakes green-yellow
QH
QU
QA
QS
QR
QW
II. Light in its various radiant forms plays important roles as radiant energy in lakes
During the Day: QB = QS + QH + QA - QR - QU - QW
QB = Net Radiation Surplus: the net amount of solar radiation affecting a lake
QS = direct solar radiation (the sun)
QH = indirect solar radiation: scattered and reflected radiation from clouds & sky
QA = long wavelength thermal radiation gain from atmosphere and surrounds
QR = reflected light - radiation reflected from the lake's surface
QU = upscatter: light scattered from water column and benthos
QW = emission from the lake of long wavelength thermal radiation
During the Night: QB = QA - QW
Light regulates the biotic components of lakes via photosynthesis.
Light also regulates the thermal properties of lakes, which in turn regulates the lake "metabolism" and
respiration of biotic components.
III. F.A. Forel was one of the first limnologists to examine light as a variable in lakes. In 1865, using
photographic plates placed at different depths, he discovered light in some lakes penetrating to 200 meters, but
in others penetrated only a few meters.
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Light decreases with depth exponentially (see figure 7.3 above)
 Attenuation
 upscatter
 absorbtion
 light energy is transformed to heat energy by collision with stuff, with 53% of all light being converted to
energy within the first meter of water
 stuff =water, dissolved salts, organic molecules, seston = bioseston (plankton) and abioseston(nonliving POM (Particulate Organic Matter))
Coefficient of extinction of light in water
w + p + d (Epsilon = coefficient of extinction, w = water, p = particulates(POM or
PIM), d = dissolved color (DOM or DIM)
 Light is quantified via absorption or transmission as in spectrophotometry
increased A means a decrease in T resulting from a HIGHER 
 Iz = I0e-z (See Horne and Goldman page 37)
 light is not monochromatic and different wavelengths are absorbed differently (each wavelength has a
different  or  is wavelength dependent)
- Distilled water ROY is strongly absorbed in the first meter while BG penetrates the most
 Various factors modify light penetration in lakes
water (w)
particulates (p)absorb ROY and BIV allowing green to penetrate, but have little impact on
transmission at low concentrations
color caused by organics absorbs R(less selectively) and IR leading to heating
dissolved color (d) absorbs UV and BG within the first meter
 Transmission of light in lakes varies with lake type and season and is diagrammed via isopleths as in the
diagram above (Figure 5-9).
In Oligotrophic lakes (Lake Lawrence)  averages 0.39/meter and is more or less constant.
In Eutrophic lakes (Wintergreen Lake)  averages 1.0 /meter and varies a lot due to changes in algal
populations
 Compensation Depth = 1% T level separating the upper
productive trophogenic zone from the lower
decompositional tropholytic zone
Trophogenic
Comp. Depth
 Algae differ by light useage depending on the pigments
Tropholytic
that they contain (Chl. A, B, C, D, E and more)
1. green algae with Chlorophyll A absorbs red
(640nm) and blue (405nm), also possess Chlorphyll
B (620nm and 440nm) and thus live in surface
waters where these wavelengths are abundant
2. Diatoms, bluegreens (and also greens) possess mainly Accessory pigments like carotenoids and
xanthophylls which absorb in the yellow-green (450-550nm) and/or phycocanin (Bluegreens and
Cryptomonads (540-583nm). These light wavelengths penetrate deeper and so these organisms can live
at depths or in winter.
 Thus lake color is an important parameter that is quantified in one of several ways.
1. Platinum units range from 0 (clear) to 300 (stained like weak tea) and are a set of standards compared
to a mixture of Potassium hexachloroplatinate and Cobalt Chloride and HCl in water.
2. Forel-Ule Scale (Based upon CuSO4 + K2CrO4 + CoSO4
3. True color must be distinguised from
apparent color (bottom/particulate
effects)
 Transmission through ice and snow
-Clear ice = 85%T
-Bubbled ice = 20%T
-Snow covered ice T may be much lower
-Decreased light leads to decreased
photosynthesis leading to decreased O2
which of course is needed by the life of
the lake from plants to consumers to
decomposers. The result may lead to
either hypoxia or anoxia leading to winter
kill typical in small, shallow lake systems
in North Temperate Regions
-Green light (480-550nm) passes best
through ice restricting life to organisms
that can use green light (flagellated
cryptomonads for example)
 Transparency is an approximation of
%T &  and A.
- is usually measured instrumentally with photometers or estimated with the Secchi Disk.
- Secchi Disk is a 20 cm white or white and black disk lowered on the shaded side of a boat
(avoid dawn and dusk) until it disappears (note distance) and then pulled up until it reappears
(note distance) and then averaged (down + up)/2.
-Secchi Disk Transparency is a function of the light reflected from the disk and is influenced by
dissolved, particulates, and water.
-Secchi Disk Depth(SDD) = 5% Transparency
-compensation depth = 1.3(ZSD) = 1%T = the as a general rule, but use with caution since
sediments and organics (color) can change the relationship.
- You can estimate the extinction coefficient for a lake:
 ≈1.7/ ZSD