Environmental Engineering Reference
In-Depth Information
where Z is the downward distance from the water surface and K e is the light extinction coeficient.
The light extinction coeficient can be calculated from the measured light over depth based on
Beer's law. Typical extinction coeficients, as well as Secchi depths, for a number of lakes are listed
in Table 12.1.
The extinction coeficient for reservoirs is frequently greater than that for natural lakes. For
example, the light extinction coeficients for Lake Houston, Texas, range from about 2.5 to 8.0 m -1
(Lee and Rast 1997) compared to 0.06-0.12 m -1 for Crater Lake, Oregon (Table 12.1).
The penetration of light through the water column is affected by the materials dissolved or sus-
pended in it. Pure water has a light extinction coeficient of about 0.03-0.04 m -1 (Lorenzen 1972;
Verdiun et al. 1976). Other factors that may increase light extinction include pigments or organic
acids dissolved in water, suspended solids, and phytoplankton (Martin and McCutcheon 1999).
The greater the extinction coeficient or the lower the Secchi depth, the less the light will pen-
etrate. Higher extinction coeficients may be due to the naturally higher color or turbidity associated
with southern waterbodies, but they may also be the result of pollution. For example, excess nutrient
loadings result in excess growth of aquatic plants.
As such, the light extinction impacts the depth of the euphotic zone, or the zone in which plants
can live. Recall from Chapter 11 that the bottom of the photic zone is usually taken to be that depth
where the radiation is 1% of that at the surface ( H / H 0 = 0.01), so that the depth of the photic zone
will be Z = 4.61/ K e . Although the Secchi depth is not a direct measure of the photic zone depth, a
common rule of thumb is that the depth of the photic zone is about twice the Secchi depth. This is
TABLE 12.1
Representative Values of Extinction Coeficients, Secchi Depths, and Photic Zone Depths
Secchi
Depth
(m)
Depth of
Photic
Zone (m)
Lake
Ke (m 1 )
Description
Crater Lake (OR)
0.06-0.12
25-45
>120
Clear, sky blue ultraoligotrophic lake
Lake Tahoe (CA/NV)
0.12
40
90-136
As above but with decreasing clarity since 1960s
due to watershed overdevelopment
Lake Superior (blue water)
0.13
15-20
46-60
Ultraoligotrophic, most oligotrophic of the
Laurentian Great Lakes
Lake Superior (green
water near Duluth)
0.3
5-12
20-30
Western arm near Duluth and St. Louis River and
harbor inputs
St. Louis River (Duluth-
Superior Harbor)
4.21
0.7
>5
Brown (bog) stained from river plus high
suspended sediments
Lake Michigan
0.19-0.24
?
19-31
Mesooligotrophic
Lake Huron
0.1-0.5
?
25-31
Mesooligotrophic
Lake Erie
0.2-1.2
2-10
12-26
Eutrophic (clarity improving recently due to
zebra mussels)
Lake Ontario
0.15-1.2
?
12-29
Mesotrophic
Lake Baikal (Siberia)
0.2
5-40
15-75
Oligotrophic
Grindstone Lake (Pine
County, MN)
0.82
3-6
8-20
Mesotrophic, water is fairly stained or colored
Ice Lake (Itasca County,
MN)
0.83
2-5
6-15
Mesotrophic
Lake Minnetonka
Halsted Bay (Hennepin
County, MN)
2.9
0.5
<2
Eutrophic
Source: USEPA, Watershed Academy Web, Understanding lake ecology, Available at: http://cfpub.epa.gov/watertrain.
 
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