Light and Heat in Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 335

based on the assumption that, at the Secchi depth, light is about 10% of the light at the surface
(ln(0.1)/ln(0.01) = 2). Representative photic zone depths are illustrated in Table 12.1.
Since the light extinction coeficient increases and the depth of the photic zone decreases as a
result of excess plant growth such as by phytoplankton, the light extinction coeficient has com-
monly been used to characterize the trophic status of lakes and reservoirs (see Table 12.1). Carlson
(1977) used the Secchi depth as a basis for his trophic status of lakes, as will be discussed in
Chapter 16.
Common trophic levels include eutrophy, mesotropy, and oligotrophy. Eutrophic lakes and reser-
voirs are those that receive excess nutrients and hence are highly productive and have excess plant
growth. Oligotrophic lakes are very low in nutrients and productivity. Mesotrophic lakes are in
between.
The light extinction coeficient ranges for lakes of varying trophic status provided by Likens
(1975) are ultraoligotrophic (0.03-0.8 m -1 ), oligotrophic (0.05-1.0 m -1 ), mesotrophic (0.1-2.0 m -1 ),
and eutrophic (0.5-4.0 m -1 ). Forsberg and Ryding (1980) based their classiication on the Secchi
depth, with oligotrophic lakes having Secchi depths greater than 13 ft., mesotrophic lakes having
Secchi depths ranging from 8 to 13 ft., eutrophic lakes having Secchi depths from 3 to 8 ft., and
hypereutrophic lakes having Secchi depths less than 3 ft.
The light extinctions illustrated in Table 12.1 refer to total light. However, the penetration of light
through the water column varies with the wavelength so that the spectral distribution varies with the
depth. Light underwater is diminished by absorption and scattering (Kirk 1994). While absorption
removes light, scattering increases the probability that light will be absorbed by increasing the path
length. Since different wavelengths are absorbed differently, each wavelength would have its own
extinction coeficient. The sorption of longwave or thermal (infrared) radiation is very rapid, and
about 53% of the total light energy is converted to that in the irst meter of water (Wetzel and Likens
2000). Of the visible light, depending on the amount of scattering and absorption, the longer wave-
lengths, such as red, yellow, and orange, are absorbed irst (resulting in large extinction coeficients)
while the shorter wavelengths (violet, blue, and green) can penetrate further (smaller coeficients)
with blue penetrating the deepest. Chlorophyll, a photosynthetic plant pigment, is most eficient in
capturing red and blue light (at 665 and 465 nm, Figure 12.4). Relatively pure water will appear
blue, but depending on the dissolved and particulate materials in the water affecting scattering and
sorption, water may appear more green or brown in color.
12.3 SURFACE HEAT BALANCE
The exchange of heat between the water surface and the overlying atmosphere (surface heat
exchange) is primarily the result of the ive processes depicted in Figure 12.10. The irst two pro-
cesses are the absorption of the longwave and shortwave radiation discussed earlier. Solar energy
may be absorbed in the water column. If light penetrates to the bottom, that energy may then be
absorbed by the bottom substrate and later reemitted. The remaining processes include longwave
back radiation from the water's surface, conduction and convection, and evaporation, all of which
are impacted by the water temperature.
The longwave radiation from the water is basically heat emission from a warm object. The radia-
tion emitted is proportional to the absolute temperature of water to the fourth power (blackbody
radiation).
Conduction is the transfer of heat due to the differences in temperatures between the water and
the overlying air, and convection is the exchange of heat due to the movement of luids. For surface
exchanges, the magnitude of both is proportional to the temperature gradient and, at equilibrium,
the water temperature would be equal to the air temperature (no gradient). The transfer of heat
is also enhanced by wind. Convection and diffusion can also transport heat from the warmed
surface layer to lower layers. Where light penetrates to the bottom, the heat may be absorbed and
reemitted.