EXERCISE 2
Light and Temperature
Solar radiation is vital to the metabolism, indeed
to the very existence, of freshwater ecosystems.
Nearly all energy that drives and controls the
metabolism of lakes and streams is derived from
solar energy, which is converted biochemically
via photosynthesis to potential chemical energy.
The photosynthetic synthesis of organic matter
occurs within the lake or river (autochthonous)
or within the terrestrial drainage basin (allochthonous) and is transported to the aquatic ecosystem in various forms of dissolved and particulate organic matter by "vehicles" for movement
[e.g., air, water, animals; cr., Likens and Bormann
(1972)]. In addition to direct biological utilization, the absorption of solar energy and its
dissipation as heat markedly affect the thermal
structure and stratification of water masses and
circulation patterns of lakes, reservoirs, and
streams. These characteristics in turn have profound effects on nutrient cycling and the distribution of dissolved gases and the biota. The
optical properties of fresh waters, therefore, exert
important regulatory controls on the physiology
and behavior of aquatic organisms.
Light impinging on the surface of water does
not penetrate completely-a significant portion
LIGHT
Units of Measurement
is reflected and back scattered [cf., Wetzel (1983)
and Exercise 4, p. 43]. Within the water, light is
rapidly attenuated with increasing depth by both
absorption and scattering mechanisms. Absorption is defined as diminution oflight energy with
increasing depth by transformation to heat [cf.,
Westlake (1965)]. Absorption is influenced by
the molecular structure of water itself, by particles suspended in the water, and particularly by
dissolved organic compounds. The result is a
selective absorption and attenuation of light
energy with increase in depth, influenced by an
array of physical, chemical, and, under certain
conditions, biotic properties of the water. These
optical properties are dynamic, changing seasonally and over geological time for individual
freshwater ecosystems.
Over half of the solar radiation that penetrates
into water is absorbed and dissipated as heat. As
we will see in subsequent analyses (Exercises 3
and 4), the distribution of this heat is influenced
greatly by wind energy. In this exercise, we will
evaluate methods for the measurement and
description of the distribution of light and
temperature in water.
Solar radiant energy that reaches the surface of the earth has a spectral range from
about 300 nm (ultraviolet) to about 3000 nm (infrared). Light is that portion, about half,
of the total radiant energy that can be detected by the human eye (380 to 780 nm).
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Light and Temperature
Solar radiation is vital to the metabolism, indeed
to the very existence, of freshwater ecosystems.
Nearly all energy that drives and controls the
metabolism of lakes and streams is derived from
solar energy, which is converted biochemically
via photosynthesis to potential chemical energy.
The photosynthetic synthesis of organic matter
occurs within the lake or river (autochthonous)
or within the terrestrial drainage basin (allochthonous) and is transported to the aquatic ecosystem in various forms of dissolved and particulate organic matter by "vehicles" for movement
[e.g., air, water, animals; cr., Likens and Bormann
(1972)]. In addition to direct biological utilization, the absorption of solar energy and its
dissipation as heat markedly affect the thermal
structure and stratification of water masses and
circulation patterns of lakes, reservoirs, and
streams. These characteristics in turn have profound effects on nutrient cycling and the distribution of dissolved gases and the biota. The
optical properties of fresh waters, therefore, exert
important regulatory controls on the physiology
and behavior of aquatic organisms.
Light impinging on the surface of water does
not penetrate completely-a significant portion
LIGHT
Units of Measurement
is reflected and back scattered [cf., Wetzel (1983)
and Exercise 4, p. 43]. Within the water, light is
rapidly attenuated with increasing depth by both
absorption and scattering mechanisms. Absorption is defined as diminution oflight energy with
increasing depth by transformation to heat [cf.,
Westlake (1965)]. Absorption is influenced by
the molecular structure of water itself, by particles suspended in the water, and particularly by
dissolved organic compounds. The result is a
selective absorption and attenuation of light
energy with increase in depth, influenced by an
array of physical, chemical, and, under certain
conditions, biotic properties of the water. These
optical properties are dynamic, changing seasonally and over geological time for individual
freshwater ecosystems.
Over half of the solar radiation that penetrates
into water is absorbed and dissipated as heat. As
we will see in subsequent analyses (Exercises 3
and 4), the distribution of this heat is influenced
greatly by wind energy. In this exercise, we will
evaluate methods for the measurement and
description of the distribution of light and
temperature in water.
Solar radiant energy that reaches the surface of the earth has a spectral range from
about 300 nm (ultraviolet) to about 3000 nm (infrared). Light is that portion, about half,
of the total radiant energy that can be detected by the human eye (380 to 780 nm).
15