Light and Temperature
19
Although theoretically the extinction coefficient (11) is constant for a given wavelength
in water, in nature underwater irradiance is a composite of many wavelengths.
Therefore the relationship is imperfect under natural conditions and represents a
composite for multi chromatic light and the various characteristics of water that affect
absorption and scattering.
The in situ or total extinction coefficient (11,) is a composite of absorption by the water
itself (l1w), by particles suspended in the water (l1p), and especially by dissolved
compounds or "color" (l1c) (Aberg and Rodhe, 1942):
11, = I1w + I1p + I1c
Suspensions of particulate matter have relatively little effect on absorption at low
concentrations. Particulate matter can be removed by filtration or centrifugation.
The attenuation of total irradiance from direct and indirect insolation within a lake
can be measured with a variety of instruments. Ideally, the total underwater irradiance
would be measured with a receptor system at a specific depth. It is this amount of
energy, whether from direct incident light or from scattered light sources, that is
collectively important to organisms for photosynthesis or as behavioral stimuli. This
amount of irradiance is the photon scalar irradiance, defined as the total number of
photons from all directions about the point of measurement when all directions are
weighted uniformly (Smith and Wilson, 1972). Instruments for measuring photon
scalar irradiance are available commercially (e.g., Li-Cor, Lincoln, NE).
Approximations of underwater irradiance commonly are made with less sophisticated instrumentation. Many older underwater photometers contain selenium
photocells; more recent instruments employ silicon photovoltaic cells. These photocells
respond to light of wavelengths between approximately 300 and 750nm and usually
have a pronounced peak in response between 500 and 600nm (Strickland, 1958;
Sauberer, 1962; Westlake, 1965).
The physical structure of the photoreception unit is important. Most underwater
photometers are flat (a 2n system) and respond largely to incident light from above.
Little or none of the laterally scattered light, as could be utilized by an organism, is
measured. Various hemispherical designs with diffusing materials permit a more
realistic measurement of incident as well as scattered light [e.g., Rich and Wetzel
(1969)]. Combined with a downward hemisphere, the photometer approaches a 4n
design that permits an estimation of both incident and scattered light from above and
reflected and scattered light from below.
Some photometers currently in use give relative measurements. That is, underwater
values are compared, as a percentage, to values measured immediately below the
surface of the water (e.g., 10-cm depth; to avoid wave-mediated reflection and
scattering). While such data are useful, more information is gained if measurements
were in absolute units. Recent photometers have quantum sensors that permit
measurement of photosynthetically active radiation within the 400- to 700-nm portion
of the spectrum directly in microeinsteins/m 2 -sec. Other sensors measure directly in
watts/m 2 or in lux (see Table 2.1).
None of these instruments measures the spectral distribution of the total underwater
irradiance. However, when filters with a specific and narrow range of wavelengths are
positioned over the photocells, the spectral attenuation of irradiance at depth can be
evaluated. Radiometers that scan the spectrum and measure the amount of energy at all
wavelengths within the visual spectrum are available commercially, and a few are
adapted for use underwater. Exemplary data using such an instrument are shown in
Fig. 2.2 and demonstrate the selective attenuation of irradiance in a mesotrophic lake.
19
Although theoretically the extinction coefficient (11) is constant for a given wavelength
in water, in nature underwater irradiance is a composite of many wavelengths.
Therefore the relationship is imperfect under natural conditions and represents a
composite for multi chromatic light and the various characteristics of water that affect
absorption and scattering.
The in situ or total extinction coefficient (11,) is a composite of absorption by the water
itself (l1w), by particles suspended in the water (l1p), and especially by dissolved
compounds or "color" (l1c) (Aberg and Rodhe, 1942):
11, = I1w + I1p + I1c
Suspensions of particulate matter have relatively little effect on absorption at low
concentrations. Particulate matter can be removed by filtration or centrifugation.
The attenuation of total irradiance from direct and indirect insolation within a lake
can be measured with a variety of instruments. Ideally, the total underwater irradiance
would be measured with a receptor system at a specific depth. It is this amount of
energy, whether from direct incident light or from scattered light sources, that is
collectively important to organisms for photosynthesis or as behavioral stimuli. This
amount of irradiance is the photon scalar irradiance, defined as the total number of
photons from all directions about the point of measurement when all directions are
weighted uniformly (Smith and Wilson, 1972). Instruments for measuring photon
scalar irradiance are available commercially (e.g., Li-Cor, Lincoln, NE).
Approximations of underwater irradiance commonly are made with less sophisticated instrumentation. Many older underwater photometers contain selenium
photocells; more recent instruments employ silicon photovoltaic cells. These photocells
respond to light of wavelengths between approximately 300 and 750nm and usually
have a pronounced peak in response between 500 and 600nm (Strickland, 1958;
Sauberer, 1962; Westlake, 1965).
The physical structure of the photoreception unit is important. Most underwater
photometers are flat (a 2n system) and respond largely to incident light from above.
Little or none of the laterally scattered light, as could be utilized by an organism, is
measured. Various hemispherical designs with diffusing materials permit a more
realistic measurement of incident as well as scattered light [e.g., Rich and Wetzel
(1969)]. Combined with a downward hemisphere, the photometer approaches a 4n
design that permits an estimation of both incident and scattered light from above and
reflected and scattered light from below.
Some photometers currently in use give relative measurements. That is, underwater
values are compared, as a percentage, to values measured immediately below the
surface of the water (e.g., 10-cm depth; to avoid wave-mediated reflection and
scattering). While such data are useful, more information is gained if measurements
were in absolute units. Recent photometers have quantum sensors that permit
measurement of photosynthetically active radiation within the 400- to 700-nm portion
of the spectrum directly in microeinsteins/m 2 -sec. Other sensors measure directly in
watts/m 2 or in lux (see Table 2.1).
None of these instruments measures the spectral distribution of the total underwater
irradiance. However, when filters with a specific and narrow range of wavelengths are
positioned over the photocells, the spectral attenuation of irradiance at depth can be
evaluated. Radiometers that scan the spectrum and measure the amount of energy at all
wavelengths within the visual spectrum are available commercially, and a few are
adapted for use underwater. Exemplary data using such an instrument are shown in
Fig. 2.2 and demonstrate the selective attenuation of irradiance in a mesotrophic lake.