Chapter 15 Remote Sensing of Seagrasses
351
higher spatial resolution. The range of airborne and
satellite remote sensing instruments now available
offers the potential for spatial, spectral, radiometric,
and temporal resolution to optimize the detection of
benthic vegetation over a wide range of environmental situations. This makes remote sensing a feasible
option for regular monitoring of seagrass meadows.
III. Optical Properties of the Overlying
Water Column
A. Introduction
The fundamental principles of the interaction of light
in water are discussed in Zimmerman and Dekker,
Chapter 12 and the interaction of the light within
the canopy is discussed in Zimmerman, Chapter 13.
In this chapter, we further develop these principles
and use them to understand how and why the remote
sensing of seagrasses and associated environments
is possible. An understanding of the way light interacts with the atmosphere, through the air–water
interface, through the water column (and vice versa)
must be obtained before it is possible to place results from scientific and applied literature, and case
studies, into an overall perspective.
A remote sensing instrument measures light from
the sun after it has passed through the atmosphere, interacted with the target, and has been reflected back
through the atmosphere to the sensor mounted on
an aircraft or a satellite. Scattering causes the light
to change direction whereas absorption captures
light and transforms it into another form of energy.
Thus at any level within the atmosphere, and specifically just above the water surface, downwelling
irradiance is composed of a direct sunlight fraction and a scattered sunlight fraction called diffuse
skylight.
At the air–water interface, two processes can occur; the downwelling irradiance E d is reflected due
to the specular or Fresnel reflectance of the water
surface, or the light is refracted (due to a density
difference between air and water) and passes into
the water column. For a flat water surface, the calculation of these Fresnel reflectance and refraction
values is straightforward. For a rough water surface
with swell, waves, and fractal wavelets, the situation becomes more complex, but it can still be calculated or may alternately be approached through
observations.
The same processes affect the passage of light energy in the water column as in the atmosphere but the
materials causing the scattering and absorption are
different. In a natural water column above a seagrass
meadow, the depth of the water itself and five water column components determine the fate of light
in the water before it reaches the substratum. Pure
water, colored dissolved organic matter (CDOM),
phytoplankton, dead organic particulates, and mineral particulates each absorb and scatter light in a
spectral (i.e. wavelength dependent) manner that is
known or may be simulated. The light reaching the
seagrass canopy is modified by the interaction of the
downwelling irradiance with these substances and
thus influences the quality of the light available for
photosynthesis (see Zimmerman, Chapter 13).
B. Optical Properties of the Water Column
Over a Seagrass Bed
Pure water has fixed absorption and scattering properties that are slightly dependent on temperature and
salinity. CDOM is by definition dissolved and thus
should not scatter light. Due to its organic composition, the color of CDOM is yellow and thus it acts
as a blue light filter. Similarly, dead organic matter
suspended in the water column usually has a yellow to brown color and also acts as a blue light
absorber. The photosynthetic light harvesting pigments of phytoplankton absorb strongly in characteristic wavelengths. The most important pigment is
chlorophyll a, which has in vivo absorption maxima
at 438 and at 676 nm although many accessory photosynthetic and photoprotective pigments contribute
to absorption over a range of visible wavelengths.
The cell components and cell walls of phytoplankton
also scatter light, particularly if cells contain large
gas vacuoles. Mineral matter (clays and silts) absorbs
light depending on the color of the source material.
This color can vary from white to gray, black in extreme cases, or it may have various hues of yellow,
brown, or red. All particulate matter (algae, dead
organic matter, and mineral matter) scatters light;
the amount and spectral shape of scattering is determined by the particle size distribution and the refractive index of the particles.
Fig. 2 shows absorption and scattering spectra of
the various components in the water column from a
coastal water site in Australia. The asterisks superscript denote a specific absorption or scattering; i.e.
where the true spectral absorption or scattering has
351
higher spatial resolution. The range of airborne and
satellite remote sensing instruments now available
offers the potential for spatial, spectral, radiometric,
and temporal resolution to optimize the detection of
benthic vegetation over a wide range of environmental situations. This makes remote sensing a feasible
option for regular monitoring of seagrass meadows.
III. Optical Properties of the Overlying
Water Column
A. Introduction
The fundamental principles of the interaction of light
in water are discussed in Zimmerman and Dekker,
Chapter 12 and the interaction of the light within
the canopy is discussed in Zimmerman, Chapter 13.
In this chapter, we further develop these principles
and use them to understand how and why the remote
sensing of seagrasses and associated environments
is possible. An understanding of the way light interacts with the atmosphere, through the air–water
interface, through the water column (and vice versa)
must be obtained before it is possible to place results from scientific and applied literature, and case
studies, into an overall perspective.
A remote sensing instrument measures light from
the sun after it has passed through the atmosphere, interacted with the target, and has been reflected back
through the atmosphere to the sensor mounted on
an aircraft or a satellite. Scattering causes the light
to change direction whereas absorption captures
light and transforms it into another form of energy.
Thus at any level within the atmosphere, and specifically just above the water surface, downwelling
irradiance is composed of a direct sunlight fraction and a scattered sunlight fraction called diffuse
skylight.
At the air–water interface, two processes can occur; the downwelling irradiance E d is reflected due
to the specular or Fresnel reflectance of the water
surface, or the light is refracted (due to a density
difference between air and water) and passes into
the water column. For a flat water surface, the calculation of these Fresnel reflectance and refraction
values is straightforward. For a rough water surface
with swell, waves, and fractal wavelets, the situation becomes more complex, but it can still be calculated or may alternately be approached through
observations.
The same processes affect the passage of light energy in the water column as in the atmosphere but the
materials causing the scattering and absorption are
different. In a natural water column above a seagrass
meadow, the depth of the water itself and five water column components determine the fate of light
in the water before it reaches the substratum. Pure
water, colored dissolved organic matter (CDOM),
phytoplankton, dead organic particulates, and mineral particulates each absorb and scatter light in a
spectral (i.e. wavelength dependent) manner that is
known or may be simulated. The light reaching the
seagrass canopy is modified by the interaction of the
downwelling irradiance with these substances and
thus influences the quality of the light available for
photosynthesis (see Zimmerman, Chapter 13).
B. Optical Properties of the Water Column
Over a Seagrass Bed
Pure water has fixed absorption and scattering properties that are slightly dependent on temperature and
salinity. CDOM is by definition dissolved and thus
should not scatter light. Due to its organic composition, the color of CDOM is yellow and thus it acts
as a blue light filter. Similarly, dead organic matter
suspended in the water column usually has a yellow to brown color and also acts as a blue light
absorber. The photosynthetic light harvesting pigments of phytoplankton absorb strongly in characteristic wavelengths. The most important pigment is
chlorophyll a, which has in vivo absorption maxima
at 438 and at 676 nm although many accessory photosynthetic and photoprotective pigments contribute
to absorption over a range of visible wavelengths.
The cell components and cell walls of phytoplankton
also scatter light, particularly if cells contain large
gas vacuoles. Mineral matter (clays and silts) absorbs
light depending on the color of the source material.
This color can vary from white to gray, black in extreme cases, or it may have various hues of yellow,
brown, or red. All particulate matter (algae, dead
organic matter, and mineral matter) scatters light;
the amount and spectral shape of scattering is determined by the particle size distribution and the refractive index of the particles.
Fig. 2 shows absorption and scattering spectra of
the various components in the water column from a
coastal water site in Australia. The asterisks superscript denote a specific absorption or scattering; i.e.
where the true spectral absorption or scattering has