Chapter 12
Aquatic Optics: Basic Concepts for Understanding
How Light Affects Seagrasses and Makes
them Measurable from Space
Richard C. Zimmerman
Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University,
Norfolk, VA 23529; e-mail:rzimmerm@odu.edu or arnold.dekker@csiro.au
Arnold G. Dekker
Environmental Remote Sensing Group, CSIRO Land & Water, P.O. Box 1666, 2601, Canberra,
ACT, Australia; e-mail: arnold.dekker@csiro.au
I. Introduction
This chapter provides an overview of aquatic optics
as it pertains to the transmission of light through the
water column and as it impinges on and reflects from
benthic substrates, such as seagrasses. This chapter
is meant as a primer for Chapter 13 (Zimmerman:
Light and Photosynthesis in Seagrass Meadows) and
Chapter 15 (Dekker et al., Remote Sensing of Seagrasses). Application of the theory presented here
will be developed further in those chapters.
Light transmitted through the atmosphere is modified by absorption and scattering before it hits the
surface of the water, where it is either specularly
reflected or transmitted across the air–water interface (Fig. 1). The water and its constituents further
modify light entering the water, again through processes of absorption and scattering, but also fluorescence and Raman scattering, before it reaches the
submerged plant canopy. Thus, it is this modified
signal that is of prime importance for photosynthesis of the seagrass meadows, macro-algal beds, coral
reefs, and benthic micro-algal mats. This modified
signal is also further modified by interactions with
the benthic substrate, thus, creating the target signal
for remote sensing of optically shallow waters. BeAuthor for correspondence, e-mail: rzimmerm@odu.edu or
arnold.dekker@csiro.au
fore the reflected light reaches the sensor, however,
it has to travel back through the water column, the
air–water interface and the atmosphere to reach the
sensor. Thus, our understanding of submarine photosynthesis and remote sensing of optically shallow
environments depends critically on the fundamental
principles of hydrologic optics.
II. A Primer on Hydrologic Optics
A. The Radiation Field
The fundamental properties describing the radiation
field are defined in precise physical terms, but their
symbolic notations in the literature are far from universal. To minimize confusion and to adhere to the
increasingly popular conventions emerging from the
field of hydrologic optics, the symbols and definitions used here (Table 1) will conform to those of
Kirk (1994) and Mobley (1994). The reader is encouraged to consult these excellent texts for a significantly deeper understanding of the fundamental
concepts of hydrologic optics introduced here. Although values for the terms and functions defined
below are spectrally dependent, the parenthetical notation (λ) has been omitted from the equations for
simplicity. Direction within the light field is generally expressed in terms of the zenith, nadir, and
295–301.
A. W. D. Larkum et al. (eds.), Seagrasses: Biology, Ecology and Conservation, pp.
c
2006 Springer. Printed in the Netherlands.
Aquatic Optics: Basic Concepts for Understanding
How Light Affects Seagrasses and Makes
them Measurable from Space
Richard C. Zimmerman
Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University,
Norfolk, VA 23529; e-mail:rzimmerm@odu.edu or arnold.dekker@csiro.au
Arnold G. Dekker
Environmental Remote Sensing Group, CSIRO Land & Water, P.O. Box 1666, 2601, Canberra,
ACT, Australia; e-mail: arnold.dekker@csiro.au
I. Introduction
This chapter provides an overview of aquatic optics
as it pertains to the transmission of light through the
water column and as it impinges on and reflects from
benthic substrates, such as seagrasses. This chapter
is meant as a primer for Chapter 13 (Zimmerman:
Light and Photosynthesis in Seagrass Meadows) and
Chapter 15 (Dekker et al., Remote Sensing of Seagrasses). Application of the theory presented here
will be developed further in those chapters.
Light transmitted through the atmosphere is modified by absorption and scattering before it hits the
surface of the water, where it is either specularly
reflected or transmitted across the air–water interface (Fig. 1). The water and its constituents further
modify light entering the water, again through processes of absorption and scattering, but also fluorescence and Raman scattering, before it reaches the
submerged plant canopy. Thus, it is this modified
signal that is of prime importance for photosynthesis of the seagrass meadows, macro-algal beds, coral
reefs, and benthic micro-algal mats. This modified
signal is also further modified by interactions with
the benthic substrate, thus, creating the target signal
for remote sensing of optically shallow waters. BeAuthor for correspondence, e-mail: rzimmerm@odu.edu or
arnold.dekker@csiro.au
fore the reflected light reaches the sensor, however,
it has to travel back through the water column, the
air–water interface and the atmosphere to reach the
sensor. Thus, our understanding of submarine photosynthesis and remote sensing of optically shallow
environments depends critically on the fundamental
principles of hydrologic optics.
II. A Primer on Hydrologic Optics
A. The Radiation Field
The fundamental properties describing the radiation
field are defined in precise physical terms, but their
symbolic notations in the literature are far from universal. To minimize confusion and to adhere to the
increasingly popular conventions emerging from the
field of hydrologic optics, the symbols and definitions used here (Table 1) will conform to those of
Kirk (1994) and Mobley (1994). The reader is encouraged to consult these excellent texts for a significantly deeper understanding of the fundamental
concepts of hydrologic optics introduced here. Although values for the terms and functions defined
below are spectrally dependent, the parenthetical notation (λ) has been omitted from the equations for
simplicity. Direction within the light field is generally expressed in terms of the zenith, nadir, and
295–301.
A. W. D. Larkum et al. (eds.), Seagrasses: Biology, Ecology and Conservation, pp.
c
2006 Springer. Printed in the Netherlands.