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When light impinges upon a molecule of water or a gas (or upon any form of matter), it may be either scattered or absorbed. When absorbed, the energy of the photon is entirely retained, at least for a while. When scattered, the energy of the photon is re‐emitted as another electromagnetic wave of the same wavelength and is scattered in all directions. Both processes are highly wavelength‐dependent.

The wavelength of light that is maximally absorbed is an inherent property of a substance, a convenient property of matter that is exploited in a variety of ways, e.g. in spectrophotometry. In contrast, scattering of light is proportional to the inverse fourth power of the wavelength ( λ ⁻⁴), which means that blue light is scattered far more readily than red. The blue sky above us is a result of the differential scattering of light by Earth’s atmosphere. Similarly, the blue water of the open ocean is partially the result of the same phenomenon, though it is also influenced by the reflection of the sky.

Both absorption and scattering contribute to the attenuation of light in the open ocean. A beam of light traveling over a distance loses part of its energy, whether it be in air or water. The relationship is expressed as Lambert’s law:

(1.15)

where I₀ is the intensity of the incident light, I _x is the intensity after traveling through distance x, and α _λ is the attenuation coefficient, a function of wavelength.

The attenuation coefficients for the different colors of light are quite different in water from those in air (Figure 1.22). Even though blue light is the most prone to scattering, it is attenuated the least with water depth. As a consequence, the character of light changes drastically with depth, a situation that must be reckoned with by the life in the open ocean. When light reaches a depth of 500 m, its wavelength has been narrowed to a very small range on either side of 480 nm in the blue and its intensity has been drastically reduced.

Three vertical zones are recognized in the ocean based on the availability of light. The first one is the euphotic zone, where the intensities and wavelengths of light are sufficient to support growth and reproduction in plant life. The bottom of the euphotic zone is defined by the compensation depth where photosynthesis (energy production) balances respiration (energy consumption) in plants. Two equations apply (after Lalli and Parsons 1993). The first is a simplification of Eq. (1.15):

Figure 1.22 Attenuation of different wavelengths of light as a function of depth.

Source: Lalli and Parsons (1993), figure 2.4 (p. 25). Reproduced with the permission of Pergamon Press.

(1.16)

where I₀ is the intensity of light at the surface, I_D is the intensity at depth, and k is the extinction coefficient of the seawater at that location (usually for the wavelength 550 nm), where ln denotes the natural logarithm. The second defines the compensation depth itself:

(1.17)

where D_c is the compensation depth, I₀ is the light at the surface, and I_c, the compensation light intensity is an experimentally determined value that ranges between 0.001 and 0.01 cal cm⁻² min⁻¹ depending on the type of dominant algae and how well it gathers light.

Figure 1.23 Light intensity and photic zonation as a function of depth in coastal and open ocean environments.

Source: Lalli and Parsons (1993), figure 2.5 (p. 26). Reproduced with the permission of Pergamon Press.

The bottom of the euphotic zone is highly variable (Figure 1.23) and is dependent upon water clarity. In clear ocean water, the bottom of the euphotic zone is at about 150 m. Below the euphotic zone is the disphotic zone, where there is still enough light to allow vision but not enough to support plant life. Like the euphotic zone, the depth of the disphotic zone varies with water clarity. In clear open‐ocean water, the disphotic zone extends to about 1000 m. The aphotic zone, where light is extinguished, makes up the remainder of the water column. Remembering that only 3% of the ocean’s volume is above 1000 m, light is therefore absent from the vast majority of the open sea.