Visible radiation, or light, from the Sun is important to the world's ocean systems for several reasons. It provides the energy necessary for ocean currents and wind-driven waves. Conversion of some of that energy into heat helps form the thin layer of warm water near the ocean's surface that supports the majority of marine life. Most significantly, the transmission of light in sea water is essential to the productivity of the oceans.
Visible wavelengths of light are captured by chlorophyll-bearing marine plants, which then make their own food through the process of photosynthesis . The organic molecules created by this process are an important energy source for many small organisms that are the base of the entire marine food chain . All life in the oceans is ultimately dependent upon the light and the process of photosynthesis that it initiates. Similarly, light transmission is a key factor in the ecology of lakes and streams, which are discussed elsewhere in this encyclopedia.
The uppermost, sunlit layer of the ocean where 70 percent of the entire amount of photosynthesis in the world takes place is called the euphotic zone. It generally extends to a depth of 100 meters (330 feet). Below this is the disphotic zone, between 100 and 1,000 meters (330 and 3,300 feet) deep, which is dimly lit. Some animals are able to survive here, but no plants. Although the amount of light is measurable at this range of depths, there is not enough available for photosynthesis to take place. The layer of the ocean where no light at all penetrates—over 90 percent of the entire ocean area on Earth—is called the aphotic zone, where depths are more than 1,000 meters (3,300 feet).
A certain amount of incoming light is reflected away when it reaches the ocean surface, depending upon the state of the water itself. If it is calm and smooth, less light will be reflected.
If it is turbulent, with many waves, more light will be reflected. The light that penetrates the surface is refracted due to the fact that light travels faster in air than in water. Once it is within the water, light may be scattered or absorbed by solid particles. Most of the visible light spectrum is absorbed within 10 meters (33 feet) of the water's surface, and almost none penetrates below 150 meters (490 feet) of water depth, even when the water is very clear.
Greater abundances of solid particles in the water will decrease the depth of light penetration. Therefore, water near the seashore that is more turbid (cloudy) due to particles will show a decrease in light transmission, even in shallow water. This is due to large numbers of particles brought in by river systems, and biological production by microorganisms , as well as waves, tides, and other water movement picking up debris on the ocean floor.
Water selectively scatters and absorbs certain wavelengths of visible light. The long wavelengths of the light spectrum—red, yellow, and orange—can penetrate to approximately 15, 30, and 50 meters (49, 98, and 164 feet), respectively, while the short wavelengths of the light spectrum—violet, blue and green—can penetrate further, to the lower limits of the euphotic zone. Blue penetrates the deepest, which is why deep, clear ocean water and some tropical water appear to be blue most of the time. Moreover, clearer waters have fewer particles to affect the transmission of light, and scattering by the water itself controls color. Water in shallow coastal areas tends to contain a greater amount of particles that scatter or absorb light wavelengths differently, which is why sea water close to shore may appear more green or brown in color.
SEE ALSO Algal Blooms in the Ocean ; Carbon Dioxide in the Ocean and Atmosphere ; Lakes: Biological Processes ; Lakes: Chemical Processes ; Lakes: Physical Processes ; Plankton ; Sea Water, Physics and Chemistry of .
Christina E. Bernal
Davis, Richard A. Oceanography: An Introduction to the Marine Environment, 2nd ed. Dubuque, IA: Wm. C. Brown Publishers, 1991.
Gross, M. Grant. Oceanography: A View of the Earth, 3rd ed. Englewood Cliffs, NJ: Prentice Hall, 1982.
Richardson, Mary Jo, and Wilford D. Gardner. "Tools of the Trade." Quarterdeck 5, no. 1 (1997):10–15.
Thurman, Harold V., and Elizabeth A. Burton. Introductory Oceanography, 9th ed. Upper Saddle River, NJ: Prentice Hall, 2001.
Scientists have developed several different methods and instruments to measure light transmission in water. The simplest measurement method involves the use of the Secchi disk, a white plate about 30 centimeters (12 inches) in diameter. It is fastened horizontally to a rope marked in meters. The disk is then lowered into the sea, lake, or other waterbody. The depth at which the disk is lost to sight is noted using the rope markings. This provides a rough estimate of the depth of light penetration.
A more sophisticated device for measuring light transmission is the nephelometer, which measures the scattering of incident (incoming) light by particles in the water. The optical backscatter meter and light scattering meter work in a similar fashion by projecting a light beam into the water. A detector on the instrument measures the amount of light that is scattered back into it.
The transmissometer measures light attenuation, or the sum of scattering and absorption of light in sea water. It projects a beam of light of a known wavelength over a known distance, and the data may be used to calculate the percentage of light that is transmitted at a certain depth.
The a-c meter has separate sensors to detect absorption of light by particles and total light attenuation. It functions in a manner like that of a transmissometer.