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What is ocean color?

Artist's drawing of the Nimbus

FIGURE 2.9: Artist's drawing of the general design of the Nimbus series of satellites. SOURCE: C. R. Madrid, ed. (1978), The Nimbus 7 Users' Guide, Goddard Space Flight Center, NASA.

The surface of the ocean changes color depending on the chemicals and particles floating in the water. In the open ocean, the water is quite clear except for times when nutrients stimulate phytoplankton (microscopic free-floating photosynthetic organisms) growth, which can color the ocean in shades of green. In addition, dissolved organic chemicals can change the color of the water and for that reason they are lumped under the term “colored dissolved organic matter.” In coastal waters, the color of the ocean can change in response to many factors such as sediment run-off from rivers and rain events, phytoplankton growth, storms that resuspend sediments from the bottom, etc. Currently, using this knowledge about ocean color is the only proven concept to observe ocean biology on a global scale from satellites. Ocean color observations from satellites can be translated to information about phytoplankton abundance, the amount of carbon taken up by phytoplankton, potentially toxic phytoplankton blooms, and much more.

Definition of Ocean Color

According to NASA’s ocean color website, ocean color is defined as follows:

The “color” of the ocean is determined by the interactions of incident light with substances or particles present in the water. The most significant constituents are free-floating photosynthetic organisms (phytoplankton) and inorganic particulates. Phytoplankton contain chlorophyll, which absorbs light at blue and red wavelengths and transmits in the green. Particulate matter can reflect and absorb light, which reduces the clarity (light transmission) of the water. Substances dissolved in water can also affect its color.

The phrase “ocean color data” refers to accurate measurements of light intensity at visible wavelengths. As ocean color data is related to the presence of the constituents described above, it may therefore be used to calculate the concentrations of material in surface ocean waters and the level of biological activity. Ocean color observations made from Earth orbit allow an oceanographic viewpoint that is impossible from ship or shore — a global picture of biological activity in the world’s oceans.

The Ocean Color Signal

  1. Electromagnetic radiation is emitted by the Sun as sunlight.
  2. The sunlight is reflected by the atmosphere and by the ocean’s surface.
  3. The sunlight that enters the ocean is absorbed, scattered, and reflected by phytoplankton and other floating materials near the ocean’s surface.
  4. The sunlight is then backscattered from the ocean’s surface and measured by the satellite in the form of colors based on the transparency of the ocean.