What determines the color of large bodies of water (i.e. Ocean/large lakes?)

Asked by: Cindy Peabody


The ocean reflects the color of the sky, but even on cloudless days the color of the ocean is not a consistent blue. Phytoplankton, microscopic plant life that floats freely in the lighted surface waters, may alter the color of the water. When a great number of organisms are concentrated in an area, the plankton changes the color of the ocean surface. This is called a 'bloom.'

Microscopic plant life is at the base of the marine food web and is the primary food and energy source for the ocean ecosystem. Phytoplankton convert nutrients into plant material by using sunlight with the help of the green pigment chlorophyll. The chlorophyll pigments in the plants absorb light, and the plants themselves scatter light. Together, these processes change the color of the ocean as seen by an observer looking downward into the sea. Very productive water with a high concentration of plankton appears blue-green. Very pure water appears deep-blue, almost black.

From space, variations in ocean color can be measured with sensitive instruments. Ocean and land plants are green because of the chlorophyll in plant cells. Chlorophyll a absorbs mainly blue-violet and red and reflects green; chlorophyll b absorbs mainly blue and orange and reflects yellow-green. Satellite instruments measure the amount of reflected light of different wavelengths. These amounts allow scientists to estimate the productivity of Earth's land masses and oceans.

The sunlit surface layer of the ocean can be full of microscopic plants and animals. As the legend shows, the red areas contain the most life, while the purple areas are nearly empty of life. The number values on the scale indicate the milligrams of phytoplankton per cubic meter of sea water. While the organisms are microscopic, large numbers result in a measurable mass when filtered from the water. Ocean areas of high productivity support more life than less productive areas. It is as simple as more food = more fish. More oxygen is produced and carbon dioxide consumed in these highly productive areas of the ocean.

The plankton populations are dependent on a variety of factors, including ocean currents, temperature, availability of nutrients, amount of sunlight, and ocean depth. Many different species of plankton contribute to ocean color, although only a few species, occurring in great numbers, are found at any one time or place. The individual plants live at various depths, from the surface to nearly 100 meters, but prefer the surface sunlit regions with sufficient light to support photosynthesis.

In the ocean, as on land, plants are the foundation of the food chain on which all other organisms depend. The productivity of the ocean is very important to human activities and to the overall health of the planet. Remote-sensing of the productivity of the oceans through the use of instruments aboard satellites helps us better understand this vast frontier.

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NOTE from the Editor: It has been pointed out to me by Joe Larsen, Researcher at USC, that the Rayleigh scattering of the light from the sun (according to which the blue light is scattered more than other colors) is also an important factor in the color of the oceans. Rayleigh scattering is also a reason behind the blue color of the sky.
Answered by: Christina Ratzinger