3: What Color is the Ocean - and why do you need a satellite to tell you?


ocean photograph If someone were to ask you what the color of the ocean was, chances are that you would answer that is was blue......and for most of the world's oceans, your answer would be correct. We see color when light is reflected by the things around us. White light is made up of a spectrum or combination of colors, as in a rainbow of many different wavelengths. The longer wavelengths of light are red, the shorter wavelengths, blue.

The order of the colors in the rainbow - Red, Orange, Yellow, Green, Blue and Violet reflect (no pun intended) the order of their wavelengths from longest to shortest. When light hits the surface of an object, these different colors can be reflected or absorbed in differing intensities depending on the unique properties of the material on which the light is shining. The color we see depends on which colors are reflected and which are absorbed. For example, a book that appears red to us absorbs more of the green and blue parts of the white light shining on it, and reflects the red parts of the white light.

The same applies to the ocean. When sunlight hits the ocean, some of it is reflected back directly (sunglint), but most of it penetrates the ocean surface and interacts with the water molecules that it encounters. Most of the light that is scattered back out of clear, open ocean water is blue while the red portion of the sunlight is quickly absorbed very near the surface. However, there are many things in addition to just water molecules in the ocean and these things can change the color that we see. In coastal areas, runoff from rivers, resuspension of sand and silt from the bottom by tides, waves and storms and a number of other things can change the color of the near-shore waters.

plankton picture However, for most of the world's oceans, the most important things that influence its color are PHYTOPLANKTON. Phytoplankton are very small, single-celled plants, generally smaller than the size of a pinhead that contain a green pigment called chlorophyll. All plants (on land and in the ocean) use chlorophyll to capture energy from the sun and through the process known as photosynthesis convert water and carbon dioxide into new plant material and oxygen. Although microscopic, phytoplankton can bloom in such large numbers that they can change the color of the ocean to such a degree that we can measure that change from space.

The basic principle behind the remote sensing of ocean color from space is this; The more phytoplankton in the water, the greener it is....the less phytoplankton, the bluer it is. Pretty simple really.

But what we really want to know is "how much phytoplankton is there"? and also "how does the distribution and abundance of phytoplankton change in time and space?". There are many reasons why we should care about the answers to these questions, and these along with some of the basic principles of remote sensing (I'll be getting to that in a minute) can be found in the Teacher's and Student's Guide at NASA's ocean color monitoring program called SeaWiFS.

test tubes

However, you don't really have to be an oceanographer or even a rocket scientist to conduct an experiment that will demonstrate how changing concentrations of phytoplankton can influence the color of the ocean. In fact, you don't even have to risk getting seasick. All you need are a few clear glass test tubes (or even a few glass jars will work), some water, an eye dropper and a bunch of phytoplankton. Just in case you can't find any phytoplankton in your local store, green food coloring will work even better.

Now for the experiment. Fill each of the test tubes with the same amount of water. Leave the first test tube untouched as this will serve as our version of clear ocean water. Fill the eye dropper with the green food coloring and carefully add one drop of the food color to the second test tube. Carefully shake the test tube so that the food color is evenly distributed throughout the water. The water should have turned a very pale green color. Now place two drops in the third test tube and repeat the procedure. You will notice that the color is much darker than the second tube. That is because the concentration of "phytoplankton" in this test tube is twice as high as in the previous one. In the last tube, add about four drops of the food coloring and see how much darker it is.

If we assume that there are about one thousand drops of water in each of the test tubes, then we can calculate the actual concentration of phytoplankton (or food coloring) in each of the test tubes and relate that concentration to the color that we see. The higher the concentration, the greener the water. Have someone take another test tube and secretly add some food coloring to it. See if you can estimate what the number of drops that were added was based on the color that you see by holding the unknown sample next to the ones whose concentration you have measured.

And just for fun, try to simulate what a phytoplankton bloom might look like by squirting a whole bunch of food coloring into the last tube and see what happens.


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gene carl feldman (gene@seawifs.gsfc.nasa.gov) (301) 286-9428