Date: Tue, 21 Aug 2001 12:51:01 -0700

Author: "Kelly V. Beck"

Subject: Re: 5 questions


Hi Anthony,

>2. Why does ocean water sometimes look blueish or greenish? Is it an
>absorption phenomenon? Does it depend on water depth or whether the Sun is

This is a great question that can have implications for current
ecological/environmental research. I'll explain in a bit of a circuitous

It is an absorption effect. You basically see the parts of the spectrum
that are reflected or scattered and not those that are absorbed. Water has
an optical window in the visible; water strongly absorbs both infrared and
uv (infrared interacts with vibrational modes of H2O molecule, uv interacts
with electron energies in H2O). All of the visible range has a much
greater absorption length. But, within the visible range, red is absorbed
much more than the shorter wavelengths; it has a much shorter absorption
length. So, blue, which has a longer absorption length has a greater
chance to scatter back to your eyes.

But water is not the only thing in the ocean. An extremely important and
abundant component of the oceans is phytoplankton. Phytoplankton are
single-celled plants that form the basis of the oceanic food chain. As
plants, they have chlorophyll. When there is a large phytoplankton bloom,
the oceans will reflect more green, just as land plants reflect more green
due to their chlorophyll content. The ocean color varies from blue to
green depending upon the size of the phytoplankton population.

This has very important implications for marine ecologists. The health of
the marine ecosystem depends on the health of the phytoplankton
balance. Phytoplankton need carbon and other nutrients. So, the size of
the phytoplankton population is an indication of the status of ocean
currents which bring nutrients up from the bottom of the ocean and of the
carbon exchange rate with the atmosphere, among many other things. The
atmospheric carbon cycle tell us a lot about global warming. So, groups
like NASA and NOAA use ocean color as an important tool for understanding
ecological issues associated with the carbon cycle.

Now, to bring this back to the classroom. NASA observes ocean color from
satellites. You can download much of this satellite data from NASA to use
in classroom imaging activities using free imaging software such as NIH
Image (Scion Image for PCs). If you go to NASA's vast educational outreach
sites, you will even find activities written up for teachers' use in the
classroom. These make great computer lab activities.


Kelly V. Beck
Academic Research Program Officer
Division of the Vice Provost for Undergraduate Education
Stanford University
(650) 725 - 9139