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Sea the Light Influences on Ocean Color

NASA's Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission is being developed. Its key technology is the state-of-the-art Ocean Color Instrument (OCI).

What influences ocean color?

The sun illuminates our ocean by emitting particles of light.

Two important processes determine the fate of these light particles in the ocean. One process is absorption.

A light particle has energy that dissipates...
...as it travels through the ocean...
...when it is taken up...
...by molecules or ocean matter ("absorbers").
"Absorption" occurs when the Sun's energy...
...is transferred to the absorbers.

Another key process is scattering.

The path of light energy...
...as it passes through the ocean...
...can scatter in multiple directions.
"Scattering" occurs when the Sun's energy...
...is redirected with no loss.

These processes determine the fate of sunlight entering the ocean.

The water itself both absorbs and scatters sunlight...
...in distinctive ways.

But why does "clear" water appear to be blue?

Clear water absorbs light differently at various wavelengths, with very high absorption of red. (Divers note that blood appears red near the surface but black at depth.)

Which colors are not absorbed efficiently? Shorter wavelengths such as blue.

But absorption is only part of the equation.

Some of the incoming sunlight gets scattered back up and out of the ocean. This effect is called backscatter... the redirection of light back to the direction from which it came.

Backscattered light is an important component of ocean color...
...since it sends light back towards your eye.

Clear water backscatters blue light with high efficiency. Orange and red, not so much.

The overall effect can be observed in clear ocean water. Deeper water is dark blue because reds are being absorbed and blues are being scattered.

At shallower depths, on the other hand, the seabed is scattering many wavelengths. So, a greater variety of colors – such as aqua and teal blue – are observed.

How do backscattering and absorption affect ocean color?

A simplified version of the ocean color equation describes their relationship:

But water is not the only component of seawater that absorbs and backscatters light!

The ocean is full of life, including...
...tiny organisms that come in many shapes, sizes, and colors...
...known as "phytoplankton."

Phytoplankton – and other such suspended matter – absorb and backscatter light differently.

Ocean color data tell us about the backscattering and absorption of seawater, including the phytoplankton within it.

This information is used to map the concentration of phytoplankton over the globe.

How do backscatter and absorption tell us about phytoplankton?

Seawater with very small phytoplankton cells strongly backscatters blue light.

But very small phytoplankton also absorb blue to some degree. Furthermore, clear water itself absorbs red.

Thus, where there's very small phytoplankton, blue-green colors are reflected back your eye.

What about other sizes of phytoplankton?

Absorption for other sizes of phytoplankton are generally similar to the curves for "Very Small Phytos" (above).

On the other hand, backscatter for large phytos – or communities with various sizes of phytoplankton – is nearly flat. This means all colors are scattered equally.

Thus, where there's large or variable sizes of phytoplankton, green-yellow colors will most likely be reflected back to your eye.

Information about phytoplankton size is one important reason to measure ocean color.

Are there other reasons why we care about backscatter and absorption?

Blooms!

Phytoplankton blooms typically have high concentrations of one cell size. For example, blooms of larger cells backscatter red strongly and may appear red to the eye.

Thus, the slope of backscatter can help pinpoint which phytoplankton species are blooming. This helps to determine whether algal blooms are potentially toxic.

Potentially toxic (Alexandrium tamarense)

Not toxic (Chaetoceros debilis)

Potentially toxic (Microcystis)

Not toxic (Rhizosolenia)

Potentially toxic (Dinophysis)

Not toxic (Emiliania huxleyi)

Not toxic (Phaeocystis)

Not toxic (Myrionecta rubra)

Potentially toxic (Protoperidinium divergens)

Not toxic (Trichodesmium)

PACE's fine-resolution measurements over a broad spectrum of light, known as hyperspectral imaging.

Scheduled to launch in 2022, PACE will extend and improve NASA's over 20-year record of observing ocean life, aerosols, and clouds.

PACE will help better identify phytoplankton communities from space. Its novel technology will keep a sharp eye on the health of our ocean.

More Wavelengths. Better Resolution.

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