Envisat captures first image of Sargassum from space (Forwarded)

ESA News
http://www.esa.int

6 June 2007

Envisat captures first image of Sargassum from space

Sargassum seaweed, famous in nautical lore for entangling ships in its dense
floating vegetation, has been detected from space for the first time thanks
to an instrument aboard ESA's environmental satellite, Envisat. The ability
to monitor Sargassum globally will allow researchers to understand better
the primary productivity of the ocean and better predict climate change.

Using optical radiance data from the Medium Resolution Imaging Spectrometer
(MERIS) aboard Envisat, Dr Jim Gower and Stephanie King of the Canadian
Institute of Ocean Sciences and Dr Chuamin Hu of the US University of South
Florida were able to identify extensive lines of floating Sargassum in the
western Gulf of Mexico in the summer of 2005.

"This appears to be the first report of a satellite image of Sargassum,"
Gower said. "It is usually associated with the area of the North Atlantic
known as the Sargasso Sea after the Sargassum encountered there by early
explorers. Our observations of Sargassum lines extending over large areas of
the Gulf show that in this area and season it represents a significant
fraction of marine primary productivity."

Marine primary production is the process by which floating vegetation, such
as phytoplankton and seaweed, absorb atmospheric carbon dioxide through
photosynthesis and convert it into organic carbon. By absorbing half of the
carbon dioxide emitted into the atmosphere, the oceans have a profound
influence on climate, making them major areas of interest for climate
modellers.

The discovery was made using the MERIS maximum chlorophyll index (MCI) which
provides an assessment of the amount of chlorophyll in vegetation to produce
detailed images of chlorophyll per unit area. MERIS is uniquely suited for
this because it provides images of above-atmosphere spectral radiance in 15
bands, including three bands at wavelengths of 665, 681 and 709 nanometres
in order to measure the fluorescence emission from chlorophyll a.

Chlorophyll is the green photosynthetic compound in plants that captures
energy from sunlight necessary for photosynthesis. The amount of chlorophyll
present in vegetation plays an important role in determining how healthy it
is. Accurately monitoring chlorophyll from space, therefore, provides a
valuable tool for modelling primary productivity.

"The 709 band used by MERIS is not present on other ocean-colour sensors. It
was essential to our detecting Sargassum," Gower said. "The MCI index has
allowed us to find so many interesting things, including Sargassum and
Antarctic super blooms. It really gives us a new and unique view of the
Earth."

Gower and King are now combining data from MERIS with a sophisticated
processing algorithm and powerful Grid computing to broaden this new view.
The basic principle behind Grid computing is that anything one computer can
do, a pool of computers can do faster and better, enabling the solution of
massively complex tasks beyond the capabilities of a single machine or local
network.

By using Grid technology, Gower and King intend to compute 5-years worth of
MERIS data to determine global estimates of Sargassum biomass and its
contribution to ocean productivity. "So far, we have found two things
(Sargassum and Antarctic superblooms) that have never been seen from space
before," said King. "It is really very exciting."

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