South Georgia: A View Through the Clouds
MODIS image of South Georgia Island, acquired on August 8, 2002.
250 meter resolution JPEG image
A short quiz: South Georgia is
A) a region in one of the United States, famous for Vidalia onions;
B) the southern region of a small country at the eastern end of the
Black Sea, whose capitol city is Tbilisi;
C) a small and remote island in the far southern Atlantic Ocean; or
D) all of the above.
(Answer: D, "all of the above").
Even considering that some of us wax poetic about Vidalia onions, this
Science Focus! article concerns South Georgia (C), the British territory
in the southern Atlantic Ocean, which is shown in the remarkably clear MODIS
image at the top of the page. This image is remarkable, because for much of
the year (particularly during the austral winter when this image was
acquired) South Georgia is persistently covered by clouds. It's rare for
spaceborne sensors to get a clear view of the island and its surrounding
waters.
However, even if the view isn't clear, a quick visit to South Georgia would
indicate that the waters around the island must be fairly productive. Large
colonies of seals and seabirds breed at South Georgia and the region supports
important commercial fisheries for krill and fish. In the past, the Antarctic
fur seal and many species of whales were hunted to near extinction.
Here are just a few of South Georgia's king penguins:
King penguin rookery on South Georgia Island.
The British Antarctic Survey has conducted a number of research cruises to
investigate why the waters around South Georgia are so productive. Large
phytoplankton blooms around the island support the base of this food chain.
However, in the remote Antarctic, shipboard surveys are restricted to
relatively small areas and short time scales (icebergs and rough weather can
also be a problem). It was only with satellite imagery, that the true size and
extent of South Georgia's phytoplankton blooms could be appreciated.
Data from SeaWiFS (using the spatial and temporal binning process that
slowly clears clouds away through repeated observations) and British Antarctic
Survey cruise data confirmed that waters to the northwest of South Georgia are
generally the most productive. SeaWiFS also revealed that large blooms
frequently occur to the south of the island, an area where few ship studies
have taken place SeaWiFS revealed that South Georgia's blooms are highly
variable, resulting in some seasons being far more productive than others. For
example, large and persistent phytoplankton blooms to the northwest and south
of the island were observed in January 2000 compared to the phytoplankton poor
waters of January 1998.
Comparison of shipboard chlorophyll
concentration measurements and SeaWiFS chlorophyll concentrations
during January 1999. Note size of ship survey area in SeaWiFS
image. (Click on the lower image for full-size version; this
image is reduced in size and quality for display on the Web page.)
SeaWiFS data also indicated that phytoplankton blooms are regular
occurrences both to the southwest and to the north of the island.
At the height of phytoplankton activity, blooms can fill the entire
area between South Georgia Island and the "Polar Front" of the
Southern Circumpolar Ocean Current, which is an area of about 300 square
kilometers. SeaWiFS also revealed that blooms can take place as
early as October and as late as March, months when rough ocean
conditions discourage scientific ships, and scientists, from
routine research operations.
SeaWiFS data from January 2000 (above, showing
southern extension of phytoplankton bloom), compared to January 1998 (below),
when there was minimal phytoplankton productivity. (Click on the images for
full-size versions; these images are reduced in size and quality for
display on the Web page.)
In addition, the satellite imagery revealed that phytoplankton growth around
South Georgia was far higher than is typical for the Southern Ocean, and
frequently extended considerable distances downstream from the island, towards
the Polar Front and beyond (sometimes more than 2,750 km).
SeaWiFS chlorophyll concentrations during December 2001. The phytoplankton bloom extended several thousand kilometers
to the east of South Georgia Island.
These widespread and long-lasting phytoplankton blooms are likely to have
major consequences for secondary producers (e.g. krill and copepods that feed
on phytoplankton), and in turn for seals and penguins that feed on krill, and
highlight the South Georgia region's potential as a major carbon sink in the
Southern Ocean. The next step is to gain a better understanding of the
environmental dynamics that make this area such a productive place. Using
SeaWiFS and MODIS ocean color data, it's possible to get a deeper, multi-level
understanding of this marine ecosystem, just as it's possible to peel off the
outer layers of an onion to reveal the tasty interior*.
Sweet Vidalia onion. (Click on the image for full-size version)
*[The author begs the forgiveness of readers of this article, as he just had to include another reference to Vidalia onions.]
Acknowledgements
This Science Focus! article was written with the substantial assistance
of Dr. Rebecca Korb and figures were produced by Andrew Fleming, both from the
British Antarctic Survey.
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