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Bleaching
on the Great Barrier Reef
Images
by Norman Kuring and Robert Simmon, based on data provided by Scarla Weeks,
University of Queensland
NASA Earth Observatory-Summer
officially came to an end in Australia on March 20, 2006. For northeastern
Australia (Queensland and New South Wales), it was the hottest summer on record,
according to the Australian Bureau of Meteorology, and that was bad news for
Australia’s Great Barrier Reef. Up to 50 percent of coral bleached at certain
inshore locations in the southern Great Barrier Reef. In general, hot water puts
coral under stress. Under such conditions, the coral expels the tiny algae,
zooxanthellae, that live in symbiosis with it. The algae give the coral its
color and produce nutrients through photosynthesis, so when the algae are
expelled, the coral turns white and eventually dies. The process is called coral
bleaching.
But coral bleaching is only one
symptom of an ecosystem in hot water: high temperatures have a negative impact
on other parts of the marine ecosystem as well. “All marine species operate
within a range of environmental parameters. Once this changes, the effects
cascade through the food-chain,” says Scarla Weeks, an ocean researcher at the
University of Queensland, Australia, funded by the Pew Institute for Ocean
Science. Ocean currents are driven in part by water temperature, and if a
current shifts, this may impact an entire ecosystem. Warmer temperatures may
result in decreased concentrations of phytoplankton, the tiny plants that grow
in the upper sun-lit layer of the ocean. Because phytoplankton form the base of
the marine food chain, their decline will cascade through the food chain. Larger
animals like fish will have little to eat and either die or move elsewhere. Loss
of fish impacts the sea birds that feed on them. In 2002, says Weeks, high sea
surface temperatures led to the worst bleaching event on record in the Great
Barrier Reef. The same year, 50 percent of seabird chicks on Heron Island in the
southern Great Barrier Reef starved because the adult birds were unable to find
enough fish.
The chain from warm water to
coral bleaching and sea bird deaths is long and complex, but satellite data may
provide some insight into the mechanisms controlling the chain. Weeks is using
NASA sea surface temperature and chlorophyll concentration data to study these
complex ocean processes. The images above illustrate sea surface temperatures
and chlorophyll concentrations as observed by the Moderate Resolution Imaging
Spectroradiometer (MODIS) on NASA’s Aqua
satellite between February 6 and February 20, 2006. Warm pink and yellow tones
show where sea surface temperatures were warm in the top image. The warmest
waters are the shallow waters over the reef near the coast, where coral
bleaching was most severe this summer.
The lower image shows chlorophyll
concentrations, where high concentrations (yellow) generally point to a high
concentration of phytoplankton in surface waters of the ocean. In this image,
the bright yellow dots actually represent the coral reefs, and not surface
phytoplankton. Ocean waters are clear off Australia’s northeast coast. The
patterns shown here reveal the movement of ocean currents, with lower
chlorophyll concentrations representing oceanic waters. By studying the changing
patterns, scientists can monitor the dynamics of the Great Barrier Reef and
every link in that marine ecosystem.
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