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Frontiers
Iron Key Unlocks Ocean Ecosystems
September/October 1998
In 1995, oceanographers
carefully released half a ton of iron into clear, lifeless waters 800
miles southwest of the Galapagos Islands. Over the next week, the electric
blue sea turned green with algae and other phytoplankton. NSF-funded researchers
measured a 30-fold increase in phytoplankton growth. The results were
clear: In equatorial waters, plant life is limited by a lack of iron.
This was only the beginning. The iron-induced plankton
bloom "drew-down" as much as 9,100 tons of carbon dioxide,
connecting iron with Earth's atmosphere and climate. The experiment
provided guidance for studies of iron-rich paleo-climatic records, as
well as phytoplankton growth.
"The confirmation of the iron hypothesis is
a paradigm-breaking discovery," says Don Rice, director of NSF's
Chemical Oceanography Program. "It links many disparate fields;
for example, changes in ocean chemistry lead to changes in ocean biology,
which lead to more changes in chemistry, which feed back to changes in
the biology and so forth."
Lead scientists Kenneth Coale and Ken Johnson, both
from California's Moss Landing Marine Laboratories, estimate that
about 20 percent of the world's oceans are high-nitrate low-chlorophyll
like the Galapagos' waters. The lack of iron is the limiting factor
to growth in these oceans.
"Life evolved in a time when iron was abundant," explains
Coale. "But one of the first effects of photosynthesis was to precipitate
iron out of the system." Areas once iron-rich lost minerals to the
seafloor. Plants that need iron were limited to the continental margins
where rivers and coastal erosion dump minerals into the sea.
As a small-scale tool, says Coale, the iron experiment
is invaluable. "We need to understand how the ocean works. This
is a way for us to probe the biological responses and resulting changes
in the geochemistry of these ecosystems."
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