Chapter 3: The Galapagos Plume
The preceding section discussed an important aspect of
physical oceanography called upwelling, and showed how upwelling
can cause the formation of an area of high primary productivity called
an upwelling zone. Now another image will be used to illustrate remarkable
interactions between the physics, chemistry, and biology of the oceans.
October 31, 1983 CZCS image of the high productivity plume induced
by the interaction of the South Equatorial Current with the Galapagos archipelago.
This image shows an area of high primary productivity
extending to the west of the Galapagos Islands in the equatorial Pacific
Ocean. The fact that the Galapagos Islands are located near the
Equator is quite important, as there are several ocean currents that
flow near the Equator that affect Galapagos waters.
The surface current is called the South Equatorial Current (SEQ), and
flows in a westward direction.
As the SEQ flows past the shallow volcanic platform of the
Galapagos Islands, it induces upwelling due to Ekman flow, just as
for the Benguela upwelling zone. The phytoplankton nourished by
the upwelled nutrients are transported westward, forming a productive plume
that extends hundreds of kilometers west of the Galapagos Islands.
One of the interesting aspects of oceanography along the
Equator in the Pacific Ocean is the fact that upwelling occurs near
the Equator (caused by the direction of the trade winds on either
side of the Equator), yet the primary productivity is lower than
would be expected based on the concentrations of nitrate and
phosphate. The equatorial belt of productivity can be seen in CZCS
global composite images or composite images of the Pacific Ocean.
An oceanographer named John Martin performed measurements
of iron concentrations in the ocean, and determined that in areas
where iron concentrations were particularly low, iron could be
the nutrient that limited the rate of primary productivity in large
regions of the ocean. In seawater, iron is chemically different from
nitrate and phosphate, so upwelling doesn't provide iron. Dust from
the continents is one of the main sources, so the greater the
distance from the continent, less iron is available. Near the
continents, iron is also available from the sediments in shallow
waters near the coast.
Martin predicted, based on observations of the waters around
the Galapagos, that if the ocean was fertilized with iron, an
increase in primary productivity would result. Though he died
before the experiment was conducted, it was performed successfully,
providing the exact results Martin had predicted. Observations of ocean color,
similar to those made by the CZCS from space, were performed from an
airplane, and documented the growth of phytoplankton in the
fertilized patch of the ocean. (see References and related sites)
The remaining question concerned the role of iron in the
Galapagos plume. It seemed clear that the same process of iron
fertilization was occurring naturally here, so oceanographers
surmised that iron was being supplied by the minerals found in the
volcanic ash blowing off of the Galapagos Islands. It turned out that
they were both right and wrong. They were right in that the
volcanic minerals supplied the iron, but it was not the ash blown off
the islands. The volcanic rocks beneath the surface, and the
deposited iron in the sediments around the islands, appear to be
the main source of the iron that nourishes the productivity in the
Galapagos plume.
Chapter 4: Seasonal upwelling in the Arabian Sea
Chapter 2: The Benguela upwelling zone
Index: Classic CZCS scenes
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Atmos Composition
