evidence that iron limits ocean primary production is exemplified
here in the Galapagos bloom, where this entire region of Equatorial
waters is bathed in about six micromolar (mM or10-6 Molar) of nitrate,
both upstream and downstream of the Galapagos Islands. This is a
high level of nitrate for the open ocean. It was this question that
led the late John Martin to suggest that downstream waters are rich
in iron and upstream waters do not have enough iron to support plant
growth. Our measurements during FeEx I cruise in 1993 substantiated
this hypothesis, indicating that water impinging this Galapagos
plateau has high levels of iron leading to increases in primary
This conclusion required several experiments. The first were bottle
enrichment experiments in which water was retrieved from high-nutrient,
low-chlorophyll regions, amended with minute, quantities of iron,
and then allowed to incubate on deck.
slide shows the difference of water to which no iron amendments
are added, and this is an example of water to which subnanomolar
(less than 10-9 Molar) levels of iron were added.
Subnanomolar levels of iron added to the incubation bottles represents
an amount that you will see during glacial times in the surface
terms of primary productivity, you can see that the addition of
iron to these samples results in dramatic increases in production.
These bottle experiments are small--they're on the order of 20 liters--and
you can tell from the biological pump diagrams that have been presented
that carbon flux takes place over a much larger scale, involving
many aspects of the marine community. So the testing of the iron
hypothesis required, essentially, a much larger experiment.
These larger experiments were conducted in the two FeEx experiments,
FeEx I and FeEx II. Both were conducted in the Equatorial Pacific
in a region characterized by high concentrations of nitrate but
otherwise low concentrations of iron and plant biomass.