Recent studies have shown that primary productivity (carbon fixation) in areas of the ocean (including the equatorial Pacific, and the South Pacific)...

Recent studies have shown that primary productivity (carbon fixation) in areas of the ocean (including the equatorial Pacific, and the South Pacific) is limited by the availability of iron to phytoplankton.

a.) Why might iron limit productive in these areas and not others? (Hint: think about where iron inputs might be coming from)

b.) Iron has been described as having "much more leverage" with regards to other nutrients. Why? (hint: think in terms of Redfield ratios)