The idea that iron might limit phytoplankton growth in large regions of the ocean has been tested by enriching an area of 64 km(2) in the open equatorial Pacific Ocean with iron. This resulted in a doubling of plant biomass, a threefold increase In chlorophyll and a fourfold increase in plant production. Similar increases were found in a chlorophyll-rich plume downstream of the Galapagos Islands, which was naturally enriched in iron. These findings indicate that iron limitation can control rates of phytoplankton productivity and biomass in the ocean.