THE ’iron hypothesis’(1,2) states that phytoplankton growth and biomass are limited by low concentrations of available iron in large regions of the world’s oceans where other plant nutrients are abundant. Such limitation has been demonstrated by experiments in which iron has been added to both enclosed and in situ (un-enclosed) phytoplankton populations(2-6). A corollary of the iron hypothesis is that most ’new’ iron is supplied by atmospheric deposition(7,8), and it has been suggested that changes in the deposition rates of iron-bearing dust have led to changes in biological productivity and, consequently, global climate(7), Here we report surface-water measurements in the equatorial Pacific Ocean which show that the main iron source to equatorial waters at 140 degrees W is from upwelling waters. Shipboard in vitro experiments indicate that sub-nanomolar increases in iron concentrations can cause substantial increases in carbon export to deeper waters in this region. These findings demonstrate that equatorial biological production is controlled not solely by atmospheric iron deposition, but also by processes which influence the rate of upwelling and the iron concentration in upwelled water.