There is compelling evidence that phytoplankton growth is limited by iron availability in the subarctic pacific(1), and equatorial Pacific(2) and Southern oceans(3). A lack of iron prevents the complete biological utilization of the ambient nitrate and influences phytoplankton species composition in these open-ocean ＇high-nitrate, low-chlorophyll＇ (HNLC) regimes(4). But the effects of iron availability on coastal primary productivity and nutrient biogeochemistry are unknown. Here we present the results of shipboard seawater incubation experiments which demonstrate that phytoplankton are iron-limited in parts of the California coastal upwelling region. As in offshore HNLC regimes, the addition of iron to these nearshore HNLC waters promotes blooms of large chain-forming diatoms. The silicic acid:nitrate (Si:N) uptake ratios in control incubations are two to three times higher than those in iron incubations. Diatoms stressed by a lack of iron should therefore deplete surface waters of silicic acid before nitrate, leading to a secondary silicic acid limitation of the phytoplankton community. Higher Si:cell, Si:C and Si:pigment ratios in diatoms in the control incubations suggest that iron limitation leads to more silicified, faster-sinking diatom biomass. These results raise fundamental questions about the nature of nutrient-limitation interactions in marine ecosystems, palaeoproductivity estimates based on the sedimentary accumulation of biogenic opal, and the controls on carbon export from some of the world＇s most productive surface waters.