IRON-LIMITED DIATOM GROWTH AND SI N UPTAKE RATIOS IN A COASTAL UPWELLING REGIME/

There is compelling evidence that phytoplankton growth is limited by i ron availability in the subarctic pacific(1), and equatorial Pacific(2 ) and Southern oceans(3). A lack of iron prevents the complete biologi cal utilization of the ambient nitrate and influences phytoplankton sp ecies composition in these open-ocean 'high-nitrate, low-chlorophyll' (HNLC) regimes(4). But the effects of iron availability on coastal pri mary productivity and nutrient biogeochemistry are unknown. Here we pr esent the results of shipboard seawater incubation experiments which d emonstrate that phytoplankton are iron-limited in parts of the Califor nia coastal upwelling region. As in offshore HNLC regimes, the additio n of iron to these nearshore HNLC waters promotes blooms of large chai n-forming diatoms. The silicic acid:nitrate (Si:N) uptake ratios in co ntrol incubations are two to three times higher than those in iron inc ubations. 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 sug gest that iron limitation leads to more silicified, faster-sinking dia tom biomass. These results raise fundamental questions about the natur e of nutrient-limitation interactions in marine ecosystems, palaeoprod uctivity estimates based on the sedimentary accumulation of biogenic o pal, and the controls on carbon export from some of the world's most p roductive surface waters.