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We investigated phytoplankton Fe limitation using shipboard incubation experiments in the high-nutrient South American eastern boundary current regime. Low ambient Fe concentrations (ϳ0.1 nM) in water collected from the Humboldt and Peru Currents were supplemented with a range of added Fe levels up to 2.5 nM. Phytoplankton chlorophyll a, photosystem II photosynthetic efficiency, and nitrate and phosphate drawdown increased in proportion to the amount of Fe added. The Humboldt Current algal community after Fe additions included colonial and flagellated Phaeocystis globosa and large pennate diatoms, whereas the Peru Upwelling assemblage was dominated by coccolithophorids and small pennate diatoms. Apparent half-saturation constants for growth of the two communities were 0.17 nM Fe (Humboldt Current) and 0.26 nM Fe (Peru Upwelling). Net molar dissolved Si(OH) 4 : NO drawdown ratios were low in both experiments (ϳ0.2-0.7), but net particulate silica to nitrogen production Ϫ 3 ratios were higher. Fe limitation decreased net NO : PO utilization ratios in the Humboldt Current incubation to

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