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There is evidence that phytoplankton growth is restricted by the availability of iron in the subarctic Pacific and equatorial Pacific and Southern oceans. Shipboard seawater incubation experiments have been undertaken which show that phytoplankton are iron-limited in The Californian coastal upwelling region. The addition of iron to nearshore high-nitrate, low-chlorophyll (HNLC) waters encourages the blooms of large chain-forming diatoms. Diatoms lacking iron will deplete surface waters of silicic acid resulting in the secondary silicic acid limitation of phytoplankton.