In situ enrichment experiments have shown that the growth of bloom-forming diatoms in the major high-nitrate low-chlorophyll (HNLC) regions of the world's oceans is limited by the availability of iron1,2,3. Yet even the largest of these manipulative experiments represents only a small fraction of an ocean basin, and the responses observed are strongly influenced by the proliferation of rare species rather than the growth of naturally dominant populations4,5. Here we link unique fluorescence attributes of phytoplankton to specific physiological responses to nutrient stress, and use these relationships to evaluate the factors that constrain phytoplankton growth in the tropical Pacific Ocean on an unprecedented spatial scale. On the basis of fluorescence measurements taken over 12 years, we delineate three major ecophysiological regimes in this region. We find that iron has a key function in regulating phytoplankton growth in both HNLC and oligotrophic waters near the Equator and further south, whereas nitrogen and zooplankton grazing are the primary factors that regulate biomass production in the north. Application of our findings to the interpretation of satellite chlorophyll fields shows that productivity in the tropical Pacific basin may be 1.2–2.5 Pg C yr-1 lower than previous estimates have suggested, a difference that is comparable to the global change in ocean production that accompanied the largest El Niño to La Niña transition on record6.
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We thank Z. Kolber, R. O'Malley, and the crew and officers of the NOAA ships Ka'imimoana and Ronald Brown. This research was funded by the National Science Foundation, the National Aeronautics and Space Administration, and the National Oceanic and Atmospheric Administration's Tropical Atmosphere Ocean array programme.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
This file contains three Supplementary Discussion sections: Part I: Field Measurements; Part II: Photoinhibition and Nocturnal Changes in the Water Column; and Part III: Potential Artifacts from Blanks. (PDF 158 kb)
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Responses of phytoplankton assemblages to iron availability and mixing water masses during the spring bloom in the Oyashio region, NW Pacific
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