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It has been suggested that production in offshore waters of the subarctic Pacific is limited by availability of dissolved Fe. Although that is not yet adequately established, the functional consequences of the limitation (if it exists) can be characterized from the results of the SUbarctic Pacific Ecosystem Research (SUPER) program. Fe limitation, or something like it, establishes a phytoplankton community dominated by very small cells. These plants are not limited by Fe availability. Rather, their production is limited by their stock and available illumination. Stock is set by microzooplankton grazers with rapid population growth rates and, thus, rapid response to increases in phytoplankton abundance. Micrograzers provide efficient recycling of nitrogen as NH4, and the ready availability of NH4 sharply limits the annual utilization of NO3. Persistently high NO3 concentrations result. Other possibly Fe-limited, oceanic ecosystems with persistently high, near-surface nutrients require similar, detailed analysis of ecosystem function.
Miller et al. (Sun,) studied this question.