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An iron limitation mosaic in the California upwelling regime

Although Fe limitation is well documented in open ocean high ‐nutrient, low ‐chlorophyll (HNLC) areas, little is known about the potential for Fe limitation in coastal environments. We present a series of four Fe addition experiments that demonstrate varying degrees of Fe limitation in the central California coastal upwelling area. Fe concentrations vary widely here (<0.1 to >8.0 nM) because inputs from rivers and resuspended shelf sediments are unevenly distributed. The biological response to Fe availability is also extremely variable. Fe‐replete areas experience extensive blooms of large diatoms and almost complete depletion of nutrients. In slightly Fe‐stressed areas, Fe limits the growth of large diatoms but does not control nutrient biogeochemistry or growth of other planktonic organisms. In severely Fe‐limited waters, Fe exerts a fundamental control on nitrate and silicic acid drawdown, particulate organic carbon production, and the growth of phytoplankton, zooplankton, and bacteria. We propose a four ‐level classification scheme for Fe limitation in coastal California waters. Each level is characterized by a set of specific biological and biogeochemical responses to Fe. Parameters that show a characteristic response to Fe addition and thus define a region's Fe limitation status include particulate Si:N and Si:C production ratios, NO3− and H2SiO3 drawdown, C fixation, large diatom and picoplankton growth, bacterial production, and zooplankton grazing and biomass. Fe limitation of coastal upwelling regions needs to be recognized as an important biogeochemical process that could profoundly affect global new production and carbon cycling. The physical, chemical, and biological complexity of coastal upwelling regimes requires that Fe limitation effects be addressed with a more sophisticated approach than has generally been used to describe oceanic HNLC regimes.