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 limitatio n in the central California coastal upwelling area. Fe concentrations vary widely here (<0.1 to >8.0 nM) because inputs from rivers and resu spended shelf sediments are unevenly distributed. The biological respo nse to Fe availability is also extremely variable. Fe-replete areas ex perience extensive blooms of large diatoms and almost complete depleti on of nutrients. in slightly Fe-stressed areas, Fe limits the growth o f large diatoms but does not control nutrient biogeochemistry or growt h of other planktonic organisms. In severely Fe-limited waters, Fe exe rts a fundamental control on nitrate and silicic acid drawdown, partic ulate organic carbon production, and the growth of phytoplankton, zoop lankton, and bacteria. We propose a four-level classification scheme f or Fe limitation in coastal California waters. Each level is character ized by a set of specific biological and biogeochemical responses to F e. Parameters that show a characteristic response to Fe addition and t hus define a region's Fe limitation status include particulate Si:N an d 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 pro foundly affect global new production and carbon cycling. The physical, chemical, and biological complexity of coastal upwelling regimes requ ires that Fe limitation effects be addressed with a more sophisticated approach than has generally been used to describe oceanic HNLC regime s.