Saharan dust and phosphatic fidelity: A three dimensional biogeochemical model of Trichodesmium on the West Florida shelf

The availability of iron within the surface waters of the broad, oligotrophic West Florida shelf (WFS) controls periodic blooms of the pelagic marine cyanobacterium Trichodesmium. Summer delivery of iron (Fe), in the form of Saharan dust, alleviates this growth constraint, shifting limitation to the efficiency of phosphorus (P) cycles. Florida\u27s rivers drain Miocene phosphorus deposits to supply the WFS with freshwater nutrient supplies at molar dissolved inorganic nitrogen/phosphate (DIN/PO4) ratios of less than 6. These diazotrophs draw upon ubiquitous stocks of dissolved nitrogen gas, once stimulated by Fe-deposition within P-replete waters of the West Florida shelf. An extensive in situ data set collected between 1998-2001 (NEGOM / ECOHAB / HyCODE) provided plankton taxonomy, hydrographic, nutrient, DOM, pigment, and optical properties on the shelf. A three-dimensional numerical model was constructed to analyze the impact of iron fertilization of the diazotroph Trichodesmium and the resultant effect upon the elemental cycles of N, P, and Fe. Based on the results of the coupled physical and ecological models, wet deposition of Fe-rich Saharan dust was necessary to stimulate enough nitrogen fixation to support the toxic red tide (Karenia brevis) of ~20 micrograms chl per liter found in October 1999. Ultimately, the magnitude and longevity of the Trichodesmium population, and therefore \u27new\u27 nitrogen production, was controlled by both phosphorus and iron availability