Saharan dust and phosphatic fidelity: A three-dimensional biogeochemical model of Trichodesmium as a nutrient source for red tides 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 Saharan dust provided adequate iron (Fe) to shift limitation of growth to the availability of phosphorus (P). Florida's rivers drain Miocene phosphorus deposits to provide the WFS with freshwater nutrient supplies at molar dissolved inorganic nitrogen/phosphate (DIN/PO4) ratios of <6. These diazotrophs draw upon ubiquitous stocks of dissolved nitrogen gas, once stimulated by Fe-deposition within P-replete waters of the WFS.An extensive in situ data set collected between 1998 and 2001 (NEGOM/ECOHAB) provided plankton taxonomy, hydrographic, nutrient, dissolved organic matter (DOM), pigment, and optical properties on the shelf. We constructed a three-dimensional numerical model to analyze the impact of iron fertilization of the diazotroph Trichodesmium and the resultant effect upon the elemental cycles of N, P, and Fe. The coupled physical and ecological models show that the wet deposition of Fe-rich Saharan dust was necessary to stimulate enough nitrogen fixation to initialize the toxic red tide (Karenia brevis) of ∼20 μg chl a l−1 that occurred in October 1999. The simulation suggests that the magnitude and longevity of the Trichodesmium population, and therefore this source of ‘new’ nitrogen, was controlled by both phosphorus and iron availability.