Iron fertilization and the Trichodesmiumresponse on the West Florida shelf

Abstract

Prior laboratory studies of Trichodesmium have shown a high iron requirement that is consistent with the biochemical demand for iron in the enzyme nitrogenase. Summer delivery of iron, in the form of Saharan dust, may provide an explanation for Trichodesmium blooms observed in offshore waters of the West Florida shelf over the last 50 yr. During ecology and oceanography of harmful algal blooms (ECOHAB) field studies, background iron levels (0.1–0.5 nmol kg −1 ) were found at the surface during periods of minimal dust delivery (May 2000 and October 1999). In contrast, total dissolved iron concentrations on the order of ∼16 nmol kg −1 were measured at the West Florida shelfߚbreak after a July 1999 Saharan dust event that was identified by advanced very high resolution radiometer (AVHRR) imagery, groundߚbased radiometers, air mass analysis, and aerosol samples (dust and nonߚseaߚsalt nitrate) collected throughout South Florida. The Trichodesmium response following this July dust event was a 100–fold increase over background biomass, reaching a surface stock of ∼20 colonies L −1 . Surface dissolved concentrations of both inorganic and organic phosphorus decreased below detectable limits during this bloom. Dissolved organic nitrogen concentrations associated with the bloom (15–20 µM) were 3–4–fold greater than background and much larger than ambient NO 3 − concentrations (<0.5 µmol kg −1 ). If all dissolved organic nitrogen (DON) is converted to urea and ammonium, this organic nitrogen could have supported the red tide of >20 µg chl L −1 of the toxic dinoflagellate, Gymnodinium breve , found along the West Florida coast during October 1999.