Boyd et al. (in press) propose a relatively simple tool that observationalists can use to constrain the extent to which biological events, such as observed episodic phytoplankton blooms, are driven by dust supply to the ocean environment. In their paper, they use this tool to examine four case studies from the literature, one of whichwas authored by Lenes et al. (2001). Boyd et al. (in press) definitively conclude that the 100-fold Trichodesmium biomass increase on the West Florida shelf (WFS) that occurred by July 1999 could not be attributed to the alleviation of iron (Fe) limitation by deposition of Saharan dust. Their calculations of iron deposition incorporate multiple assumptions related to atmospheric dust concentration, solubility, wet deposition, and air column height. In all iron deposition studies, large uncertainties are associatedwith each of these variables. For example, as acknowledged in their work, iron solubility can range over 5 orders of magnitude with changes in medium, pH, and particle size. While dissolution rates for soil dust are low (b1–2%; Jickells and Spokes, 2001), solubility can be substantially increased by organic complexation, and the proximity of the WFS region to industrial centers can greatly enhance the acidity of mineral aerosols due to mixing with SO2 and NOx (Jickells et al., 2005). Boyd et al. (in press) calculated that 600 μmol Fe m−2 was available for wet deposition in one day during the 27 June–4 July dust event. Given a 5-m surface mixed layer (SML) and a 2% dissolution rate, a net concentration increase of 2.4 nmol L−1 would result in the SML. This is similar to the ~3 nmol L−1 mean offshore iron concentration reported on 5–7 July (Lenes et al., 2001). In addition, this estimate does not account for increased SML iron concentrations from a prior dust event on 15–17 June. Similarly, Boyd et al. (in press) misunderstand the properties of the Saharan Air Layer (SAL). The SAL is an elevated layer of hot, dry, dust-laden air that originates over the Sahara (Prospero and Carlson, 1972). Over the Caribbean and southern Florida the layer is typically 2 km thick and extends to an altitude of 3–4 km, sometimes higher (Gatz and Prospero, 1996; Reid et al., 2003). Furthermore, Boyd et al. (in press) assume that the measured surface dust concentration is applicable to the SAL; in reality, dust concentrations in the layer are on average about three times higher than concentrations at the surface (Prospero and Carlson, 1972; Reid et al., 2003). Considering these