Iron in the Sargasso Sea (Bermuda Atlantic Time‐series Study region) during summer: Eolian imprint, spatiotemporal variability, and ecological implications

We report iron measurements for water column and aerosol samples collected in the Sargasso Sea during July‐August 2003 (summer 2003) and April‐May 2004 (spring 2004). Our data reveal a large seasonal change in the dissolved iron (dFe) concentration of surface waters in the Bermuda Atlantic Time‐series Study region, from ∼1–2 nM in summer 2003, when aerosol iron concentrations were high (mean 10 nmol m −3 ), to ∼0.1–0.2 nM in spring 2004, when aerosol iron concentrations were low (mean 0.64 nmol m −3 ). During summer 2003, we observed an increase of ∼0.6 nM in surface water dFe concentrations over 13 days, presumably due to eolian iron input; an estimate of total iron deposition over this same period suggests an effective solubility of 3–30% for aerosol iron. Our summer 2003 water column profiles show potentially growth‐limiting dFe concentrations (0.02–0.19 nM) coinciding with a deep chlorophyll maximum at 100–150 m depth, where phytoplankton biomass is typically dominated by Prochlorococcus during late summer.