Iron in the Sargasso Sea

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.