Spatial and temporal distribution of iron in the surface water of the northwestern Atlantic Ocean

The spatial and temporal distribution of three size fractions (<0.2 mu m, <0.4 mu m, and unfiltered) of total Fe were studied in four cruises (in October 1991, July 1992, March 1993, and May 1993) along a transect from the mouth of Delaware Bay across the Gulf Stream to the Sargasso Sea. Results show that Fe in all fractions decreased dramatically with increasing distance from shore and with increasing salinity (0.3-14 nM for <0.2 mu m Fe, 0.15-1.3 nM for 0.2-0.4 mu m Fe, and 0.8-11100 nM for >0.2 mu m Fe fraction). Concave <0.2 mu m Fe-salinity curves may result from several processes, including Fe removal by particle scavenging and phytoplankton uptake, shelf sedimentary Fe input, atmospheric Fe deposition, and multiple water endmember mixing. Significant removal of >0.2 mu m Fe in the shelf water may have important implications for estimating riverine Fe inputs to the open ocean. Maximum concentrations of >0.2 mu m Fe (1.5-3.5 nM) and major nutrients were observed at the shelf/slope front, probably due to enhanced vertical mixing and convergence circulation at the frontal zone. Significant temporal variations in Fe concentrations were also observed. In March 1993, enhanced riverine discharge, vertical mixing, and bottom resuspension contribute to significantly higher Fe concentration in shelf waters than in other seasons. Seaward advective Fe transport from the continental shelf leads to elevated Fe concentrations in offshore waters at this time.