Abstract Previous coastal marine studies have concluded that the total dissolved Fe may influence the initiation of algal blooms, including Brown Tides of Aureococcus anophagefferens. However, the existence of unavailable colloidal and organically complexed Fe make the dissolved pool a poor indicator of what is bioavailable for assimilation by phytoplankton. To evaluate the role of Fe in the formation of coastal algal blooms, we measured its physicochemical speciation—dissolved, high molecular weight (HMW), low molecular weight (LMW), labile particulate, refractory particulate, organically complexed, and labile dissolved factions—in the Peconic Estuary, NY, USA, which hosts sporadic Brown Tides. Across the Brown Tide-prone, high salinity portion of the estuary dissolved Fe varied from 9 to 240 nM, which is above and below levels reported to cause physiological Fe stress in this species (100 nM). Dissolved Fe in the more saline portions of the Peconics was, on average, 80% organically complexed and 73% high molecular weight (HMW=colloidal, 0.2 μM–10 kDa). Within an intensively studied embayment, decreases in labile particulate Fe, along with increases in low molecular weight (LMW) Fe, coincided with peaks in algal biomass. A significant positive correlation between LMW Fe and chlorophyll a ( r 2=0.62, P<0.01) and a significant negative correlation between labile particulate Fe and chlorophyll a ( r 2=0.69, P<0.01) during this study suggested phytoplankton may have mediated the observed transfer between size classes. High levels of labile dissolved Fe (57 nM) and LMW Fe (62 nM) observed during a Brown Tide bloom is consistent with efficient Fe reduction by Aureococcus, possibly accounting for its Fe-replete growth during blooms. Dissolved Fe inputs observed to precede Brown Tides are hypothesized to be a diagenetic indicator of organic matter input (e.g., DON, DOC) from sediments, which may promote heterotrophic growth by blooms.