Trace metals influence phytoplankton both as limiting nutrients and as toxicants. Until recently, phytoplankton productivity in the ocean was thought to be primarily limited by major nutrients (N, P, and Si). However, enrichment experiments in bottles and in mesoscale patches of surface water have shown that iron limits algal growth in major regions of the ocean (Coale et al. 1996; Tsuda et al. 2003; Boyd et al. 2000; Coale et al. 2004), and some coastal upwelling systems (Hutchins et al. 1998, 2002). In addition, it now appears that iron limits N2-fixation by cyanobacteria in the ocean, and thus may control oceanic nitrogen inventories (Rueter 1983; Falkowski 1997; Kustka et al. 2003; Mills et al. 2004). Several other micronutrient metals (Zn, Co, Mn, and Cu) have also been shown to stimulate phytoplankton growth in ocean waters, but their effect is usually less than that of iron (Coale 1991; Crawford et al. 2003; Franck et al. 2003). However, these metals may play an important role in regulating the composition of phytoplankton communities because of large differences in trace metal requirements among species (Brand et al. 1983; Coale 1991; Sunda and Huntsman 1995a). For example, the addition of zinc to surface waters of the subarctic Pacific stimulated the growth of coccolithophores over that of diatoms, while the addition of iron preferentially favored the growth of large diatoms (Crawford et al. 2003). In general, iron can have a critical influence on the composition and structure of algal communities because of differences in requirements among species, particularly coastal and oceanic ones (Brand et al. 1983; Sunda and Huntsman 1995b) and large-celled and small-celled species (Price et al. 1994; Sunda and Huntsman 1997). Trace metals are believed to be less important in limiting algal growth in most coastal and freshwater systems. An important issue in the influence of trace metals on harmful blooms is the relative sensitivity of HAB species to trace metal limitation or toxicity relative to that of competing non-HAB algae. Although such differences are likely, they are as yet largely unknown.