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In September 2004 a large, nearly monospecific diatom bloom of Pseudo-nitzschia cuspidata off the coast of the state of Washington reached cell concentrations of 6.1 x 106 cells L-1 and produced maximum particulate domoic acid (pDA), dissolved domoic acid (dDA), and cellular domoic acid concentrations of 43 nmol L-1 1, 4 nmol L-1, and 63 pg cell-1, respectively. This bloom co-dominated the phytoplankton assemblage with the euglenoid Eutreptiella sp. in the Juan de Fuca eddy region, a known initiation site for toxigenic Pseudo-nitzschia blooms. Two isolates of P. cuspidata collected during separate cruises produced domoic acid (DA) in culture. During the September 2004 survey, 84% of the stations (n = 98) had detectable Pseudo-nitzschia and 78% had detectable pDA. There were no significant correlations between either pDA or cellular DA and ambient concentrations of macronutrients; however, when considering only those stations where Pseudo-nitzschia was present, pDA was positively correlated with chlorophyll a and negatively correlated with temperature (p < 0.01) at both 1- and 5-m depths. Correlations between cellular DA concentrations and total bacteria or cyanobacteria abundances were not significant. Variable ratios of pDA:dDA in the eddy region suggest that DA release was under cellular regulation by Pseudo-nitzschia. Stations where dissolved Fe concentrations were limiting (,0.5 nmol L-1) had the highest Pseudo-nitzschia abundances and pDA and cellular DA values. These results provide enticing field evidence of the role of Fe limitation in controlling cellular DA levels. [ABSTRACT FROM AUTHOR]