The known history of the genus Pseudo-nitzschia [mostly reported in error as Nitzschia seriata (PT Cleve) H. Peragallol on the West Coast of the USA is traced in three time periods: the early observations of 1920-1943, later publications in 1960-1990, and recent publications after known domoic acid events in 1991-1996. The genus has been widespread in these coastal waters, and more than a dozen species have been identified, with the number still increasing. Dominance shifts among species on time scales of weeks, months, and years. There is direct evidence of increases of cell numbers with even small amounts of upwelling, whereas some blooms have coincided with times of low expectation of upwelling. Original data from W.E. Allen's notes provided by W.H. Thomas, Scripps Institution of Oceanography, suggest a possible explanation, borne out by recent culturing work. Viable cells can sink from the surface waters and rise in a different growth stage. Geographically, species in this genus have been studied most intensively recently in Monterey Bay, California, and it is possible that this bay functions as a center of seed stock that can be transported both north and south by seasonal currents. Presently known distribution of species is documented, with abundant stock of four species known to produce the toxin. In some areas, a domoic acid event could result from even moderate numbers of the large-celled P. australis Frenguelli at any life stage, or large numbers of the more slender P. multiseries (Hasle) Hasle in the stationary growth phase, or even from some populations of the ubiquitous P. pungens (Grunow ex P.T. Cleve) Hasle.
