Phytoplankton composition and abundance in the Golfo Nuevo, Argentina, have been studied from 1995 to 2001 along with water-temperature, salinity, nutrient and chlorophyll a concentrations. The pattern of seasonal phytoplankton distribution has shown recurrent blooms of Alexandrium tamarense during late winter and spring characterized by strong interannual fluctuations in magnitude. During October and December 2000, a large number of dead penguins (Spheniscus magellanicus) and other marine birds were found along the shore of Chubut. Coincident with those episodes of bird mortality, a bloom of A. tamarense occurred from October through December 2000. The spatial distribution of A. tamarense was patchy and the population density increased near the coast (maximum 22 x 10 3 cells L-1 in the surface layer). A. tamarense cysts were observed in bottom sediments, and increased in abundance from coastal stations (0-5 cysts cm(-3) of sediment) to the offshore, deeper areas (100300 cysts cm(-3)). Multiple regression analysis Suggests that variations in irradiance may be the major source of temporal variability of A. tamarense in Golfo Nuevo. The relationship of A. tainarense cell abundance and that of the dominant phytoplankton species during the spring was investigated using Pearson correlation. A. tamarense cell abundance was significantly correlated with cell abundances of the dinoflagellates Scrippsiello trochoidea and Prorocentrum micons and negatively correlated with phytoflagellates. The morphological variability of A. tainarense from field populations and in clones grown under different conditions was examined. There was great variability in the presence/absence of the ventral pore (an important character used to distinguish species of Alexandrium). (c) 2005 Published by Elsevier B.V.
