Observations on Alexandrium tamarense (Lebour) Balech and Other Dinoflagellate Populations in Golfo Nuevo, Patagonia (Argentina)

The toxic dinoflagellate Alexandrium tamarense and other dinoflagellate species were studied, along with water temperature and nutrient concentrations, from September 1995 to December 1998 in the Golfo Nuevo, Chubut, Argentina. Nutrient concentrations were low, showing a peak of high concentration in winter and a phase of depletion in late spring and summer. Dinoflagellates tended to be abundant during spring and summer, when Prorocentrum micans was the most important species. Other dinoflagellates were Pyrophacus horologium and Dinophysis acuminata. Ceratium tripos, C. fusus and C. horridum were present during the autumn, and a C. tripos peak up to 5.9 × 103 cell l–1 was observed in May 1997. Alexandrium tamarense showed strong interannual variation, the highest concentration being found in spring (September–October) 1995, with densities up to 15 × 103 cells l–1. The second A.tamarense peak was observed during October–November 1998 with maximal densities up to 5 × 103 cells l–1. Moderately high A. tamarense cyst densities, up to 300 cysts cm–3 of sediment, were found in the deep zone of the Golfo Nuevo basin. Among meteorolog-ical variables, increased late winter rain and higher solar radiation during spring may have influenced A. tamarense blooms.