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

The toxic dinoflagellate Alexandrium tamarense and other dinoflagellatespecies were studied, along with water temperature and nutrientconcentrations, from September 1995 to December 1998 in theGolfo Nuevo, Chubut, Argentina. Nutrient concentrations werelow, showing a peak of high concentration in winter and a phaseof depletion in late spring and summer. Dinoflagellates tendedto be abundant during spring and summer, when Prorocentrum micanswas the most important species. Other dinoflagellates were Pyrophacushorologium and Dinophysis acuminata. Ceratium tripos, C. fususand C. horridum were present during the autumn, and a C. tripospeak up to 5.9 x 10³ cell l^–1 was observed in May 1997.Alexandrium tamarense showed strong interannual variation, thehighest concentration being found in spring (September–October)1995, with densities up to 15 x 10³ cells l^–1. The secondA.tamarense peak was observed during October–November1998 with maximal densities up to 5 x 10³ cells l^–1. Moderatelyhigh A. tamarense cyst densities, up to 300 cysts cm^–3of sediment, were found in the deep zone of the Golfo Nuevobasin. Among meteorolog-ical variables, increased late winterrain and higher solar radiation during spring may have influencedA. tamarense blooms.