Thursday 25 August 2005

PB6

PTH0143
Occurrence and abundance of the heterotrophic dinoflagellate Gyrodinium fusus (Meunier) Akselman in the Bahía Blanca Estuary, Argentina
Blanc, Silvia⁵, Barria De Cao, M. Sonia^1,2, Hoffmeyer, Monica^1,3, Piccolo, M. Cintia^1,2, Beigt, Debora¹
1 Instituto Argentino De Oceanografía, Argentina
2 Dpto. Biología, Bioquímica y Farmacia, Universidad Nacional del Sur. Bahía Blanca., Argentina
3 Universidad Tecnológica Nacional. Facultad Regional Bahía Blanca, Argentina
4 Dpto. de Geografía.- Universidad Nacional del Sur, Argentina
5 Argentinean Naval Service of Research & Development, Argentina
Author email: silblanc@yahoo.com
The hetrotrophic dinoflagellates are key protists in the transfer of energy in the pelagic ecosystem. Heterotrophic dinoflagellates are predators on some phytoplankton and microzooplankton organisms and even, on copepods eggs and early naupliar stages. They are in turn important prey for micro and metazoa zooplankters, inclusive they can be predators of and simultaneously prey for other dinoflagellates. The microplankton from the Bahía Blanca estuary (38°42'S-61°50'W) has been studied for 25 years and Gyrodinium fusus had never been observed before the year 2002, although it had been reported for the Argentinean shelf between 36° and 3°40' by Akselman (1985). The aim of this work was to analyse the presence of G. fusus in the inner part of the Bahía Blanca estuary in relation to physical and biochemical variables during an annual cycle. Sampling was carried out at a fixed station with a Van Dorn bottle every 15 days. Temperature, salinity, turbidity, chlorophyll "a" and solar radiation were measured in each occasion. Samples were fixed with Lugol's solution. Cells were enumerated following the Utermhöl's method. The maximum abundance of G. fusus registered was 1,529 individuals L-1 during the winter. The presence of G. fusus was related to the lowest values of temperature and solar radiation and the highest values of chlorophyll "a" and was restricted to the winter-early spring when a diatom dominated bloom of phytoplankton occurs annually in the estuary. Its occurrence was also related to the presence of Scrippsiella trochoidea and Protoperidinium sp. A possible trophic coupling between G. fusus and some phytoplankton species during this period is inferred. The finding of G. fusus in the estuary enlarges its geographical distribution in the Argentina coast.

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