CONICET | Buscador de Institutos y Recursos Humanos

ABSTRACT The inner zone of Bahía Blanca estuary is a shallow, well mixed, highly turbid and nutrient-rich temperate ecosystem. The phytoplankton annual cycle is characterized by a recurrent winter/early-spring diatom bloom. From May to August 2002 a researching programme aimed to describe the potential relationships between the dynamics of phytoplankton and nutrients was carried out. Phytoplankton composition, abundance, chlorophyll a, and dissolved inorganic nutrients (nitrate, nitrite, ammonium, phosphate and silicate) were measured. The species involved were classified in blooming (including Thalassiosira spp. and Chaetoceros spp.) and non-blooming species (including other diatoms and dinoflagellates). The results show: (1) a pre-bloom period, which includes low biomass values and a strong decrease of NO3-, (2) an initial bloom period characterized by high diatom abundance and chlorophyll a, a marked dominance of Thalassiosira curviseriata and a strong decrease of NH4+, (3) an intermediate bloom period of high abundance, co-dominance of blooming species and a strong decrease of PO43- and (4) a final bloom period with a decrease in abundance of blooming species and a general nutrients recovery. Thalassiosira curviseriata seems to be a species associated with high nutrient stock and low light intensity. Its growth mainly concurrent with DIN decrease was related with new production in this area. The obtained data suggest that low phosphate concentrations, more than DIN and silicates, contributed to the diatom bloom collapse. According to our results, this high production during winter can be explained as a net growth of phytoplankton because of a successful low light and temperature acclimated species, possibly favoured by low depredation and high nutrient availability.