Dissolved nutrient availability during winter diatom bloom in a turbid and shallow estuary (Bahl'a Blanca, Argentina)

The inner zone of Bahia 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 research program aimed to describe the potential relations between the dynamics of phytoplankton and nutrients was carried out. Phytoplankton composition, abundance, chlorophyll a (Chl 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 nonblooming species (including other diatoms and dinoflagellates). The results show: (1) a prebloom period, which includes low biomass values and a strong decrease of NO3-; (2) an initial bloom period characterized by high diatom abundance and Chl a, a marked dominance of Thalassiosira curviseriata, and a strong decrease of NH4+; (3) an intermediate bloom period of high abundance, codominance 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 nutrient 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 to 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 favored by low predation and high nutrient availability.