Nitrate and ammonium uptake and ammonium regeneration rates were measured with 15N incubations during the austral summer period of 1995–1996 in the Bransfield Strait region (Antarctica). The objective was to quantify new and regenerated production in three zones that included stations with high phytoplankton biomass dominated by large and chain-forming diatoms (Strait of Gerlache) or colonies of Phaeocystis (Bellingshausen Sea), and stations with low phytoplankton biomass and high abundance of Cryptophyceae and other flagellates (western Bransfield Strait). All zones were characterized by high nitrate (>10 mmol N m-3) and low ammonium (generally <1 mmol N m-3) concentrations. Phytoplankton production in the high-biomass zones was sustained mainly by ammonium, and ammonium regeneration was enough to supply microplankton demands at daily scales. The average values of f ratio for Bellingshausen Sea and Gerlache Strait stations were 0.39 and 0.42, respectively. Despite the high biomass observed, chlorophyll-specific inorganic nitrogen uptake was low in these areas when compared with stations in the western Bransfield Strait, where a new bloom (based on nitrate) was developing (mean f ratio of 0.64). Dominance of flagellates and small diatoms, accumulations of nitrite, and ammonium regeneration rates exceeding upate rates in the western Bransfield Strait suggest that the bloom was a secondary succession stage. The variability in phytoplankton composition and nitrogen dynamics can be interpreted as a consequence of the diversity of environments in this region, but also as the result of the different temporal stages of seasonal succession of microplankton. Our results show that instead of a gradual change from nitrate-based to ammonium-based production as the summer season progressed, secondary blooms using nitrate as the primary nitrogen source may develop in areas like the western Bransfield Strait during mid summer. Rapid nitrogen uptake and growth efficiencies during active phytoplankton growth periods in these areas may produce large differences between short-term and seasonal estimations of nitrate consumption during the ice-free season

This work was funded by the Spanish Antarctic Programme project number ANT94/1010

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