Prokaryotic plankton biomass and heterotrophic production in western Antarctic waters during the 1995–1996 Austral summer

Abstract

Abstract We examined prokaryotic biomass (PB) and prokaryotic heterotrophic production (PHP) during spring and summer in three Antarctic areas: Bellingshausen Sea, Bransfield Strait and Gerlache Strait in December 1995 and in January–February 1996. The data gathered allowed comparisons among areas, between seasons, and with the RACER study, carried out 10 years earlier in the same zone (Deep-Sea Res. 38 (1991) 1029). Prokaryotic numbers ranged from 0.67 to 13.3×105 cells ml−1, with a mean of 3.86×105 cells ml−1. In order to obtain accurate estimates of PHP, the effect of temperature on leucine incorporation was studied and conversion factors (CF) were determined empirically. Several experiments were carried out to determine the influence of temperature on leucine incorporation rate. In 5 out of 7 experiments, leucine incorporation increased on increasing temperature from −2 to 4°C, but remained constant at higher temperatures (up to 11°C). In one experiment no response of leucine incorporation rate to temperature was found. CF were determined in filtration-dilution cultures. Parallel cultures from different areas were incubated with different organic matter amendments and at different temperatures. These treatments did not affect the conversion factor except for the incubation at −1.5°C in one experiment. The remaining cultures produced CF that were not significantly different. The average value was 0.81 kgC (mol leucine)−1. The PHP was higher in December (between 25.2 mgC m−2 d−1 in Bransfield Strait and 47.7 mgC m−2 d−1 in Gerlache Strait) than in January (between 8.0 mgC m−2 d−1 in Bransfield Strait and 22.9 mgC m−2 d−1 in Gerlache Strait) in all the areas studied. As a result, doubling times in Bellingshausen Sea and Gerlache Strait increased from 14–17 days in December to 32–75 days in January. In Bransfield Strait doubling times were always around 45 days. In Gerlache Strait this decrease in activity between December and January occurred despite a doubling of the chlorophyll concentration. Prokaryotic abundance was lower than that expected from chlorophyll concentrations and general relationships between chlorophyll and prokaryotic numbers. The prokaryotic plankton was not capable of processing all the organic matter provided by phytoplankton and was, therefore, not limited by resource supply. Lysis due to viruses, on the other hand, was shown to be an important loss factor for prokaryotes (Guixa-Boixereu et al., Deep-Sea Res. II (2002) 827–845). Temperature and predation by viruses might be the two main factors controlling the activity of prokaryotes in these waters.