Bacterial abundance, production and organic carbon limitation in the Southern Ocean (39–62°S, 4–14°E) during the austral summer 1997/1998

Bacterial abundance and production were studied in different zones in the Southern Ocean (39–62°S, 4–14°E) during a cruise in December–January 1997/1998. The role of potential growth limitation of bacteria due to limited availability of organic carbon (glucose) or inorganic N and P was studied in parallel. A positive correlation between surface water temperatures (−2 to 18 °C) and bacterial abundance (<0.1×106–1.5×106 cells ml−1) was observed. Bacteria were studied in vertical profiles, concentrated to three areas close to 6°E: the former Spring Ice Edge (SIE, 60°S, high chlorophyll a), the former Winter Ice Edge (WIE, 56°S, low chlorophyll a) and the Antarctic Polar Front at 51°S (APF, moderate chlorophyll a levels). Bacterial abundance was uniformly low south of the APF, and for the upper 50 m generally below 0.3×106 bacteria ml−1. In deeper water, bacterial abundance decreased dramatically for WIE and APF stations, but less markedly for SIE stations. The average volumetric bacterial production in the mixed layer was highest for APF stations (0.04 μg C l−1 h−1), but only half of this value for SIE stations (0.02 μg C l−1 h−1), with WIE in between (approximately 0.03 μg C l−1 h−1). Below 100 m, bacterial production decreased to values close to the detection limit. None of the three areas demonstrated any systematic diurnal variations in bacterial production in surface water (2 m) or at the chlorophyll maximum (situated between 30 and 66 m). We observed a positive correlation between bacterial production and in vivo chlorophyll a fluorescence, but there was no correlation between this parameter and bacterial abundance, possibly indicating different control mechanisms for these two parameters. Unfiltered water samples from 20 m depth were incubated at in situ temperatures and amended with ammonium, phosphate or glucose. In all the three experiments, from warm waters (relatively poor in inorganic nitrogen and phosphorus) south of Cape Town (38°S, +18.6 °C), in the colder and inorganic nutrient-rich waters north of the APF (45°S, +7.0 °C) as well as in the cold, nutrient-rich waters at the SIE (61°S, −0.13 °C), organic carbon additions resulted in a significant increase in bacterial production. Bacterial growth rates were very different between the three regions, and the growth response in the bacterial communities to the carbon additions was very slow at low temperatures.