Bacterioplankton composition in the Scotia Sea, Antarctica, during the austral summer of 2003

Physical ocean processes (ice-melt, island run-off and upwelling of nutrients) were hypothesised to affect the bacterioplankton composition in the surface mixed layer of the Scotia Sea during the austral summer of 2003, and this was investigated using flow cytometry and catalysed reporter deposition fluorescence in situ hybridisation (CARD-FISH) techniques. The bacterio- plankton was composed predominantly of Alphaproteobacteria (PB), comprising SAR11, Roseobacter spp. and SAR116 groups, followed by Sphingobacteria/Flavobacteria and Gammaproteobacteria, including SAR86. Two distinct bacterioplankton communities were identified, largely based on bac- terioplankton abundance, which varied from 0.3 ± 0.06 × 10 6 cells ml -1 in the west to 0.8 ± 0.3 × 10 6 cells ml -1 in the east, and a corresponding difference in SAR11 percentages of 30 ± 15% in the west compared to 5 ± 5% in the east. The western community was present in waters that were largely in an over-wintered, pre-bloom condition. The eastern bacterioplankton community was associated with phytoplankton blooms developed within the eastern Scotia Sea nutrient upwelling zone, where the Antarctic Circumpolar Current (ACC) encounters the shallow bathymetry associated with the Scotia Arc, in combination with seasonal ice-melt and island effects that enabled surface water stratification.