The seasonal development of the bacterioplankton bloom in the Ross Sea, Antarctica, 1994-1997

We report on investigations of bacterioplankton growth dynamics and carbon utilization in the full water column of the Ross Sea, Antarctica carried out on six cruises in 1994-1997, using epifluorescence microscopy, thymidine and leucine incorporation to estimate bacterial abundance and production, respectively. The Ross Sea experienced a bacterial bloom with an amplitude equaling similar blooms observed in the North Atlantic and North Pacific, reaching 3 × 10⁹ cells 1^-1 or 35 mmol Cm^-2 in late January. Increases in bacterial biomass were driven both by increases in abundance and in cell volume. Cell volumes ranged from 0.03 μm³ cell ^-1 in early spring to over 0.15 μm³ cell^-1 in midsummer. Larger cells were associated with faster division rates. Bacterial growth rates ranged 0.02-0.3 divisions d^-1, equal to rates at lower latitudes. Bacterial biomass accumulated steadily in the upper water column at a net rate of 0.03 d^-1. While there is clear evidence of a bacterial bloom in the Ross Sea, equal to bacterioplankton blooms observed in other oceanic systems, the magnitude of bacterial response relative to the phytoplankton bloom was modest. For example, euphotic zone bacterial production (BP) rates were equivalent to 1-10% of particulate primary production (PP) except in April 1997 when PP was very low and BP:PP was sometimes > 1. BP integrated over the upper 300 m was a more substantial fraction of the overlying PP than BP in the euphotic zone alone, with bacterial carbon demand in the upper 300 m about 30% of the seasonal PP. There was significant seasonal variation of bacterial biomass below the euphotic zone, indicating dynamic bacterial growth in the lower layer, and a supply of labile organic matter for bacteria. Bacterial metabolism is apparently limited by DOC flux in the upper layer. There is little evidence of temperature limitation, independent of substrate concentration. The relatively small diagenesis of phytoplankton biomass in the euphotic zone implies that there is relatively more organic matter available to support bacterial metabolism in the lower water column.