Seasonal Production and Bacterial Utilization of Doc in the Ross Sea, Antarctica

Heterotrophic picoplankton, including Bacteria and Archaea, form a major, often dominant component of the euphotic zone biomass throughout much of the world ocean. In Antarctic waters however, the bacterial biomass is a much smaller fraction of the total plankton stock, at least during the Austral summer. In the Ross Sea during the 1996-1997 growing season, bacterial abundance reached a peak of 2-3 x 10 cells liter-!, comparable to peak levels in the fertile regions of the world ocean, but bacterial biomass was a paltry 5% of the phytoplankton stock. Bacterial production ranged from <1 to 22% of the daily primary production, averaging 6% during October-April. Bacterial abundance and production were significantly correlated with the accumulation of semilabile dissolved organic carbon (DOC) over the growing season, indicating the importance of this carbon pool as a source of bacterial nutrition. Bacterial production increased prior to any increase in temperature. The semilabile DOC is entirely consumed by the end of the growing season. A microbial carbon budget based on these observations suggests that ca. 30% of the total annual primary production is metabolized by bacteria. Bacterioplankton biomass accumulation and production rates are regulated by the amount of labile dissolved organic carbon produced on seasonal times cales in the upper 150 m of the Ross Sea. This relationship argues for the predominance of bottom-up control of bacterial standing stocks in these waters, possibly in contrast to other Antarctic coastal regions.