Bacterial standing stock and production and their relation to phytoplankton in the Southern Ocean

In the Atlantic sector of the Southern Ocean bacterial numbers, biomass and production were measured in the upper 200 m of the water column on six successive transects along the 6°W meridian between 47°S and 60°S in October–November 1992. The transects extended from the Polar Front region (PFr) across the southern part of the Antarctic Circumpolar Current (southern ACC) to the retreating Marginal Ice Zone (MIZ). During this austral spring period, bacterial standing stock and production were highest and showed a gradual increase in the PFr where a phytoplankton bloom started to develop. No indication of elevated bacterial parameters were detected at the MIZ. Bacterial numbers and chlorophyll a as well as bacterial and primary production were highly positively correlated and were similar to the relationships found in other oceanic regions. Bacterial production amounted to 12–22% of primary production on average, and the turnover time of bacterial biomass was 5–70 days. These spring values are lower than reported data found during austral autumn or in most other oceanic regions. In the PFr and the MIZ, primary production was particularly high relative to bacterial production, resulting in an average bacterial consumption of 27–28% of primary production (at a conversion efficiency of 30%). In contrast, in the waters of the southern ACC in between these two regions, 66% of primary production on average was consumed by bacteria. The different levels of bacterial consumption may be explained by an uncoupling or time lag between the onset of the phytoplankton spring bloom development in frontal areas and bacterial processes.