MICROBIAL PLANKTON ACROSS DRAKE PASSAGE

We determined biomass and activity of microbial plankton across the Po lar Front (PF) in Drake Passage during January 1994. Temperature was a round 0 degrees C south and between 3 and 5 degrees C north of the PF. Both biomass and activities of microorganisms were significantly lowe r in the Antarctic waters south of the PF than in the sub-Antarctic wa ters north of it. Thus, values of chlorophyll alpha, integrated betwee n 0 and 200 m, reached 150 mg m(-2) north, but only 25 mg m(-2) south of the PF. Likewise, bacteria varied between 10(14) and 4 x 10(13) cel ls m(-2). However, the abundance of heterotrophic nanoflagellates was extremely low throughout Drake Passage (around 3 x 10(10) cells m(-2)) . Bacterial doubling times were long (mean of 25 days). Bacterivory wa s estimated from the abundance of predators and prey and from temperat ure. The grazing impact on bacterioplankton biomass was insignificant (less that 0.05% per day) and low on bacterial heterotrophic productio n (15% per day). Neither biomass nor the activities of microorganisms were found to increase at the PF. The microbial food web was uncoupled and the bacteria did not seem to be controlled by predation.