Bacterial growth in experimental plankton assemblages and seawater cultures from the Phaeocystis antarctica bloom in the Ross Sea, Antarctica

A series of seawater culture experiments was carried out during the Phaeocystis antarctica bloom in the Ross Sea polynya (76S0S, 180W; November to December 1994 and December 1995 to January 1996) to examine bacterioplankton growth and derive empirical factors for estimating bacterial production rates. Bacterial growth was exponential over 3 to 10 d in all experiments, at rates of ca 0.1 to 0.7 d-', even in persistently cold waters (-2 to + 1C) Growth rates were lower in the early part of the bloom (early to mid-November) and highest during the period of peak primary productivity (2 to 4 g C m-' d-' in late November through December). Apparent lag phases in the growth curves lasting 1 to 7 d could be accounted for mathematically by subpopulations in the bacterial assemblages growing exponentially at different rates, with no need to invoke inactive, nondividing or nonviable populat i o n ~. Lags were absent dunng the period of peak primary production, suggesting adaptation of the bacteria to ambient DOM Growth was not stimulated by small temperature increases (A+2 to 4"C), and was not balanced by removal processes in untreated 'whole' water samples. Growth rates were broadly s~rmlar to other directly observed bacterial growth rates in the Antarctic and did not appear to Mfer from rates in warmer waters. Conversion factors for thymidine and leuclne averaged 8 and 0.8 X 10'' cells mol-l, respectively, not dissimilar to estimates from temperate waters. These findings suggest that bacteria were growing actively in 0 to -ZC waters under rich bloom conditions, and lend strong support to the hypothesis that bacterioplankton metabolism controls DOC accumulation in Antarctic waters, at least at the low rates of DOM supply we infer from field and experimental observations. Bactenoplankton responded within 10 to 20 d to the evolving P. antarctica bloom and did not appear to behave substantially differently from lower latitude bloom systems.