STOCKS AND DYNAMICS OF BACTERIOPLANKTON IN THE NORTHWESTERN SARGASSO SEA

We examined seasonal variations of bacterioplankton stocks and distrib utions in the upper 250 m in the Sargasso Sea near Hydrostation S and the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) site during 1 987-1988 and 1991-1994. Mean vertical profiles of bacterial abundance, cell volume, and H-3-thymidine and H-3-leucine incorporation rates va ried seasonally, and distribution patterns were correlated with physic al mixing in the fall and winter. Conversion factors for H-3-thymidine and H-3-leucine incorporation were determined empirically to be 1.63 and 0.078 x 10(18) cells mol(-1), respectively. Integrated bacterial b iomass and production within the euphotic zone were low compared to ot her oceanic sites and ranged between 241-411 mg C m(-2) and 11-36 mg C m(-2) day(-1), respectively. Seasonal variation in bacterial biomass and production was observed; however, the range of variation was less than two-fold despite a five-fold range in primary production. Bacteri al biomass (BE): phytoplankton biomass (PB) ratios remained high durin g the summer and fall, with bacterial biomass dominating the chi a-C e stimates at times, and BB:PB ratios decreased in the winter and spring due to increased phytoplankton production. Low bacterial production ( BP):phytoplankton production (PP) ratios were observed for all seasons . Although BP:PP ratios were low, growth efficiencies observed in this region indicate that carbon flux through seasonal BP could account fo r 17->100% of seasonal PP. The small response of bacterial production during and after a phytoplankton bloom may indicate that the majority of dissolved organic carbon (DOG) that accumulates in post bloom condi tions is of semi-labile quality, resulting in slow bacterial oxidation of DOG. Copyright (C) 1996 Elsevier Science Ltd