Stocks and dynamics of bacterioplankton in the northwestern Sargasso Sea

Craig A. Carlson^1,4, Hugh W. Ducklow² and Thomas D. Sleeter³

¹Horn Point Environmental Laboratory, Box 775, Cambridge, MD 21613, U.S.A.

²The College of William and Mary, Virginia Institute of Marine Sciences, Box 1346, Gloucester Point, VA 23062

³Ministry of the Environment, Government Administration Building, 30 Parliment St., Hamilton HM-12, Bermuda

⁴Present Address:
Bermuda Biological Station for Research, Ferry Reach GE 01, Bermuda

(Received 4 October 1994; in revised form 30 May 1995; accepted 12 September 1995)

Abstract

We examined seasonal variations of bacterioplankton stocks and distributions 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 1987-1988 and 1991-1994. Mean vertical profiles of bacterial abundance, cell volume, and ³H-thymidine and ³H-leucine incorporation rates varied seasonally, and distribution patterns were correlated with physical mixing in the fall and winter. Conversion factors for ³H-thymidine and ³H-leucine incorporation were determined empirically to be 1.63 and 0.078 x 10¹⁸ cells mol^-1, respectively. Integrated bacterial biomass and production within the euphotic zone were low compared to other 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. Bacterial biomass (BB): phytoplankton biomass (PB) ratios remained high during the summer and fall, with bacterial biomass dominating the chl a-C estimates 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 for 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 (DOC) that accumulates in post bloom conditions is of semi-labile quality, resulting in slow bacterial oxidation of DOC.