AQUATIC MICROBIAL ECOLOGY 19: 13-27

Uncoupling of bacteria and phytoplankton during the austral spring bloom in Gerlache Strait, Antarctic Peninsula

D. F. Bird¹ and D. M. Karl²

¹Departement des sciences biologiques, Universite du Quebec a Montreal, CP 8888, Succ. Centre-Ville, Montreal, Quebec H3C 3P8, Canada

²SOEST, University of Hawaii. 1000 Pope Road. Honolulu, Hawaii 96822. USA

The response of the bacterial (Bacteria and Archaea) community to vernal phytoplankton blooms was examined over a grid of stations in Gerlache Strait. Antarctic Peninsula, during the RACER II program (29 October to 26 November 1989). Total bacterial production (0.13 to 10.6 mg C m^-3 d^-1), based on the incorporation of ³H-leucine into protein, increased with increasing chlorophyll a (Chl a) concentration. Bacterial cell-specific growth rate also increased with increasing primary production among stations. Nevertheless, bacterial cell abundance was greatest at the sites that had the lowest chl a concentrations, and declined wherever phytoplankton bloomed. Early bloom communities had few nanoprotist grazers; grazing was undetectable by the Landry-Hassett dilution method during this period. Fully developed bloom communities (chl a > 10 mg ^-3) had a profusion of nanoprotist grazers (median 3000 cells ml^-1). Despite relatively low ingestion rates per individual (0.9 bacteria cell^-1 h^-1), the abundant grazing community kept bacterial biomass very low in Gerlache Strait, to the point that the metabolism of the pelagic bacterial surface community was only a minor fraction of total ecosystem metabolism. Grazing was the apparent cause, although biomass limitation of the bacteria due to lack of resources (e.g. bioavailable dissolved organic matter) may be the ultimate cause of the uncoupling of bacterial and phytoplanktonic communities in these habitats.