AME 43:255-266 (2006)  -  doi:10.3354/ame043255

ABSTRACT: The spatial distribution of heterotrophic bacterioplankton abundance (BA) and bacterial colony forming units (CFU) plus additional biological (abundance of autotrophic and heterotrophic pico- and nanoplankton, diatoms and chlorophyll a concentration), physical (temperature and salinity) and chemical (nutrient concentrations) variables were investigated along the Galician coast (NW Iberia) after the cessation of a strong spring upwelling event. BA and abundance of autotrophic and heterotrophic pico- and nanoplankton tended to increase with distance from the coast, while diatoms were more abundant near to the coast. Over 70% of the variance in BA could be explained by a regression equation with 3 variables, in which abundance of autotrophic nanoflagellates (61%) and abundance of heterotrophic nanoflagellates (7%) accounted for 68% of the total explained variability. The remaining 2% was related to chlorophyll a variations. Variability in CFU abundance (67%) was attributable to a negative relationship with salinity and to a lesser but significant degree by a positive relationship with diatom abundance. These data suggest that a number of mechanisms regulated bacterial abundance in the NW Iberian margin after spring upwelling: CFU was related to continental influence and diatoms, and BA was associated with the biomass of autotrophic nanoflagellates. The relationships between BA, autotrophic nanoflagellates (ANF) and chlorophyll a (Chl) in the oceanic samples suggest that a change from bottom-up to top-down control of BA would occur at concentrations higher than 2500 ANF ml^–1 and 1.72 mg Chl m^–3.

KEY WORDS: Bacterial abundance · Phytoplankton · Upwelling relaxation · NW Iberia