Spatial variability in bacterial abundance and other microbial components in the NW Iberian margin during relaxation of a spring upwelling event

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.This work was supported by the EU project MAST-CT90-0017 (The control of phytoplankton dominance). M.K.Z. thanks the Ministerio de Educación y Ciencia and Ministerio de Asuntos Exteriores of Spain for financial support. B.G.C. was funded by an I3P fellowship of CSIC-ESF.Peer reviewe