Spatial distribution of heterotrophic bacteria in the northeast Atlantic (POMME study area) during spring 2001

Abstract

Heterotrophic bacteria abundances, total chlorophyll a (Tchla), and nitrate concentrations were determined during the spring cruise (23 March–13 April 2001) of the Programme Océan Multidisciplinaire Méso Echelle (POMME) in the northeastern Atlantic between 39.0°–44.5°N and 16.6°–20.6°W. Sampling covered a grid of 81 stations regularly spaced. Three bacteria subpopulations (HNA1, HNA2, and LNA) were resolved by flow cytometry on the basis of their nucleic acid content, after staining with SYBR Green II (molecular probes), and by their scatter properties. The bacterial distribution was investigated down to 600 m depth. HNA2 were essentially observed in the upper 200 m and were not present at all stations. HNA1 dominated in the surface layer and were positively linked to Tchla. This relationship exhibited some heterogeneity due to the latitudinal evolution of the phytoplankton bloom and the seasonal thermocline formation already occurring in the south. In contrast, LNA dominated the bacterial subgroups below 100 m depth, and their distribution bore the fingerprint of the geostrophic current field and the mesoscale features identified in the study area, i.e., cyclonic and anticyclonic eddies and frontal structures.