Microbial community structure along 18°W (39°N–44.5°N) in the NE Atlantic in late summer 2001 (POMME programme)

The spatial and vertical distribution of heterotrophic prokaryotes and ultraphytoplankton (< 10 μm) was investigated by flow cytometry in the NE Atlantic Ocean (Programme Océan Multidisciplinaire Méso Echelle, POMME 3 cruise, 39°N–45°N) with the aim to document the relationships between these distributions and the hydrodynamic features prevailing in the NE Atlantic. A particular interest was given to the north–south transect 18°W (12 stations) because it crossed an anticyclonic cold-core SWODDY-type eddy (slope water oceanic anticyclonic eddy) and was sampled within less than 3 days. Four ultraphytoplankton (< 10 μm) groups (Prochlorococcus, Synechococcus, pico- and nanoeukaryotes) were distinguished by flow cytometry on the basis of their optical properties (scatter, natural fluorescence). Three heterotrophic prokaryotes groups were resolved (two High Nucleic Acid: HNA1 and HNA2 and one Low Nucleic Acid: LNA) after nucleic acid staining. Results showed significant latitudinal variations in biogeochemical parameters as well as in ultraphytoplankton and heterotrophic prokaryote distribution. The inner domain of the eddy also showed variations in heterotrophic prokaryote abundances with respect to the spring situation [Thyssen, M., Lefèvre, D., Caniaux, G., Ras, J., Dugrais, L., Fernández I.C., Denis, M., 2005. Spatial distribution of heterotrophic bacteria in the North East Atlantic (POMME study area) during spring 2001. J. Geophys. Res., 110, C07S16, doi:10.1029/2004JC002670], with higher values of HNA2 during late summer. The same trend was observed on the abundance of cyanobacteria (Prochlorococcus, Synechococcus) and small size ultraphytoplankton. Despite the observed differences in microbial distributions inside and outside the eddy, the biological features appeared insufficient to independently characterize this anticyclonic eddy and provide information on its microbial dynamics.