The response of planktonic heterotrophic prokaryotes to the NW Mediterranean winter-spring offshore phytoplankton bloom was assessed in 3 cruises conducted in March, April-May and September 2009. Bulk measurements of phytoplankton and bacterioplankton biomass and production were complemented with an insight into bacterial physiological structure by single-cell analysis of nucleic acid content [low (LNA) vs. high (HNA)] and membrane integrity (>Live> vs. >Dead> cells). Bacterial production empirical conversion factors (0.82±0.25SEkgCmolleucine-1) were almost always well below the theoretical value. Major differences in most microbial variables were found among the 3 periods, which varied from extremely high phytoplankton biomass and production during the bloom in March (>1gCm-2d-1 primary production) to typically oligotrophic conditions during September stratification (Live> cells (47-97%) were temporally opposite in the study periods, with maxima in March and September, respectively. Different relationships were found between physiological structure and bottom-up variables, with HNA bacteria apparently more responsive to phytoplankton only during the bloom, coinciding with larger average cell sizes of LNA bacteria. Moderate phytoplankton-bacterioplankton coupling of biomass and activity was only observed in the bloom and post-bloom phases, while relationships between both compartments were not significant under stratification. With all data pooled, bacteria were only weakly bottom-up controlled. Our analyses show that the biomass and production of planktonic algae and bacteria followed opposite paths in the transition from bloom to oligotrophic conditions. © 2014 Elsevier Ltd.

This work was financially supported by grant FAMOSO (CTM2008-06261-C03/MAR) and, partially, also by grant HotMix (CTM2011-30010/MAR) and the Grup de Recerca de la Generalitat de Catalunya (2009SGR/1177). A.G. had a fellowship from the Portuguese FCT and L. F.-V. was supported by an IEO Fellowship

10 pages, 5 figures, 5 tables

Peer Reviewed