Gross community production (GCP), dark community respiration (DCR), and the biomass of the different size classes of organisms in the microbial community were measured in the northeastern Atlantic basin as part of the Programme Oc¿an Multidisciplinaire M¿so Echelle (POMME) project. The field experiment was conducted during three seasons (winter, spring, and late summer--fall) in 2001. Samples were collected from four different mesoscale structures within the upper 100 m. GCP rates increased from winter (101 ¿ 24 mmol O2 m-2 d-1) to spring (153 ¿ 27 mmol O2 m-2 d-1) and then decreased from spring to late summer (44 ¿ 18 mmol O2 m-2 d-1). DCR rates increased from winter (-47 ¿ 18 mmol O2 m-2 d-1) to spring (-97 ¿ 7 mmol O2 m-2 d-1) and then decreased from spring to late summer (50 ¿ 7 mmol O2 m-2 d-1). The onset of stratification depended on latitude as well as on the presence of mesoscale structures (eddies), and this largely contributed to the variability of GCP. The trophic status of the POMME area was defined as net autotrophic, with a mean annual net community production rate of +38 ¿ 18 mmol O2 m-2 d-1, exhibiting a seasonal variation from +2 ¿ 20 mmol O2 m-2 d-1 to +57 ¿ 20 mmol O2 m-2 d-1. This study highlights that small organisms (picoautotrophs, nanoautotrophs, and bacteria) are the main organisms contributing to biological fluxes throughout the year and that episodic blooms of microphytoplankton are related to mesoscale structures.