Annual variability of photosynthetic parameters and primary production (PP), with special focus on large (i.e. >2 µm) phytoplankton was assessed by monthly photosynthesis-irradiance experiments at two depths of the southern Bay of Biscay continental shelf in 2003. Integrated chl a (22-198 mg m -2 ) was moderately dominated by large cells on an annual basis. The March through May dominance of diatoms was replaced by similar shares of dinoflagellates and other flagellates during the rest of the

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... ar. Variability of photosynthetic parameters was similar for total and large phytoplankton, but stratification affected the initial slope α B [0.004-0.049 mg C mg chl a -1 h -1 (μmol photons m -2 s -1 ) -1 ] and maximum photosynthetic rates P B m (0.1-10.7 mg C mg chl a -1 h -1 ) differently. P B m, correlated positively with α B only for the large fraction. P B m tended to respond faster to ambient irradiance than α B , which was negatively correlated with diatom abundance in the >2 µm fraction. Integrated PP rates were relatively low, averaging 387 (132-892) for the total and 207 (86-629) mg C m -2 d -1 for the large fraction, likely caused by inorganic nutrient limitation. Although similar mean annual contributions of large phytoplankton to total values were found for biomass and PP (58%), water-column production to biomass ratios (2-26 mg C mg chl -1 d -1 ) and light utilization efficiency of the >2 µm fraction (0.09-0.84 g C g chl -1 mol photons -1 m 2 ) were minimum during the spring bloom. Our results indicate that PP peaks in the area are not necessarily associated to maximum standing stocks.