Photosynthetic parameters and primary production, with focus on large phytoplankton, in a temperate mid-shelf ecosystem

• P-E relationships were measured monthly in 2003 for total and large phytoplankton.
• Different photosynthetic performance showed little photoacclimation in large cells.
• Water-column production to biomass was only predictable for large phytoplankton.
• Low annual primary production (139 g C m−2 y−1) was due to nutrient limitation.

Annual variability of photosynthetic parameters and primary production (PP), with a 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 year. Variability of photosynthetic parameters was similar for total and large phytoplankton, but stratification affected the initial slope αB [0.004–0.049 mg C mg chl a−1 h−1 (μmol photons m−2 s−1)−1] and maximum photosynthetic rates PmB (0.1–10.7 mg C mg chl a−1 h−1) differently. PmB, correlated positively with αB only for the large fraction. PmB tended to respond faster to ambient irradiance than αB, 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, probably the result of 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 m2) were minimum during the spring bloom. Our results indicate that PP peaks in the area are not necessarily associated to maximum standing stocks.