Basic photosynthetic characteristics determining the shift in phytoplankton species and size composition from coccolithophores and dinoflagellates to diatoms were investigated. The study revealed a slight seasonal decrease in the light utilization efficiency (cr,,,) from 0.078 to 0.056 mg C (mg chl a-'] h-' (W m-') from winter to early spring, a s well as a small increase in the photosynthesis light saturation (Ik; 29.4 and 45.5 W m-2, respectively) and in the diurnal assimilatory number [DAN;19.6 and 24.0 mg C (mg chl a)-' d-l, respectively]. At the same time, respiration (R) fell drastically: in wmter, when phytoplankton was dominated by dinoflagellates and coccolithophores, R was 29 O/O of maximum production (P,,,), while in spring, with diatoms dornlnating, R was 2.5 % of P,,,,,. Chlorophyll a concentration also showed a drastic increase: from 0.73 to 2.15 mg m-3 in the surface layer and from 20 to between 100 and 150 mg m-' on an area1 basis. These data suggest that the nearly 2-fold growth of primary production from winter to spring (275.2 t 163.5 and 581 4 & 232.4 mg C m-* d-l, respectively) is accounted for by larger phytoplankton biomass and lower respiration. These changes were mainly brought about by an increase in diatoms: the increase of both production and chlorophyll a from winter to spring features an ever greater diatom contribution, reaching > 50 % in spring. A close relationship is suggested between the increase in primary production and the changes in the structure of seawater from winter to spring.