Photosynthetic parameters of phytoplankton from 50 degrees N to 50 degreesS in the Atlantic Ocean

We conducted 150 photosynthesis-irradiance (P-E) experiments along 2 Atlant ic meridional transects from 50 degrees N to 50 degrees S in April-May and October-November 1996. The latitudinal and vertical distributions of the ma ximum chlorophyll a-normalized rate of photosynthesis (P-m(B)) and the init ial slope of the P-E curve (alpha(B)) were examined in relation to the Vari ations in relevant physical, chemical and biological variables. P-m(B) rang ed from <1 mg C mg chl(-1) h(-1) in the central oligotrophic gyres to >10 m g C mg chl(-1) h(-1) in temperate regions and the upwelling area off Maurit ania. The dynamic range of the observed variations in the P-E parameters wa s 3 to 4 times higher than assumed in productivity models that divide the o cean into biogeochemical provinces. Variability in the physiological parame ters of phytoplankton was as high as that of chlorophyll concentration. We obtained a model of multiple linear regression to calculate integrated prim ary productivity from data of surface temperature, chlorophyll a and P-m(B) . Changes in P-m(B) accounted for 30% of the total variability in productiv ity, whereas variations in chlorophyll a explained only 5%, which indicates that phytoplankton photophysiology is more relevant than biomass in the co ntrol of primary productivity. We found a significant, negative correlation between the latitudinal changes in P-m(B), and those in the depth of the n itracline, suggesting an important role for the nutrient supply from below the thermocline in the regulation of photosynthetic efficiency over large s patial scales. A large degree of temporal variability was observed in the s ubtropical gyres: P-m(B) and alpha(B) varied by a factor of 3 between the 2 cruises, whereas phytoplankton biomass remained constant. The differences in the photosynthetic parameters between seasons were larger than between b iogeochemical provinces. We emphasize the need to include nutrient-driven c hanges of phytoplankton photophysiology in models of primary productivity.