E. Maranon et Pm. Holligan, Photosynthetic parameters of phytoplankton from 50 degrees N to 50 degreesS in the Atlantic Ocean, MAR ECOL-PR, 176, 1999, pp. 191-203
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.