Photosynthetic parameters of phytoplankton from 50°N to 50°S in the Atlantic Ocean

Abstract

We conducted 150 photosynthesis-irradiance (P-E) experiments along 2 Atlantic meridional transects from 50°N to 50°S in April-May and October-November 1996. The latitudinal and vertical distributions of the maximum chlorophyll a-normalized rate of photosynthesis (P(m)/(B)) and the initial slope of the P-E curve (α(B)) were examined in relation to the variations in relevant physical, chemical and biological variables. P(m)/(B) ranged from <1 mgC mg chl-1 h-1 in the central oligotrophic gyres to > 10 mg C mg chl-1 h-1 in temperate regions and the upwelling area off Mauritania. The dynamic range of the observed variations in the P-E parameters was 3 to 4 times higher than assumed in productivity models that divide the ocean into biogeochemical provinces. Variability in the physiological parameters of phytoplankton was as high as that of chlorophyll concentration. We obtained a model of multiple linear regression to calculate integrated primary 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 productivity, whereas variations in chlorophyll a explained only 5%, which indicates that phytoplankton photophysiology is more relevant than biomass in the control of primary productivity. We found a significant, negative correlation between the latitudinal changes in P(m)/(B) and those in the depth of the nitracline, suggesting an important role for the nutrient supply from below the thermocline in the regulation of photosynthetic efficiency over large spatial scales. A large degree of temporal variability was observed in the subtropical gyres: P(m)/(B) and α(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 biogeochemical provinces. We emphasize the need to include nutrient-driven changes of phytoplankton photophysiology in models of primary productivity.