Phytoplankton carbon and chlorophyll distributions in the equatorial Pacific and Atlantic: A basin-scale comparative study

Satellite datasets show higher chlorophyll concentration in the surface water of the equatorial Atlantic relative to that of the equatorial Pacific. Is the phytoplankton biomass also higher in the Atlantic? To answer this question, a basin-scale ocean circulation-biogeochemistry model that has a phytoplankton dynamic model is utilized to compare the spatial and temporal variations of phytoplankton carbon biomass. We use field data collected in the equatorial Atlantic to derive a new set of biological parameters so that the model can reproduce the general features of the phytoplankton biomass and chlorophyll in this region. For instance, the model reproduces the observed deep chlorophyll a maximum (DCM) that is much deeper to the south (~ 100 m) of the equator than to the north (~ 70 m). The simulated surface chlorophyll also compares well with the satellite derived chlorophyll at basin scale for the equatorial Atlantic. Our comparative analyses demonstrate that for the upwelling region, the phytoplankton biomass in the surface water is considerably higher in the equatorial Pacific than in the equatorial Atlantic. However, for the entire euphotic zone, the integrated phytoplankton biomass is much higher in the equatorial Atlantic than in the Pacific. The difference in the surface water simply reflects larger phytoplankton carbon to chlorophyll ratios in the equatorial Pacific, due to strong iron limitation. The difference in the subsurface water is due to a pronounced deep biomass maximum (DBM) existing in the equatorial Atlantic, which is associated with higher nitrate in the lower euphotic zone. This modeling study emphasizes the importance of using variable C:Chl ratios to estimate carbon biomass at regional to global scales.

► A model reproduces the general features of phytoplankton C and chlorophyll.
► Phytoplankton C in the surface water is higher in the Pacific than in the Atlantic.
► Phytoplankton C in the subsurface is higher in the Atlantic than in the Pacific.
► Large C:Chl ratios cause higher surface C biomass in the equatorial Pacific.
► Deep biomass maxima yield higher C biomass in the euphotic zone of the Atlantic.