Spatial and temporal variability of the phytoplankton carbon to chlorophyll ratio in the equatorial Pacific: A basin-scale modeling study

The relationship between phytoplankton carbon biomass and chlorophyll is nonlinear because of the complex impacts of light, nutrient conditions, and temperature. A basin-scale ocean circulation-biogeochemistry model implemented with a dynamic phytoplankton model is employed to explore the spatial and temporal variability of the phytoplankton carbon to chlorophyll (C:Chl) ratio in the equatorial Pacific Ocean. The dynamic model computes the phytoplankton C:Chl ratio as a function of light, nitrate, iron, and temperature. The model reproduces well the general features of phytoplankton dynamics in this region, e.g., the deeper chlorophyll maximum (DCM) in the western warm pool and shallower DCM in the upwelling region. The model predicts large spatial and temporal variations of the C:Chl ratio. The mixed layer C:Chl ratio increases from < 100 in the eastern upwelling region to > 150 in the warm pool, whereas subsurface ratio is similar to 50 below 100 m. The model produces a weak seasonality in the mixed layer C:Chl ratio but strong interannual variability that is associated with the El Nino-Southern Oscillation (ENSO) events. The warm pool has strong anomalies during the cold phase of the ENSO with a reduced C:Chl ratio and a shoaled DCM. However, the upwelling region reveals strong anomalies during the warm phase of the ENSO, showing an increased C:Chl ratio in the euphotic zone and a deepened DCM. The predicted large spatial and temporal variations of the C:Chl ratio have potential implications for the carbon uptake in the equatorial Pacific.