Seasonal Variability of Phytoplankton Biomass Revealed by Satellite and BGC‐Argo Data in the Central Tropical Indian Ocean

The variability of phytoplankton biomass related to ocean's biological carbon pump in the central tropical Indian Ocean is not fully understood. Using satellite and biogeochemical Argo data, we found that phytoplankton biomass exhibits significantly different and even opposite changes at the near‐surface (0–10 m) and subsurface layer (50–100 m). Results showed that the shoaling/deepening of the mixed layer and thermocline dominated the phytoplankton biomass change in different layers. In February–May, the shallow mixed‐layer depth (MLD) exacerbated the oligotrophic environment in the mixed layer, thus resulting in low near‐surface phytoplankton biomass (Chl a 50 m), hence increasing the supply of both nutrients and phytoplankton from the subsurface to maintain the near‐surface phytoplankton blooms (Chl a > 0.2 mg m−3) in June–October. Below the mixed layer, wind‐induced upwelling/downwelling modulated the thermocline, which significantly affects the supply of nutrients for phytoplankton growth in the subsurface layer. The shallower thermocline during February and March provided favorable conditions for the subsurface blooms (Chl a > 0.5 mg m−3) with a prominent subsurface chlorophyll maximum layer (SCML). Conversely, the deeper thermocline in June and July was related to the lowest phytoplankton biomass with the disappearance of the SCML. These results may help explain the response of vertical phytoplankton to changes driven by multiple atmospheric and physical forcing factors in future climate‐change scenarios. Plain Language Summary Satellite and biogeochemical float are essential platforms for quantifying phytoplankton biomass in the upper ocean, which is critical for understanding ocean's biological carbon pump that removes carbon dioxide from the atmosphere. Phytoplankton variation in the upper‐water column, however, is not fully understood. We showed that the seasonal variation of phytoplankton in the near‐surface layer differs from that in the subsurface layer in the central tropical Indian Ocean (CIO) using satellite and float data. We demonstrated that various responses of phytoplankton in the upper water column are triggered by seasonal wind bursts, including the deepening of the mixed layer and shoaling of the thermocline. In February–May, weak wind and enhanced stratification resulted in low near‐surface phytoplankton biomass. However, the s