Seasonal variability of the phytoplankton biomass and its underlying processes in contrasted regions of the South Pacific Ocean based on BioGeoChemical-Argo observations

The South Pacific Ocean stands out as a dynamical region with contrasted biogeochemical (BGC) regimes. Among others, it encompasses mesotrophic areas as well as the most oligotrophic waters of the global ocean. Within these regions, the existing thousands of islands can locally or regionally disrupt the oceanic circulation and, thereby, thus the nutrient availability in the upper sunlit layer and the phytoplankton growth. Yet, the phytoplankton seasonal dynamics in these contrasted BGC regions remain largely unknown and understood. Indeed, most existing studies on phytoplankton seasonal variability from observations are either dedicated to the global ocean or based on remotely sensed data due to a lack of in-situ observations in the water column, preventing the consideration of 3D processes.

Here we took advantage of in situ observations from 13 BGC-Argo profiling floats that have drifted from 2015 to 2023 in five subregions of the South Pacific Ocean: the Tasman and Coral Seas, the Fiji island region, the oligotrophic and equatorial mesotrophic areas. We used measurements of temperature, salinity, chlorophyll-a fluorescence (Chl), particulate backscattering at 700 nm (b_bp) used as a proxy of particulate organic carbon and Photosynthetically Active Radiation. The seasonal variations of Chl and b_bp vertical distributions are characterized among the subregions and physical and biogeochemical processes likely involved have been investigated. To do so, we considered isolume and nutricline depths, the Mixed Layer Depth (MLD) as well as the maximum Brunt-Vaissala depth as an indicator of the ocean stratification stability. The latter appears more suitable than the MLD when related to the phytoplankton seasonal dynamics.