Biogeochemical float deployment strategy in the Southwestern Indian ocean

The recent roadmap IndOOS-2 has stressed the need to expand the biogeochemical-Argo observing system in the Indian Ocean. The Monaco Explorations Indian Ocean expedition offered a unique opportunity to meet this goal in the southwestern sector which was, in this regard and at that time, one of the least covered oceanic regions. We designed a deployment strategy for the biogeochemical float array grounded on past experiences, existing knowledge, and the analysis of historical datasets to cover the contrasting biophysical regimes from the Seychelles Chagos Thermocline Ridge to the subtropical gyre. Aligning with IndOOS-2 recommendations, a denser float distribution was set in the tropical band to enhance biogeochemical observations in upwelling zones. Following this strategy, a fleet of seventeen biogeochemical floats was successfully deployed during the expedition in October–November 2022. After two years of operations, the spatio-temporal distribution covered by the fleet confirmed that the goals of the deployment strategy have been reached, revealing seasonal modulations of the meridional trophic gradient with respect to phytoplankton biomass from tropical mesotrophy to subtropical oligotrophy.