Primary productivity and N ₂ -fixation in the eastern Indian Ocean: bottom-up support for an ecologically and economically important ecosystem

Oligotrophic regions of the global ocean are characterized by strong nutrient limitation, low standing phytoplankton biomass, and highly efficient nutrient recycling. We quantified nutrient inventories, primary productivity and N ₂ fixation during the BLOOFINZ-IO expedition (February 2022) in the Argo Basin located in the eastern Indian Ocean, the sole known spawning ground for Southern Bluefin Tuna. Surface nitrate concentrations were near depletion (<0.02 µmol L ^-1 ), with low but persistent residual phosphate (P) concentrations suggesting nitrogen as the major limiting nutrient. Depth-integrated net primary production (NPP), from ¹⁴ C-based in-situ incubations during 4 Lagrangian cycles, averaged ∼460 mg C m ^-2 d ^-1 , in good agreement with satellite-based NPP estimates. Nitrogen fixation provided a consistent new nitrogen source, contributing ∼16% to local NPP in the upper euphotic zone.

Gross primary production (GPP), derived from fast-repetition-rate-fluorometry-based electron transport estimates, revealed significant autotrophic respiration losses, with GPP:NPP ratios averaging ∼1.8, consistent with metabolic costs under nutrient limitation. Net community production (NCP), estimated from O ₂ /Ar ratios, remained positive across all cycles, averaging ∼20% of NPP in the upper 30 m. This result, in combination with N ₂ fixation measurement indicates that N ₂ fixation supports most of the export production in this region. Together, the multi-method approach revealed a recycling-dominated ecosystem affected by episodic mixing events, where primary productivity is maintained primarily through efficient nitrogen recycling and physiological photoacclimation. These results provide a comprehensive baseline of bottom-up support on ecosystem productivity for the Argo Basin for assessing future climate-driven changes in stratification, nutrient cycling, and food-web dynamics.