Nutrient Vertical Flux in the Indonesian Seas as Constrained by Non-Atmospheric Helium-3

The Indonesian seas are a renowned global biodiversity hotspot, yet nutrient sources and fluxes (especially the vertical flux) sustaining this richness remain unclear. Here, we used non-atmospheric helium-3 (³He) to constrain the vertical diffusion coefficient (K_d) in the Indonesian seas, which ranges from 5.2 × 10⁻⁵ to 2.3 × 10⁻³ m² s⁻¹ and averages 6.6 × 10⁻⁴ m² s⁻¹, a value notably higher than those found in the open ocean and in most marginal seas. We estimated that 6.9 ± 7.9 mmol m⁻² d⁻¹ of nitrate (NO₃⁻) is vertically transported into the surface mixed layer, that is, >90% of the total NO₃⁻ required to support a net community production (NCP) of 470 ± 467 mg-C m⁻² d⁻¹. Regions with narrow straits, steep topography and dynamic circulation with strong vertical mixing display high NCP and chlorophyll-a, suggesting that vertical nutrient transport dominates biological productivity. Findings highlight the importance of vertical mixing in supplying nutrients and maintaining the extraordinary biological productivity and diversity in the Indonesian seas.