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 (Kd) 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.