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

Abstract 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 ( 3 He) to constrain the vertical diffusion coefficient ( K d ) in the Indonesian seas, which ranges from 5.2 × 10 −5 to 2.3 × 10 −3 m 2 s −1 and averages 6.6 × 10 −4 m 2 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 (NO 3 − ) is vertically transported into the surface mixed layer, that is, >90% of the total NO 3 − 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.