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 (He-3) to constrain the vertical diffusion coefficient (K-d) in the Indonesian seas, which ranges from 5.2 x 10(-5) to 2.3 x 10(-3) m(2) s(-1) and averages 6.6 x 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 (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. The Indonesian seas, at the center of the Indo-Pacific Intersection, are recognized as a global hotspot of marine biodiversity. However, nutrient (e.g., nitrate) cycling in the Indonesian seas is poorly understood, such that nutrient sources and fluxes sustaining biological production remain unknown. The Indonesian seas lie on a plate tectonic belt, where intense submarine hydrothermal venting releases abundant primordial isotopic helium (He-3) into the ocean interior that outgasses at surface providing an ideal tracer of vertical transport. We find that vertical diffusion in the Indonesian seas is notably stronger than those found in the open ocean and in most marginal seas, with a mean vertical diffusion coefficient (K-d) of 6.6 x 10(-4) m(2) s(-1). Nitrate is vertically transported into the surface mixed layer at a rate of 6.9 +/- 7.9 mmol m(-2) d(-1), which supports >90% of net community production (NCP) in the Indonesian seas. Here, narrow straits, steep and irregular topography and dynamic circulation with strong vertical mixing result in high chlorophyll-a concentrations (a measure of primary producers' biomass) and NCP. This vertical nutrient transport supplies essential conditions for algal growth and "fuels" food web biological productivity. Thus, we suggest that strong vertical mixing plays a key role in making the Indonesian seas a global biodiversity hotspot.