Widespread increase of nitrate in subsurface waters over recent decades revealed by observations

The oceans are under increasing anthropogenic pressure, experiencing warming, acidification and deoxygenation. Nutrient concentrations are likely also changing, still no large-scale changes have yet been reported below the surface. Here, observations reveal a widespread and previously unreported increase in subsurface nitrate across extratropical oceans over the last six decades. The strongest increase occurs in the intermediate waters (∼100-800 m) that connect the surface and deep ocean, reaching up to 1.0 𝝁mol kg −1 decade −1 in the Southern Ocean and 0.4-0.6 𝝁mol kg −1 decade −1 in other extratropical basins-substantial changes given background concentrations of 10-30 𝝁mol kg −1. The enrichment results primarily from enhanced respiration, combined with increased surface nitrate in the Southern Hemisphere. This detected nitrate enrichment exposes profound disturbances to ocean physics and biogeochemistry, and alters the delivery of nutrients regulating primary productivity and carbon cycling.