Biogeochemical models predict that ocean warming is weakening the vertical transport of nutrients to the upper ocean, with severe implications for marine productivity. However, this prediction has not yet been observed due to the challenge of detecting low nutrient concentrations across the ocean surface.
We analyzed over 30,000 nitrate and phosphate depth profiles observed between 1972 and 2022 to quantify nutricline depths. These depths accurately represent nutrient fluxes, allowing us to assess long-term trends in supply rates. Over the past five decades, upper ocean phosphate has declined worldwide due to stratification. However, nitrate has remained mostly stable. Model simulations suggest that this difference is likely due to nitrogen fixation replenishing upper ocean nitrate, whereas phosphate has no equivalent biological source.