Observed declines in upper ocean phosphate-to-nitrate availability

Climate warming is increasing ocean stratification, which in turn should decrease the flux of nutrients to the upper ocean. This may slow marine primary productivity, causing cascading effects throughout food webs. However, observing changes in nutrient concentrations at the ocean surface is challenging because they are often below detection limits. The nutricline depth, where nutrient concentrations reach well-detected levels, is related with productivity and indicates upper ocean nutrient availability. Here, we quantified nutricline depths from a global database of observed vertical nitrate and phosphate profiles (1972 - 2022) to assess contemporary trends in global nutrient availability. We found strong evidence that the P-nutricline (phosphacline) is mostly deepening, especially throughout the southern hemisphere, but the N-nutricline (nitracline) remains mostly stable. Earth System Model simulations support the hypothesis that reduced iron stress and increased nitrogen fixation buffer the nitracline, but not phosphacline, against increasing stratification. These contemporary trends are expected to continue in the coming decades, leading to increasing phosphorus but not nitrogen stress for marine phytoplankton, with important ramifications for ocean biogeochemistry and food web dynamics.