Data for: 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 to 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.