There are multiple possible biogeochemical drivers for shifting nutricline depths, including changes in phytoplankton resource demands, external nutrient inputs (riverine or atmospheric), or biological nitrogen fixation. First, it is commonly proposed that phytoplankton have an increasing N:P demand in a warming ocean due to a reduction of P-rich ribosomes needed for protein synthesis (25). We find that this biochemical mechanism is unlikely to explain our results as it implies a faster deepening of the nitracline relative to the phosphacline. Second, changes in riverine inputs and atmospheric deposition of predominantly nitrogenous material can increase the demand for phosphorus relative to nitrogen. This mechanism is also unlikely to explain our results, as increasing nitrogenous input is primarily occurring in the northern hemisphere , 5 whereas a deepening phosphacline is mostly observed in the southern hemisphere (Fig. and Table ). Hence, neither changes in phytoplankton demands nor external inputs can explain the observed nutricline trends.