To test our hypothesis of a deepening nutricline, we estimated the temporal trends for the annually averaged nitracline and phosphacline depths using two separate linear regressions (Fig. ). The nitracline regression shows no significant change over time (p = 0.07). However, the phosphacline 5 significantly deepened (p = 9 x 10 -7 ) by 23 m over the past 5 decades (TPO4 slope = 0.47 m yr -1 , slope SE = 0.10 m yr -1 ) (Fig. ,B and Table ). To put this rate into perspective, the average phosphacline depth in southern oligotrophic waters (45°S to 1°S) is only 62 m, and the average difference between the nitracline and phosphacline depths (ZNO3 -ZPO4) is only 31 m throughout the global oligotrophic ocean (45°S to 45°N) (Table ). To account for seasonality in sampling, we corrected observed nutricline depths by subtracting their respective monthly climatological value. Again, only the phosphacline exhibits a deepening trend, whereas the nitracline displays no temporal trend (Fig. ). However, using different global data sets or threshold concentrations does sometimes indicate nitracline deepening, although this deepening rate never exceeds the rate for phosphacline depths (Table ). Also, nitracline and phosphacline slopes were not significantly 15 different for the lowest threshold concentrations, likely due to poorly resolved phosphacline depths at these low concentrations. Nonetheless, these global regressions consistently show that the phosphacline is deepening faster than the nitracline.