Outside of seasonality, variability in the site-specific trends could also arise from measurement error or interannual climate forcings. When we simulated measurement uncertainty to the observed nutrient concentrations, we found that the interquartile range of TNO3 only increased about 5% (Fig. ). In contrast, the interquartile range of TPO4 increased about 40% due to the low threshold 5 concentration (Fig. ). This suggests that measurement error is a considerable factor for the variability of TPO4 but not TNO3, despite both having nearly equal total variance. However, accounting for sampling error to nutrient concentrations still yielded a median TNO3 indicative of shoaling, and a median TPO4 indicative of deepening (median TNO3 = -0.18 m yr -1 , CI95% = [-0.27, -0.03]; median TPO4 = 0.26 m yr -1 , CI95% = [0.09, 0.43]). Part of the total variance could also be 10 attributed to ENSO or other climate modes, which can affect upper ocean nutrient profiles on interannual timescales . We found that during some El Niño years, the global median nitracline and phosphacline depths were relatively deeper (Fig. ). Thus, the combination of seasonal and interannual changes in nutrient concentrations together with measurement uncertainty explain most of the observed variance in nutricline trends.