We also observed extensive variability among site-specific trends (Fig. ), including both deepening and shoaling nutriclines across smaller ocean regions. Multiple mechanisms, including seasonality, likely contribute to the observed variation among sites. However, we will next show that none of the tested mechanisms can account for the long-term deepening in the phosphacline, but not nitracline. Simulating the actual sampling dates onto a fixed monthly climatology (WOA18 5 with no long-term change) resulted in interquartile ranges of TNO3 and TPO4 that are about 50% of the observed variances. This suggests that monthly variability in nutrient concentrations is an important source of variation in site-specific trends (Fig. and Fig. ). Next, we constructed 10,000 random populations of the fixed monthly climatology to robustly quantify the effect of seasonal variance without any long-term changes (Fig. ). The median trends in nutricline 10 depths are indistinguishable from zero, showing that monthly variability does not lead to an overall bias in median trends. We then introduced an artificial fixed long-term trend at all sites in the random populations. This resulted in the same variance with different locations showing a mix of deepening or shoaling trends in agreement to what we found in the actual observations. However, the median shifted to match the fixed imposed trend (Fig. ). We interpret these analyses as 15 the observed median capturing the actual underlying tendency in trends, whereas the variance in site-specific trends is mostly attributed to seasonality. To match both the observed median trends and overall variance in depths, each nitracline trend had to be sampled from a normal distribution with a shoaling tendency (mean = -0.63 m yr -1 , SD = 1.5 m yr -1 ), and each phosphacline trend came from one with a deepening tendency (mean = 0.20 m yr -1 , SD = 1.0 m yr -1 ) (Fig. ). This 20 demonstrates that the median trend robustly captures the underlying tendency of the site-specific trends despite their high variability. In summary, these analyses show that sampling timing contributes to the observed variability in trends between sites, but any overall shift in the median must come from a long-term change in nutricline depths.