To explore potential temporal biases in the cruise observations, we analyzed nutrient concentrations from the National Oceanic and Atmospheric Administration's World Ocean Atlas data product version 2018 (WOA18). WOA18 is an objectively-analyzed global climatology of nutrient concentrations per 1° of latitude and longitude for every month (61). The vertical profiles reached down to 800 m deep with depth bin sizes ranging 5 -50 m. For every geographic coordinate, we interpolated nitrate and phosphate concentrations per 1 m and found the average nutricline depths.