A third possibility is that changes in biological nitrogen fixation compensate for a declining supply 10 of nitrate from stratification, but not for phosphate (22). Marine nitrogen fixation is often limited by iron (28 -30), and for many reasons, this iron stress may be decreasing . found that Trichodesmium cells use iron more efficiently at elevated temperature (32). Moreover, increasing stratification results in an increase in the Fe:N supply ratio by moderating the relative importance of vertical vs. aeolian nutrient supply . Deposition of soluble iron from combustion 15 sources, including wildfire and fossil fuel burning, has been increasing in recent decades , and significantly impacting marine biogeochemistry . Considering these findings, diazotrophs are more commonly limited by iron in the southern and phosphorus in the northern hemisphere (36). Decreasing iron stress would therefore mostly stimulate nitrogen fixation and phosphate drawdown in the southern hemisphere. Hence, we hypothesize that the regulation of nitrogen 20 fixation is important for the observed decline in phosphate-to-nitrate availability and can explain the hemispherical difference in depth trends.