Planetary Diagnosis of Phytoplankton Iron Stress

Iron limits phytoplankton growth across vast ocean areas, particularly in high-nutrient, low-chlorophyll regions. Yet, the global dynamics of iron stress remain poorly understood. Here, we integrate satellite-derived chlorophyll fluorescence observations, diverse field measurements, and a biogeochemical model to evaluate global phytoplankton iron stress over the past two decades. Satellite, field, and model observations show strong agreement and indicate vast ocean regions experiencing iron stress, including areas beyond traditional high-nutrient, low-chlorophyll regions. We observe climate-driven trends in iron stress associated with rising surface temperatures, with stress intensifying in mesotrophic and easing in oligotrophic waters. These contrasting trends are linked to shifts in co-limiting resource availability. Consequently, iron stress varies out of phase between the core upwelling zone and the broader equatorial Pacific during El Niño/Southern Oscillation cycles. Our findings suggest that future climate change will have opposing effects on iron limitation in nutrient-rich versus nutrient-poor marine biomes.