Climate sensitivity of Equatorial Pacific nutrient limitation

Projected responses of ocean net primary productivity (NPP) to climate change are highly uncertain. The climate sensitivity of phytoplankton nutrient limitation in the low-latitude Pacific plays a crucial role, but field measurements are insufficient to provide suitable constraints. Here we quantify two decades of nutrient limitation in the Equatorial Pacific with satellite observations. Using field nutrient addition experiments, proteomics, and above-water hyperspectral radiometry, we demonstrate that physiological responses of phytoplankton to iron limitation led to ~3-fold increases in chlorophyll-normalized phytoplankton fluorescence. Extension to the >18-year satellite fluorescence record showed that Equatorial Pacific iron limitation was robust to changes in physical forcing through multiple El Niño–Southern Oscillation cycles, despite coherent fluctuations in limitation strength. In contrast, these iron limitation changes were overestimated 2-fold by a state-of-the-art climate model. Such synoptic constraints provide a powerful new approach for benchmarking the realism of model NPP projections to climate changes.