Persistent equatorial Pacific iron limitation under ENSO forcing

Projected responses of ocean net primary productivity to climate change are highly uncertain(1). Models suggest that the climate sensitivity of phytoplankton nutrient limitation in the low-latitude Pacific Ocean plays a crucial role(1-3), but this is poorly constrained by observations(4). Here we show that changes in physical forcing drove coherent fluctuations in the strength of equatorial Pacific iron limitation through multiple El Nino/Southern Oscillation (ENSO) cycles, but that this was overestimated twofold by a state-of-the-art climate model. Our assessment was enabled by first using a combination of field nutrient-addition experiments, proteomics and above-water hyperspectral radiometry to show that phytoplankton physiological responses to iron limitation led to approximately threefold changes in chlorophyll-normalized phytoplankton fluorescence. We then exploited the >18-year satellite fluorescence record to quantify climate-induced nutrient limitation variability. Such synoptic constraints provide a powerful approach for benchmarking the realism of model projections of net primary productivity to climate changes.

Automated summary

The impact of climate change on ocean net primary productivity is highly uncertain, with phytoplankton nutrient limitation in the low-latitude Pacific Ocean playing a crucial role, but poorly constrained by observations. We found that changes in physical forcing resulted in consistent fluctuations in the strength of equatorial Pacific iron limitation through El Nino/Southern Oscillation (ENSO) cycles, which were overestimated by a state-of-the-art climate model. Our assessment provides a powerful approach for evaluating the realism of model projections of net primary productivity to climate changes.